By the end of March this year, every council in England had to publish a report on what it is doing for biodiversity. Councils that are also local planning authorities — Bristol among them — must go further, and give details of the biodiversity net gain resulting, or expected to result, from the biodiversity gain plans they have approved.
Some time earlier this year, Bristol City Council published its Biodiversity Duty Report (the Report) covering the period from January 2020 to December 2025. It is a handsome document, and much of it is genuinely positive: it lists 43,523 trees planted, canopy cover up to 18.2%, and pesticide use down 42%.
On the question of BNG, though, it publishes only a single table which can’t answer the question a local resident might ask: Is new development in Bristol leaving our local nature better off or worse off?
What BNG is meant to do
Since February 2024 most new developments in England have had to deliver a 10% increase in biodiversity. Developers measure what’s on a site before they build — using a government metric calculation which converts various habitat types into ‘biodiversity units’ — and then show how they’ll deliver at least 10% more biodiversity than was there before they started.
They can do this in three ways: improve habitat on the site itself; pay for habitat to be created somewhere else; or — as a last resort — buy statutory credits from the Government. Before building can start, the Council must approve the developer’s gain plan showing how the minimum 10% BNG will be achieved.
Each gain plan is a precise record (backed by a metric calculation) of what a development will achieve. Bristol has approved a number of these so far but the Report tells you almost nothing about what’s in them.
What Bristol actually published
Here’s everything the Council’s published about its approved gain plans, as listed in the Report:
Look at the column headed ‘No. of Units post-intervention’. These are the biodiversity units expected after a development is completed. There’s no column for the value of the habitats that were there before (called the baseline). Without a baseline, we can’t know what the net gain percentage is. In the row labelled ‘On-site BNG only’ there’s a reported 275% average % net gain but the other rows are left blank, so it appears not to be an average across the seven plans reported. Three small schemes with very little habitat to begin with can easily produce a number like this: if a site starts with almost nothing, almost any new habitat can produce a large percentage increase. Published on its own, without the baseline values that would put it in proportion, it reads as a triumph but is meaningless because it lacks essential information.
What the gain plans we’ve examined say
Bristol Tree Forum has been collecting biodiversity gain plans from councils across England — 301 of them so far, from 34 authorities. We’ve found 18 for Bristol, 16 of which record a figure for the change in area habitat units on the development site. Twelve of the 16 lose habitat on site. Added together, the 18 Bristol plans show a net loss of 10.9 area habitat units within the red lines of the sites themselves. The four that gain contribute 0.24 units between them. One scheme, at Broomhill Road, records the loss of every unit on the site — 100%.
To be fair to the Council, most of those plans were approved during 2026 and so fall outside the period the Biodiversity Duty Report covers. However, six of them are dated on or before 31 December 2025 and belong squarely inside it; all of these six lose habitat on site, by 4.68 units between them.
Now, an on-site loss is not by itself a breach of anything. That’s exactly what the off-site habitat unit purchase option is for: where a site cannot accommodate the lost habitat on site, the developer is allowed to buy habitat units elsewhere to meet the 10% plus obligation. Nine of Bristol’s 18 plans deliver through a combination of on-site and off-site compensation, and four go entirely off-site. So, on paper, the obligation is met.
But a resident reading the Council’s report would have no idea that any of this was happening. The published table doesn’t state what the baseline habitat was or distinguish between gains and losses. It doesn’t say where the off-site habitat is. And it doesn’t name a single application, so there is no way to look one up.
Where does Bristol’s biodiversity go?
The report does contain one sentence that deserves more attention than it’s had. There’s no registered biodiversity gain site anywhere within the Bristol boundary. The Council’s own candidate sites ‘remain unregistered while a viable solution to procuring a legal agreement is worked towards’, and are still at scoping stage. As far as we’re aware, no Biodiversity Gain Site has yet been registered for the city.
That means every off-site unit bought to offset a Bristol development creates habitat outside the city. Thirteen of our 18 plans use off-site delivery in whole or in part. The habitat lost is in Bristol; the habitat created to compensate for it is not. So far, that compensation amounts to 14.60 units allocated to sites outside the city — 14.55 area units, 0.01 hedgerow units and 0.04 watercourse units. To see them, open https://bgs.bristoltrees.space/bgs-bodies, select LPA Allocation Maps and click on City of Bristol LPA on the map.
Other councils can manage it
Publishing a schedule of all the approved BGP approvals is not difficult. Other authorities are already doing it. For example, Cotswold District Council has published an appendix listing all 24 of its approved gain plans — application reference, date, delivery route, and the units and percentage change for habitats, watercourses and hedgerows on each one. Bolton publishes the same for its four, including the name of the habitat bank each one bought into.
County Durham has not published a schedule of all its approved applications, but it has published the full Government reporting tables, which show, without flinching, that on-site habitat units across its 16 approved plans fell by an average of 30.85%.
Durham’s report is uncomfortable reading for Durham, but it is for that reason worth something; a report that cannot embarrass its author is not really a report.
What we’d like to see
Bristol has met the letter of its duty but it could have gone further and produced a meaningful schedule that would have made sense to the ordinary reader and been used to help inform a proper debate about how the city is (or is not) benefiting from the BNG regime. On our analysis, it’s not.
Three changes would fix this, and none of them requires new data — the Council already holds all of it:
Publish the baseline alongside the post-development figure, so a net change can be worked out.
List the approved gain plans individually, with the application reference, so anyone can find the application and check it.
Say where the off-site units are, and how many have left the city.
There’s some urgency to this. The reporting cycle is five years, by which time a new Local Plan will probably have been adopted. If we have the information now, this might be used in the development of any new Local Plan so that it provides a real benefit to Bristolians. Unless Bristol chooses to publish an update voluntarily, the next time it’s legally required to tell us anything about biodiversity net gain will be 2031 — by which point seven years of development will have been permitted, built and largely finished, on the strength of a table that serves no practical purpose.
Bristol City Council’s Biodiversity Duty Report 2020–2025 is published at bristol.gov.uk. The figures attributed to gain plans in this article are drawn from Bristol Tree Forum’s own collection of biodiversity gain plans obtained from planning registers and freedom of information requests, and are ours rather than the Council’s. We’d be glad to be corrected on any of them.
The cover image above is extracted from the Council’s Biodiversity Duty Report.
Since February 2024 biodiversity net gain has been mandatory for major development in England – and for small sites since April of that year. Two and a half years on, there’s still no publicly available national dataset of the gains that developers promised and say they’re delivering.
While the Biodiversity Gain Sites Register records off-site allocations, these are a small element of the whole BNG regime. The required biodiversity gain plans themselves — the information and documents that state, scheme by scheme, what the baseline biodiversity was and what biodiversity gain the development will deliver — sit dispersed across individual planning portals. Each portal is managed by each of over 300 planning authorities and the information is not nationally collated.
So far, we’ve collected 263 of these biodiversity gain plans from 31 Local planning authorities (about 10% of authorities). We’ve audited them field by field against the published source documents. This is what they show.
How we created the dataset
Every gain plan here was found on a local planning authority’s portal as an approved application, with a published Biodiversity Gain Plan. To date, the earliest is dated 2022; 27 are from 2024, 129 from 2025 and 91 from 2026. The authorities range from Bristol, BaNES and East Suffolk, with 18 or 19 gain plans each, down to several with just one. Site areas, where a statutory metric calculation is published and the baseline habitat area can be read from it, run from a few hundred square metres to several hectares, with a median of 0.225 ha across the 133 gain plans that we have been able to locate so far.
The sample is not representative, and depended on our being able to locate gain plans on planning portals using a variety of search approaches. These searches did not always succeed and we have probably failed to identify all the approved gain plan applications on the portals we were able to interrogate.
How we found the documents
Collecting these 263 plans was harder than analysing them. Once the right application is found, the documents are usually easy to identify: they are labelled ‘Biodiversity Gain Plan’, ‘Net Gain’, ‘metric’ or something equally simple, and the labelling is broadly consistent between planning authorities. The real challenge is finding the application on the authority’s planning portal in the first place.
A gain plan must be submitted to discharge the statutory ‘deemed’ planning condition required under the BNG regime, so it sits within a related discharge application rather than within the original permission. Finding it means knowing which discharge application to look for, and under which application type (assuming there is one). Save for the regulations, there is no guidance on how the biodiversity gain condition should be discharged or what should be published when, so every authority has settled the question differently. A minority have a dedicated application type — e.g. ‘Approval of Details – BNG’ — and one search usually returns everything, but not always : on some planning portals this search returns nothing.
For example, the Bristol City Council planning portal Advanced search option returns no decided results when the ‘Discharge Deemed BNG Condition’ Application Type is used. Only two pending applications are found, but on closer examination, even though one contains the required Biodiversity Gain Plan document, there is no required statutory metric calculation, while the other one relates to an application that predates the BNG regime.
In all other cases the only method is to guess using search terms like ‘biodiversity’, ‘BNG’, ‘gain plan’, ‘net gain’, and hope this produces results. At one authority a search for ‘biodiversity’ returned 16 results, but fewer than half were gain plan applications. At another, three search terms yielded three plans, and we still don’t know whether that’s all of them.
The problem is not that the documents are concealed; it’s that there’s no reliable way to find them, and therefore no way to distinguish an authority with few gain plans from an authority whose search term we’ve not guessed correctly. We recently submitted 41 Freedom of Information requests asking for a list of biodiversity gain plan applications, but so far only one authority has given us the information we requested (Greenwich Borough Council). The others that have replied so far, such as North Somerset Council, have refused our request on the basis that ‘The information you have requested can be found on our Planning Portal’. We have asked for an Internal Review of these refusals.
There are two further obstacles: some plans are published as scans carrying no text layer, so they cannot be searched even once located, and in four decided cases from one authority the metric is published while the gain plan is not. This is not unusual.
The practical consequence is that a member of the public who wants to know how biodiversity net gain is being delivered, say on a development near them — and even when they know the original application reference — will in many cases still not find the answer. The regime may be ‘transparent’ as a matter of law but it is unsearchable as a matter of practice.
How developers deliver the gain plan
Question 4.6 of each gain plan asks the developer how it will reach the target percentage . Across the 263 gain plans we found these answers:
4.6 answer
Gain Plans
On-site
142
Both on and off site
69
Off-site
43
Both boxes ticked (contradictory)
2
Unanswered
7
Total
263
So 112 gain plans — 43% — state that all or some part of the BNG required will be delivered away from the development site. That is a much higher rate of off-site reliance than the Biodiversity Gain Sites Register alone would suggest, because a development that delivers everything on site never appears on the register . The register can tell you where off-site provision is delivered, but not how often off-site provision is used, so it systematically understates it.
Is all the BNG required delivered?
The statutory requirement requires a minimum 10% BNG increase in area, hedgerows and watercourses habitats (it applies separately to each of these habitat modules). A scheme that adds, say, 30% of area units but fails to achieve at least a 10% increase in hedgerow units has not met the BNG requirement.
Taking area habitats, where 224 gain plans state figures we can analyse, the median net gain is +15.1%. That sounds sufficient, and for most schemes it is, but we’ve found that this value is not representative of the whole dataset. Twenty-seven gain plans record an area net gain below 10% on their own figures. Another ten record a negative area net change. The largest recorded loss is −62.7%.
There is also a long tail of very small sites recording enormous percentages: 18 gain plans exceed +100% BNG and two exceed +1,000%. These are almost all sites with a baseline close to zero — say a car park or a concrete garage base — where a handful of new planters produces a percentage gain that means very little. Any analysis that averages percentages rather than pooling units will be dominated by them.
The hedgerow and watercourse modules are where the failures concentrate. Eighty-four gain plans state hedgerow figures; the median is +72.8%, but eight fall below 10%. Only 14 gain plans state watercourse figures at all — and of those 14, five fall below 10%, one of them negative. Watercourses are rare in this sample and poorly served when they appear, so it is hard to draw conclusions from this limited dataset.
Applying the test across all modules shows that 190 gain plans meet 10% everywhere, 35 do not, and 38 cannot be assessed based what the gain plan discloses.
Where do the off-site units go, and do they exist?
One hundred and four gain plans record post-intervention units at section 7.5 of the gain plan — that is, they state an off-site position in figures. Only 73 of those can be matched to an allocation on the Biodiversity Gain Sites Register, totalling 77.88 area habitat units.
The gap between those two numbers is the most consistently troubling thing in the dataset. It has several causes, and we have separated them in the register that accompanies this piece. Some gain plans record the whole habitat bank rather than the units allocated to the development, inflating the figure by a factor of ten or more. Some repeat their section 6 on-site figures in section 7, so the same units are counted twice. Some record a positive off-site uplift with no gain site reference at 7.2 and no allocation on the register at all. Twenty-seven gain plans fall into that last category — an off-site gain asserted in an approved document, with nothing on the statutory register to correspond to it. However, it may be that these sites have yet to purchase their offsite requirements.
The reverse case also occurs: 11 gain plans have units allocated on the register while the plan itself records no off-site gain.
Unit density: more habitat, no more biodiversity
Pooling the metrics we hold — 143 metrics covering 134.5 hectares of on-site area habitat — produces a finding that percentages conceal.
Baseline
Post-development
Habitat area (ha)
134.53
138.25
Biodiversity units
424.65
424.97
Units per hectare
3.157
3.074
This table covers on-site area habitats only and excludes hedgerows and watercourses, which are linear habitats, and off-site purchases. It also predates the most recent additions to the dataset, so treat it as indicative of the 143 metrics we hold rather than of all 263 plans.
Across these sites, habitat area rises by 2.8% while total biodiversity units are flat, and unit density falls by 2.6%. The 10% gains recorded scheme by scheme do not aggregate into a 10% gain in on-site units, because a large share of the recorded gain is bought off site rather than created on it, and because what is created on site tends to be lower in distinctiveness and condition than what was there before. Greener, but with a lower habitat value.
What the documents show
Alongside the dataset, we maintain a register of defects — matters visible in the approved document itself, without site visits or ecological judgement. As of today it records 143 defects across 104 gain plans, from 263 held: a defect rate of 40% across 28 of the 31 authorities.
These are not close calls. They include gain plans dated before the survey they rely on; mandatory fields left blank; section 6 net-change figures that contradict the section 6.3 and 6.4 values above them; post-development values entered in the pre-development box; section 7 left empty on a gain plan that states the gain will be achieved partly off site. Every entry cites the figures it rests on, so any authority that thinks an entry is wrong can check it against its own document.
A distinct category is worth separating out. In four cases from one authority, a search of decided applications returned a statutory metric calculation but no biodiversity gain plan at all. At one of those sites the metric records the development losing 32% of the baseline area units. Off-site provision could in principle make that up — but the document that would say so, the gain plan, was not published. Under paragraph 13 of Schedule 7A, approval of the gain plan is the condition on which lawful commencement rests. It is the document that matters, and it’s the one that’s missing.
That case is worth dwelling on, because it’s the visibility problem meeting the compliance problem . A metric published without a gain plan is not a filing error. Under paragraph 13 of Schedule 7A the approved gain plan is the document on which lawful commencement depends, and it is the one document a reader cannot find. Whether it exists and was not published, or was never required, or was never produced, cannot be determined.
Why the numbers have fallen in our audit
Two months ago this analysis would have reported a defect rate of 49%. It reports 40% because we audited our own dataset before publishing it, and because we corrected it again afterwards when an authority showed us we were wrong. Both times the effect was to remove findings, not to add them.
Machine transcription fails silently. A field the extraction misses looks exactly like a field the applicant left blank, and a blank on our spreadsheet had been turning into a finding against a named authority. We compared 2,409 fields against the source documents and found 58 errors — a rate of 2.4% — of which the largest group were fields recorded as blank that were in fact fully answered.
The worst of it concerns tick boxes. Question 4.6 is a tick box, and so are 4.1, 4.7, 4.13 and 5.2. None of these PDFs stores its ticks as form data: the tick is drawn on the page, and every text extractor reports a ticked box and an empty one identically. A gain plan that answered the question looked exactly like a gain plan that ignored it. We had recorded 30 gain plans as leaving 4.6 blank and had made findings against 27 of them. Re-reading every box by measuring the ink inside it showed that all but one had answered. Thirty-one register entries were withdrawn.
We mention this because anyone building a dataset from these forms by text extraction has the same problem and may not know it. If your BNG data has tick-box fields in it, they are incorrect.
The full method, what was tested, what was found and what remains unverified, is set out in the audit statement published alongside the register. It includes the things we still cannot vouch for: only 4.6 was read in full across the whole set, and 11 of the gain plans are not on the Defra template at all, so questions cannot be said to be unanswered on them.
What this evidence fails to show
Six filters sit between this dataset and the population of approved gain plans in England:
The authorities are self-selecting, being those whose portals expose gain plan applications in some sort of searchable form.
The search of each portal may not have found every application.
Only decided applications are included.
Only published documents can be tested.
Fewer than half the gain plans have a metric published alongside them.
A blank in the dataset is not proof of a blank on the form.
The figures here should therefore be read as a floor on the rate of defects among approved biodiversity gain plans, not an estimate of it. A 40% defect rate among gain plans that were found, decided, published in full and then audited downward says nothing reassuring about the gain plans that were not found.
It is worth being explicit about what the first filter does. The authorities in this dataset are here because their portals let us find gain plans. That is not a neutral criterion. An authority that publishes its BNG documents in searchable, consistently named form is, on the face of it, an authority paying attention to the regime and the obligations it imposes. If anything, this dataset over-represents the diligent.
The dataset, the register and the audit statement are published in full below. In the meantime: we’d like to know which authorities do this well? If your portal makes gain plans easily findable, please tell us. Good practice is easier to copy than to invent.
How to fix this
Two things could make work like this unnecessary:
National guidance from Defra and Ministry of Housing, Communities and Local Government on how the biodiversity gain condition should be discharged and what should be published — the sort of specification the LGA’s Planning Advisory Service could write if funded.
A pre-submission checklist, ideally endorsed by CIEEM, covering the handful of errors that account for most of what we found.
Neither of these solutions would be difficult and both would do more for biodiversity net gain than yet another round of metric revisions. They might also, perhaps, make the life of the poor, beleaguered planning officer a lot easier!
The full dataset (263 gain plans, 31 authorities), the non-compliance register, the BGP Full Data Extract spreadsheet and the audit statement are available via the link below. The register details every defect found, with the figures it rests on.
An earlier version of this piece reported 159 defects across 116 gain plans, a rate of 44%. Nine plans have since been withdrawn from the register because the planning applications they discharge were made before 12 February 2024, when the biodiversity gain condition took effect: the condition did not apply to them, so no requirement of it could have been breached. Following a query from one of the authorities listed (for which, our thanks), the figures above have been amended to 143 defects across 104 gain plans, 40%. Three further entries were withdrawn on the same day after the same authority checked the off-site allocations: in two cases the units are registered and our matching had failed to find them, and in the third an apparent discrepancy is an artefact of the plan reporting to one decimal place. The dataset figures elsewhere in this piece are unaffected, as they record what the plans state rather than whether a legal requirement was met.
The cover image shows the development at Baltic Wharf which was located on the Floating Harbour in Bristol — a former caravan park that has lost 78 well-established mature trees to make way for the development.
So far, most of the public alarm about this expansion has centred on Felton Common, the well-loved local nature reserve on the other side of the runway, where the airport wants to plant 8.5-metre approach lights and fencing across protected grassland. Campaigners under the Save Felton Common banner have campaigned hard against that ‘land grab’, and rightly so. But the fierce fight over the Common has largely overshadowed a second, quieter yet important threat next door — the loss of irreplaceable habitat at Cook’s Farm, which has drawn far less attention. That is what our objection is chiefly concerned with.
Bristol Airport wants to grow. Its planning application (ref. 26/P/0686/OU2) seeks permission to expand from 12 million passengers a year to 15 million. To do this, the airport needs more land — including Cook’s Farm, some 24 hectares of long-established agricultural countryside where hedgerows have stood since at least the 1840s and where some trees have been growing for centuries. The airport now proposes to compulsorily purchase the site so it can carry out its plans.
However, it first needs to persuade the planners that its proposal is acceptable. We say it’s not, and we’ve lodged a detailed objection. It is a technical document, full of hectares and habitat codes, but underneath the detail is a simple story: irreplaceable habitat is at risk, the evidence supporting the plans doesn’t add up, and the law says that should matter.
Here is what our objection says, in plain terms.
The heart of it: three veteran trees
The most emotive part of the case concerns three veteran trees growing in an ancient hedgerow on the boundary of Cook’s Farm — two Ash and one Sycamore. The airport doesn’t propose to simply cut them down. Instead, it plans to translocate them: lift them with a giant tree spade and replant them on the northern edge of the development.
That might sound like a reasonable compromise, but our objections explain why it’s not in careful detail.
A veteran tree isn’t just an old tree, especially those in long-established hedgerows. It’s a whole ecosystem — cavities, rot, deadwood and fungal growth that specialist insects, fungi and other wildlife depend on and cannot find anywhere else. These features take centuries to form. You cannot manufacture them, and you cannot move them.
Worse, veteran trees are extremely unlikely to survive being moved. Their biology works against them. Over centuries they’ve spent their reserves coping with drought, storms, disease and damage, leaving little in the tank to survive the shock of relocation. Their roots are extensive but fragile, they depend on specialised underground fungal networks built up over generations, and much of their internal wood is effectively dead but structurally essential — it can’t be repaired if damaged. Successful relocation needs around two full growing seasons of careful root preparation beforehand. Crucially, the airport’s plan makes no proposal to reduce the trees’ roots at all. As our objection bluntly puts it: if the trees are translocated without root reduction, they will die.
Our conclusion is that, for the purposes of a planning decision, these three veteran trees should be treated as lost. The airport itself seems to half-accept this: it promises a ‘compensation’ package of ten new future veteran trees and the artificial ‘veteranisation’ of ten existing trees. But you can’t conjure a veteran tree on demand. Even with the right conditions, it takes two or three decades for a ‘veteranised’ tree to start resembling the real thing — and it will still lack the fungal decay that gives a true veteran its ecological value.
The evidence doesn’t add up
Beyond the trees themselves, we found that the environmental evidence supporting the application is riddled with inconsistencies. This means that the application can’t properly be decided as it stands.
The airport is required to demonstrate that it will deliver ‘Biodiversity Net Gain’ — a calculation meant to prove development leaves nature in a measurably better state (at least 10%) than it was before development. But our objection shows the numbers in the official metric calculation simply don’t match the airport’s own ecological survey. The two documents use different habitat categories, different reference codes, and don’t cross-reference each other. Some habitats that appear in the survey are missing from the metric altogether. Habitat parcels aren’t sized, and their locations aren’t shown on the map. In short, it’s impossible to reconcile the airport’s own datasets.
The tree surveys are a particular problem. Three surveys tucked into the appendices fail to meet BS5837:2012, the British Standard for trees in planning. Between them they’re missing the surveyors’ names and qualifications, survey dates, crown-spread data, branch heights and directions, life-expectancy estimates, soil assessments, and even the number of trees in some groups. One key survey dates from June 2024 and is now out of date. And when we asked for the tree schedule in a usable spreadsheet format so we could check it, our request was refused — leaving them, in their words, to analyse the data ‘as best we can’.
Even working with that inadequate evidence, their interim analysis is striking. Of roughly 1,618 individual trees on the site, some 367 — more than a fifth — are set to be removed, along with a share of the site’s hedgerows, which the airport’s own figures and the survey record in wildly different numbers (10 hedgerow parcels in one document, 71 in another).
Why this is a matter of law, not just sentiment
This is where our objection turns from ecology to policy — and it’s the part that could decide the outcome.
National planning rules are clear about irreplaceable habitats. Paragraph 193(c) of the National Planning Policy Framework says development that causes the loss or deterioration of irreplaceable habitats — and it names ancient and veteran trees specifically — ‘should be refused, unless there are wholly exceptional reasons and a suitable compensation strategy exists’.
We make two arguments about this test:
First, it’s a two-part test that has to be met in sequence: you need both wholly exceptional reasons and a suitable compensation strategy. A compensation package — however good, and here they say it isn’t good — cannot on its own make the reasons ‘wholly exceptional’. The decision in Juden v London Borough of Tower Hamlets [2021] EWHC 1368 (Admin) backs this interpretation.
Second, we argue that this scheme doesn’t clear the ‘wholly exceptional’ bar at all. The Framework’s own example of exceptional reasons is nationally significant infrastructure — the kind of once-in-a-generation project decided by government. This application, we say, is simply about increasing the capacity of an existing airport. That’s not the sort of wholly exceptional circumstance the rules have in mind.
Our objection also points to local policy. North Somerset Council’s own adopted Core Strategy (Policy CS4) requires it to protect, connect and enhance important habitats, ‘particularly designated sites, ancient woodlands and veteran trees’. The same protection is included in the Council’s emerging 2041 Local Plan. Approving the loss of these veteran trees would go against the Council’s own rules.
What it comes down to
Strip away the appendices and the acronyms, and our case is straightforward. The airport wants to expand on to land that includes centuries-old, irreplaceable habitat. Its plan to ‘move’ three veteran trees is, on the evidence, a plan to kill them slowly. The paperwork meant to justify all this is inconsistent, incomplete and in places non-compliant with the relevant standards. National and local planning policy both say irreplaceable habitat like this should be protected, not traded away for extra passenger numbers.
Our central demand is modest but firm: these problems need to be resolved before the application can properly be decided. For anyone who cares about Cook’s Farm, its ancient hedgerows and the veteran trees growing in them, that’s the line worth holding.
In a boost for the environment, new legislation now obliges most developers to set out how they will achieve at least 10% more biodiversity than already exists on their proposed development sites.
Developers must now show how they will improve the biodiversity of their development site as a result of their works. Developers must record the (baseline) on-site habitats that exist before development starts and show how they will either enhance or replace these on site. If their proposals fail to reach the 10% threshold, the developer may provide the shortfall elsewhere. This post-development mitigation should be done as closely as possible to the development site, or at least within the Local Authority. However, if this isn’t possible, they can use approved sites anywhere in England.
This approach is called the Biodiversity Gain Hierarchy (found in Schedule 7A of the Town and Country Planning Act 1990 at section 37A). The Hierarchy says it’s a priority to avoid the ‘adverse effects’ to ‘onsite habitat with a habitat distinctiveness score … equal to or higher than four.’ If this can’t be avoided, only then is mitigation permitted. In our experience, nearly all developers ignore the requirement to avoid adverse effects and move straight on to mitigation.
Since the introduction of the Bristol Tree Replacement Standard in 2013, developers (and planners) have ignored the policy requirement to avoid tree removal where at all possible. Instead, they’ve moved straight on to providing compensation for the trees lost to the development. As a result, the money set aside for replacement tree planting was not spent (on occasion reaching nearly £1 million) and many of the lost trees were never replaced.
Under the Hierarchy, habitats that score four are designated as having medium distinctiveness. While many habitats have medium distinctiveness, many don’t. For example, managed grassed areas (called ‘modified grassland’) are often found on urban sites but have a low distinctiveness score. So, too, do many other urban habitats such as allotments and gardens. Developers are not required to avoid harming these, though losses to these habitats must still be mitigated.
There’s no definition of what an ‘adverse effect’ is or any guidance on how it is assessed. However, recent advice from the Bristol’s Chief Planner about the meaning of ‘harm’ suggests that this could be interpreted very broadly or simply ignored because some sort of mitigation will always be available.
In the last extreme, developers may purchase biodiversity credits. We wait to see how this and the offsite biodiversity mitigation market evolves, but a 2012 paper published in the Harvard Environmental Law Review suggests that such environmental markets are prisoners of their own geography because the space available is always constrained:
Markets for water quality, biodiversity, endangered species, fisheries, air quality, and aquatic resources, to name a few, must recognize that the commodities they trade exist at particular geographic scales, and set appropriate spatial limits on the redistribution of environmental quality. The size of geographic trading areas has significant implications for the economic viability of markets and the ecological quality of their offsets.
This will be a particular challenge when providing biodiversity mitigation in urban areas.
The squeeze on green spaces
Land use in Bristol is subject to intense competition by many stakeholders. This is especially true for our green and open spaces, which offer many ‘services’ beyond just habitat provision. There is very little, if any, space available for new biodiversity to be created. At best, some green spaces might be enhanced, but opportunities to do this are likely to be very limited.
For example, Bristol Tree Forum’s examination of the three proposals to develop Bedminster Green shows that, if these proposals are allowed, then nearly 400 new trees will need to be planted to compensate for the lost tree habitat – a medium distinctiveness habitat. There’s no room to plant these trees on site, so offsite provision will be needed. There are very few opportunities for doing any new tree planting (as opposed to replacing lost trees) in the surrounding wards or even across Bristol, let alone, as is usually required, within a mile of a development site.
Instead, these replacement trees will have to be planted somewhere else: ‘in some foreign field that is forever Bristol’. This will inevitably lead to a net loss of biodiversity across the city as nature is ‘hollowed out’. This is unacceptable. The whole purpose of the new biodiversity gain regime is to improve overall local biodiversity, but it seems inevitable that Bristol will instead see a steady, inexorable decline.
We are disappointed that the current draft of the new Local Plan addresses none of these issues and have said so in our responses to the latest consultation:
We believe the time has come to revise the Bristol Tree Replacement Standard (BTRS), to reflect our changing understanding of the vital importance of urban trees to Bristol and how they contribute to biodiversity gain.
The current BTRS Standard, adopted nearly a decade ago in July 2014, provides a mechanism for calculating the number of replacements needed for any trees that are removed for developments. It was ground-breaking in its time as it, typically, required more than 1:1 replacement of trees lost to development.
Since then, Defra has published the statutory version of the Biodiversity Metric (SM) (on 29 November 2023), which became mandatory on 12 February 2024. In addition, Bristol has adopted Climate and Ecological Emergency Declarations, so an updated BTRS would be an important part of implementing these declarations. It would require all new developments, subject to some exceptions, to achieve a Biodiversity Net Gain (BNG) of at least 10%. Where developments are exempt, BTRS will still apply.
Although Councillors rejected our proposals for a new Standard at their meeting on 31 October last, we’ve revisited our July 2023 proposals and recast our calculations. These proposals, set out below, provide a mechanism for complying with the new requirements and align the BTRS with the BNG provisions of the EA 2021.
The purpose of the BTRS is that it should only ever be a last resort and not the default choice – which, unfortunately, it has become. When considering any development involving established trees, the presumption should always be that trees will be retained. If this is not possible, then the impact of the proposed development must be mitigated. Only if this is impossible, should compensation for their loss be considered. This is the meaning of the Mitigation Hierarchy, as set out in paragraph 180 a) of the National Planning Policy Framework, which states:
If significant harm to biodiversity resulting from a development cannot be avoided (through locating on an alternative site with less harmful impacts), adequately mitigated, or, as a last resort, compensated for, then planning permission should be refused.
This is reflected in the Bristol Core Strategy, policy BCS9(page 29),which states that:
Individual green assets should be retained wherever possible and integrated into new developments.
This is repeated in the proposed replacement for BCS9 – Policy BG1: Green infrastructure and biodiversity in new development(page 124) – which ‘aims to ensure that green and blue infrastructure and provision for nature is incorporated into new development’ so that, among other things:
The provision of green infrastructure in new development should … Retain and incorporate important existing green infrastructure such as trees (Policy BG4 ‘Trees’), hedgerows and water features …
It is a shame that the requirement is only an aspiration, not an obligation.
Background
Under the new proposed policy – BG4: Trees (page 131) – trees lost to development will be replaced using this table:
Table 1 The proposed BG4 tree replacement table.
However, when the balance of EA 2021 takes effect, the current version of the BTRS will not, in most cases, be enough to achieve the 10% BNG minimum required for nearly all developments. A new Section 90A and Schedule 7A will be added to the Town and Country Planning Act 1990 and will set out the level of BNG required.
Under the statutory framework for biodiversity net gain, subject to some exceptions, every grant of planning permission is deemed to have been granted subject to the condition that the biodiversity gain objective is met (“the biodiversity gain condition”). This objective is for development to deliver at least a 10% increase in biodiversity value relative to the pre-development biodiversity value of the onsite habitat. …
Many development proposals will aim to achieve more than the minimum 10% gain voluntarily. Others may not but will still need to achieve much more in order to comply with the SM trading rules (page 140). This is based on the habitat type lost and its distinctiveness. In the case of Individual tree habitats – Urban or Rural – losses must be replaced within the same broad habitat (i.e. more Individual trees) or with a habitat of a higher distinctiveness.
However, for the sake of certainty, we propose only using the minimum 10% BNG required.
Our proposed new BG4 (BTRS) model
We propose that the table in BG4 be amended to reflect the requirements of the EA 2021 and SM and that the BG4 table (Table 1 above) be replaced with Table 2 below:
Table 2 Our proposed BG4 tree replacement requirement
The Replacement Trees Required number is based on the habitat area of each of the four SM tree category sizes (Table 13 below), divided by the area habitat of one BNG 4.0 Small category tree (see section 3 below) plus a 10% net gain. This is rounded up to the nearest whole number (since you can’t plant a fraction of a tree).
The reasoning for our proposal is set out below:
1. Applying the Biodiversity Metric to Urban trees
Use the broad habitat type ‘Individual trees’ to record trees where:
they are found as an individual or as part of a group;
are over 7.5cm in diameter at breast height (DBH).
Individual trees should also be recorded where they meet the definition of an irreplaceable habitat but would not otherwise be recorded.
Do not otherwise record individual trees if they occur within an area habitat type characterised by the presence of trees, examples of these are:
woodlands
orchards
wood-pasture and parkland
Individual trees are classed as ‘urban’ or ‘rural’. You should consider the degree of ‘urbanisation’ of habitats around the tree and assign the best fit for the location.
2. Calculating Individual trees habitat
Table 13 in the SM User Guide is used to calculate the ‘area equivalent’ of individual trees:
The biodiversity metric uses set values to represent the area of trees depending on their diameter at breast height. This value is a representation of canopy biomass, and is based on the root protection area formula, derived from BS 5837:2012.
You should report the number of individual trees within your project and input tree count into the ‘tree helper’ within the biodiversity metric tool to generate area values for data input. For multi-stemmed trees, use the DBH of the largest stem. You should:
account for each individual tree within a group or block of trees.
record the habitat underneath the tree canopy separately.
not reduce any area generated by the tree helper.
not deduct the area of individual trees from other habitats.
make clear in the user comments how many trees contribute towards the total area.
Recording trees within private gardens
You should assess most individual trees that are recorded in private gardens. You should record:
any medium, large and very large trees as individual trees
any small trees that are ancient or veteran
Recording trees within hedgerows
You should assess most individual trees that are recorded within hedgerows. You should record:
any medium, large and very large trees as individual trees
any small trees unless they are ancient or veteran.
You must assess the linear value of hedgerows within the hedgerow module separately.
Individual Tree habitats have medium distinctiveness and so, under Rule 1 of SBNG, ‘Losses must be replaced by area habitat units of either medium band habitats within the same broad habitat type or, any habitat from a higher band from any broad habitat type.’
3. Forecasting the post-development area of Individual trees
The SBNG User Guide provides this guidance:
You should use the tree helper to calculate the area for created trees.
You should categorise most newly planted individual trees as ‘small’, unless the tree is medium sized or above at the time of planting.
You should not factor in the age of nursery stock when using the ‘creation in advance’ function. The ‘creation in advance’ function should only be used where trees are planted in advance of the development (for example, as screening or as structural landscaping).
Exceptions
You cannot count:
newly planted trees within private gardens
natural size increases of baseline trees
trees planted as part of hedgerow creation or enhancement as individual trees.
Our calculations are based on Small category replacement trees being planted as per the SM guidance.
4. The likely impact of this policy change
We have analysed tree data for 1,038 surveyed trees taken from a sample of BS:5837 2012 tree surveys submitted in support of previous planning applications. Most of the trees in this sample, 60.5%, fall within the SM Small tree category, 32.9% are within the Medium tree category, 5.4% are in the Large tree category with the balance, 1.3%, being categorised as Very Large.
Table 4 below sets out the likely impact of the proposed changes to BG4. It assumes that all these trees were removed (though that was not the case for all the planning applications we sampled) and replaced with SM Small category trees:
There’s a climate emergency and we need to act. With higher temperatures and more severe weather events than just a decade ago, we must take action at the local as well as the global level.
Bristol City Council declared a Climate Emergency in 2018, reflecting the need to reduce the city’s contribution to the causes of climate change, and to adapt and be resilient to further expected climate impacts. For the declaration to be meaningful, it has to result in practical changes, for example the protection of existing trees on development sites. With important urban trees being routinely felled, there is no evidence that this is the case. If Bristol continues in this way, the city will become unliveable in the climate crisis.
The Council is now drafting a tree strategy for the city, which we hope will become a key element of the forthcoming revised Local Plan. We hope that the strategy will protect existing trees and prioritise the planting of replacement and new trees across the city. We have asked for 18 principles to be included in the strategy.
If our urban environment is going to be liveable in the long term, we need to create new developments that can cope with the changes in the local climate expected in the future. The benefits of trees in the fight against climate change are now well understood: trees lock up carbon, reducing pollution and flooding. They are also the best way of reducing the urban heat island effect, decreasing the temperatures of heatwaves by up to 10°C . It’s therefore vital that green infrastructure forms part of any proposed development. This is particularly crucial in the city centre.
On every occasion that trees are felled, we’re told it will be all right, as they will be replaced. Often these replacement trees are never planted because there is nowhere to plant them, or if planted, they die and are not replaced. At any rate, we need tree canopy and shade now, not in 50 years’ time when any new trees that might survive will replace the canopy lost. This is why we must protect existing trees, and if trees must be lost, local tree replacements must be planted and not just promised.
A warmer climate increases the risk of overheating and heat-related illness, even death. In the heat wave of 2003, around 70,000 people died across Europe due to the extreme heat, with older people and children particularly vulnerable. However, we can reduce much of the risk without the need for active cooling, by incorporating effective measures into development proposals from the earliest design stage. New buildings and external spaces must be designed to provide year-round comfort and support well-being. On-site tree planting for shade will contribute to this by minimising the amount of heat entering buildings. All new developments will be expected to demonstrate through ‘sustainability statements’ how they would incorporate such measures into their design from the outset.
How green (and blue) infrastructure reduces climate impacts
Developers must take into account that changes in the local climate are likely to: increase flood risk and water stress; change the shrink-swell characteristics of clay soils affecting foundations and pipework; affect slope stability; and affect the durability of building materials. Incorporating green and blue infrastructure, such as trees and water features, in developments will help to reduce all these effects. Green and blue infrastructure should be multifunctional, that is, provide ecology and biodiversity benefits as well as climate adaptation in developments. Where appropriate, this should include the use of living roofs with a sufficient substrate depth to maximise cooling benefits. However, the cooling effect of green roofs is a fraction of that afforded by trees.
Long-term thinking
As we build more homes, businesses and communities, it’s essential that we retain and integrate important existing trees within any new development. We must also consider carefully the size, species and placement of new trees provided as part of any planned landscape treatment, for example in terms of:
ensuring that any new streets are tree-lined
focusing once again on large-form trees that will be long-lived and provide substantial shade, rather than small, short-lived trees such as Rowan or Amelanchier
reducing or mitigating run-off and flood risk on the site
increasing on-site canopy cover and providing shade and shelter
ensuring that newly planted trees will be maintained in the long term and replaced if necessary.
Where tree loss or damage is unavoidable, and not merely expedient, within a development site, new replacement trees of an appropriate species must be provided either on or off site and their long-term management and maintenance secured.
