Category: Biodiversity Net Gain

When a city builds, where does nature go?

Since February 2024, developers in England have had a legal duty to improve the biodiversity of their development site by at least 10%. This Biodiversity Net Gain — BNG — is calculated in ‘habitat units’ using a government spreadsheet. If a scheme cannot achieve this gain on the land it’s actually building on, it can buy the shortfall from a habitat bank anywhere else in England.

The BGS register

The public register of these off-site purchases is called the Biodiversity Gain Sites (BGS) register. It lists both the authority where a development sits and the authority where the compensating habitat is purchased.

We looked at all 2,441 allocations on the register, covering 301 local planning authorities. We found that, across the board, 79% of the allocations send the compensation to a local authority far from the development itself. That is striking enough. But the pattern sharpens dramatically in the more urban areas.

For example, in London, 98% of allocations are delivered outside the borough where the building happens. Just 3% stays within it. London’s boroughs have generated external demand for 46 habitat units and host barely three in return. Metropolitan districts — such as Manchester, Sheffield, Leeds and Birmingham — send 88% of their compensation elsewhere.

By contrast, non-urban authorities send 74%, and keep habitat within their own boundaries in 42% of cases, against London’s 3%.

Taken together, urban authorities are net exporters of habitat. The countryside is where it lands – often in remote rural locations few will ever visit.

How far away does nature go?

The register also records the distance between each development and the habitat bought to compensate for it. The typical distance is 37 km — roughly 23 miles. One allocation in five is more than 100 km away from its development site.

Median distance from development to compensating habitat, by type of authority

For London the median is 86 km, and nearly half of the capital’s allocations are more than 100 km away. Two-thirds fall into the register’s most distant category — the same category the rules are meant to discourage.

Who loses out?

Each site can be placed on a national deprivation scale, from decile 1 (the most deprived tenth of neighbourhoods) to decile 10 (the least deprived). Comparing where a development happens with where its habitat ends up gives a rather uncomfortable picture.

Average deprivation decile of the development, and of the habitat created to compensate for it

Development in metropolitan districts sits, on average, in decile 4.0 — among the more deprived parts of England. The habitat created to compensate for it sits in decile 6.3. Nationally, the shift is from 6.0 to 6.6, and it is almost entirely driven by cities.

Of the 504 allocations where the development is in one of the three most deprived deciles, 99% send their habitat somewhere less deprived — landing, on average, in decile 6.5. For non-urban authorities there is essentially no movement at all.

Displacement, not disappearance — but that is not the whole story

It is worth being careful here. Habitat units are being created. This is displacement, not straightforward destruction, and the register shows real habitat banks being funded and monitored.

Nor is this purely an urban phenomenon. The largest single net exporters on the register are Cherwell in Oxfordshire and Westmorland and Furness — not cities, but high-growth areas under development pressure. Land scarcity, not city status, is the underlying driver.

But displacement still matters, for reasons the metric does not capture. A hectare of habitat in a dense neighbourhood serves thousands of people who can visit it, but the same hectare 50 miles away, on a farm almost nobody visits, scores identically in the register. Access, shade, cooling, birdsong on the way to school — none of these elements appear in the data.

The rules do include a brake: compensation delivered further away is discounted, by a quarter if it’s in a neighbouring area and by half if it’s farther still. On this evidence, though, the brake is not holding, and the latest plans propose to remove it altogether for many developments, making it still worth buying habitat miles away.

What the gain plans show

We also read 205 individual gain plans in detail. The contrast is just as clear on the ground.

Developments in urban authorities lost area habitat on site — down 8% overall between the ‘before’ and ‘after’ figures in their own calculations — while developments in rural authorities gained 16%. Yet both groups have to demonstrate the same 10% or more net gain. Urban schemes get there by buying it: 54% of them use off-site compensation, against 33% elsewhere. Often this is simply because there are just no local Biodiversity Gain Sites, so the developer has no local options.

This is not a story about small development sites. The average development we measured was almost exactly the same size in urban and rural authorities — around 0.7 hectares. It is about the scarcity of available urban land, which is reflected in its cost, not the size of the plots.

The question worth asking

BNG is a real advance on what came before, and the register is a genuine piece of public transparency. This analysis is only possible because the data is published.

But if the policy’s promise is that development leaves nature better off, it is fair to ask: better off for whom? On current evidence, the answer in our biggest cities is someone else, somewhere else — on average 37 kilometres away, in a less deprived place.

Here is our detailed analysis:

Where does the biodiversity go?

Analysis by Bristol Tree Forum, using the Biodiversity Gain Sites register (2,441 allocations to 28 July 2026) and 205 biodiversity gain plans transcribed from local planning authority records. Charts and underlying data at bgs.bristoltrees.space.

Ancient trees, missing data, and the fight over Bristol Airport’s planned expansion

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.

Here is our full response to the application:

BTF Consolidated Comments – 06 May 2026

Understanding Biodiversity Gain Plans

Since BNG became mandatory in February 2024, qualifying developments in England must show how they will deliver at least a 10% gain, to be set out in a Biodiversity Gain Plan (BGP) submitted to the local planning authority. We’ve transcribed 160 BGPs from 21 LPAs into a single, comparable dataset.

What the data shows so far:

  • Just over under 60% (95 of 160) deliver their gain entirely on site. Another 34 use a mix of on-site and off-site measures, while 17 rely wholly on off-site units and 14 did not state an approach. Two bought Statutory Credits.
  • Off-site reliance is a recurring theme. A number of plans record an on-site biodiversity loss offset by purchased habitat units — a legitimate route, but one that moves the gain away from the community where the development happens.
  • Net gain has to be met habitat type by habitat type (area, hedgerow, watercourse), not as a single blended figure. Several plans that pass overall are marginal — or fall short — on an individual module.

Accessing this data was harder than it should be:

  • There is no central register of gain plans – why can’t Defra or Natural England include them in their BGS allocation dataset? These documents should be easily accessible.
  • Each council’s portal has to be searched separately, with no consistent way of identifying the relevant application. Only a few authorities (City of York, Bristol and Cotswold among them) have a dedicated application category for BNG discharges. Otherwise the BGPs sit inside generic “discharge of conditions” applications and are very difficult to find.
  • BGPs are produced in a variety of forms – as clean PDFs, fillable forms, bespoke versions or scanned images requiring OCR – which is challenging, especially if they are hand-written.
  • They are often internally inconsistent: off-site figures just copied from the on-site section, credits confused with habitat-bank units, or headline numbers that do not reconcile.
  • Off-site claims frequently differ from what the Biodiversity Gain Sites register records as allocated.
  • Many applications do not include the required BNG Metric calculation.
  • Some applications can’t be analysed because all that is produced (and approved) is the certificate of the off-site HU purchase – and sometimes not even that. As a result, we’ve been unable to include these in our analysis.

The information exists, but we can only conclude that the planning approval process makes meaningful public scrutiny far harder than it needs to be. Using the available standard Defra form and creating a single searchable register of BGPs would be a straightforward improvement.

The full dataset and analysis are here — comments, corrections and more data are welcome:

BGP Full Data Extract

NB. This analysis will grow as we add more data.

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