Trading rules and bespoke compensation in a BNG assessment

The biodiversity net gain trading rules decide whether the units you lose on a site can be replaced with lower grade habitat somewhere else, and in most cases they cannot. The statutory metric applies a distinctiveness band to every habitat parcel, and the band sets what you are allowed to use as replacement. Lose high distinctiveness grassland and you must create or improve the same habitat type. Lose very high distinctiveness habitat and the metric stops helping you altogether.

Developers who treat the 10 per cent gain as a single arithmetic target usually discover the trading rules late, after the baseline survey has already fixed the answer.

Why trading rules exist in the metric

Without them, a scheme could clear species-rich grassland and buy the deficit back with amenity grassland at a fraction of the cost. The unit arithmetic would balance. The ecology would not.

So the statutory biodiversity metric constrains substitution by habitat type and condition rather than by units alone. The rules sit inside the metric calculation itself, which means a trading rule failure shows up as a red flag in the tool rather than as a negative unit balance. A scheme can show a surplus overall and still fail, because the surplus is in the wrong habitat.

That distinction matters for programming. A trading rule failure is a design problem, not a budget problem, and it is not solved by buying more units.

Distinctiveness bands and what each band requires

The metric sorts habitats into five distinctiveness bands. The replacement obligation tightens as you move up.

The step between medium and high is the one that catches schemes out. At medium you have room to move within a broad habitat group. At high the requirement narrows to the same habitat, which on a site carrying lowland meadow or species-rich hedgerow means recreating that specific habitat rather than something in the same family.

Hedgerows and watercourses are assessed on separate modules with their own unit types. You cannot offset a lost hedgerow with area habitat units, and you cannot offset lost watercourse units with hedgerow units. Three separate balances have to clear.

Bespoke compensation for very high distinctiveness habitat

Very high distinctiveness habitat is not tradeable through the metric. The tool records it and then tells you that bespoke compensation has to be agreed with the local planning authority.

In practice that means a negotiated package: like for like recreation where recreation is possible, a much larger area than the arithmetic would suggest, a longer management period, and usually Natural England input where a designated site or a priority habitat is involved.

Irreplaceable habitat is a separate and harder case. Ancient woodland, ancient and veteran trees, blanket bog, lowland fen, limestone pavement, coastal sand dunes and spartina saltmarsh swards sit on the statutory list of irreplaceable habitats. The biodiversity gain condition does not require a 10 per cent gain on them, because no gain can replace them. National policy sets a strong presumption against loss, and where loss is unavoidable the authority must secure bespoke compensation. If your baseline survey finds irreplaceable habitat inside the red line, the layout has to change.

A Preliminary Ecological Appraisal at the earliest possible stage is what stops this becoming a late discovery. The appraisal tells you which bands you are dealing with before the layout is fixed.

The spatial risk multiplier and off-site location

Off-site units are discounted according to where they sit relative to the development. The statutory metric applies a spatial risk multiplier of 1.0 for units delivered inside the development's local planning authority area or National Character Area, 0.75 for a neighbouring area, and 0.5 for anywhere further away.

That 0.5 figure is the one to model early. A scheme needing 4 off-site units within its own authority area needs 8 units of the same habitat if the only available habitat bank sits two counties away. Off-site delivery that looked affordable at scoping can double in cost once the actual location of available habitat is known.

Statutory biodiversity credits sit behind both. They are deliberately priced to be the last resort, and the published tariff rises steeply with the distinctiveness band of the habitat being replaced.

Temporal risk and advance creation

The metric discounts units for the time habitat takes to reach its target condition. Each year of delay reduces the value of the created habitat, so a woodland that needs thirty years to reach good condition delivers far fewer units than its mature area suggests.

Two levers help. Creating habitat in advance of development removes the time penalty, because the clock starts when the habitat is created rather than when the scheme completes. Choosing habitats with short establishment periods does the same thing at lower cost, though it pushes you towards grassland and scrub rather than woodland.

Advance creation needs land control and a willingness to spend before consent. On a phased scheme where the developer owns the later phases, it is often the most affordable route to a clean balance. On a single-phase site with an option agreement, it rarely is.

Where the rules make a scheme undeliverable

Three situations recur:

1. A high distinctiveness habitat covering most of the developable area, where on-site retention and the required layout cannot both happen.
2. Irreplaceable habitat inside the red line, where the answer is a different red line.
3. A habitat type with no realistic off-site market, so the only compliant route is statutory credits at a price the scheme cannot carry.

Each of these is visible from the baseline survey. None of them is visible from a desk study. That is the argument for commissioning the habitat survey in the growing season before you fix the masterplan, not after the layout has been drawn.

And there is a fourth, quieter case: a scheme where the arithmetic works but the thirty year management obligation attaches to land the developer intends to sell. The biodiversity gain plan has to show how significant on-site enhancement will be secured and managed for thirty years, usually by planning obligation or conservation covenant. A management company that does not yet exist is not an answer an authority will accept.

Evidence the planning authority checks

The validation stage looks for the completed statutory metric file, the habitat baseline with condition assessments, and a statement of how the trading rules have been satisfied. After permission, the biodiversity gain plan must be approved before development begins.

Officers and their ecology advisers routinely check:

• Whether condition scoring in the baseline is justified by the survey notes rather than asserted.
• Whether the survey was carried out in an appropriate season for the habitats present.
• Whether each trading rule has cleared in the metric rather than only the headline unit balance.
• Whether off-site units are secured through a legal mechanism and registered.
• Whether the spatial multiplier applied matches the actual location of the receptor site.

Getting the narrative right matters as much as the spreadsheet. The Biodiversity Net Gain Assessment has to explain the choices behind the numbers: why a habitat was scored as moderate rather than good, why on-site delivery was limited, and what was considered before off-site units were proposed. The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and a draft is a starting point for that person rather than a substitute. For specialist technical reports, qualified professional input may be required.

If you are scoping the wider evidence base at the same time, the full list of report types by category shows what else a major application of this kind usually carries.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.