Trading down and off-site sections of a BNG plan

The off-site sections of a biodiversity gain plan are where a scheme either balances or stops. Once the on-site design has been pushed as far as it will go, the remaining shortfall has to be met by units bought from a registered off-site gain site or, failing that, by statutory biodiversity credits bought from the government at a deliberately unattractive price. The trading rules decide which of those is even available, and they are stricter than most developers expect the first time they meet them.

Write these sections as a record of a sequence you actually followed, because that is how the local planning authority will read them.

The hierarchy inside the metric

The biodiversity gain hierarchy runs in a fixed order: avoid harm to existing habitat, minimise what cannot be avoided, deliver the gain on site, then off site, then buy statutory credits. Each step down carries a penalty in units, in cost or in credibility.

The section should open by showing the steps taken in order, with the design change made at each one. Retaining the mature hedgerow along the north boundary rather than replacing it. Reducing the developable area to keep a wet ditch. Moving attenuation into an area of low distinctiveness grassland. Where nothing was avoided or minimised, say so and explain why, because a plan that jumps straight to buying units invites the question.

An honest sequence also protects the applicant later. If the council's ecologist asks why the on-site figure is so low, the answer is already in the document.

How the trading rules block substitutions

The statutory metric applies trading rules that stop high-value habitat being replaced by anything convenient. The higher the distinctiveness of what is lost, the narrower the range of what can replace it.

Habitat of very high distinctiveness has to be replaced with the same habitat type. High distinctiveness requires the same broad habitat at the same or higher distinctiveness and the same or better condition. Medium distinctiveness allows replacement within the same broad habitat group. Only low distinctiveness habitat can be replaced by any habitat of equal or higher distinctiveness.

That has a practical consequence people meet late. A scheme that removes a stretch of species-rich hedgerow cannot compensate with amenity grassland however many units it buys, because the units have to be of the right kind. The search for off-site provision is therefore a search for a specific habitat type, and the market for some of those is thin.

Check the trading position before the layout is fixed, not after. On a site with any medium or higher distinctiveness habitat, the preliminary ecological appraisal and the baseline metric together tell you whether the compensation problem is going to be routine or difficult.

When off-site units are the realistic route

Off-site units come from a gain site registered on the biodiversity gain sites register, where the habitat enhancement is secured for at least 30 years by a conservation covenant or a planning obligation and allocated to your development.

Three things need saying in this section:

1. The habitat types and unit quantities being purchased, matched to the shortfall by type rather than as a single total.
2. The location of the gain site and its relationship to the development, because the metric applies a spatial risk factor that reduces the value of units delivered further away.
3. The mechanism securing them, with the allocation to your development recorded on the register.

Units within the same local authority area or national character area count for more than distant ones, which is a reason to search locally first even where the price is higher. The register entry, not a contract or a letter of intent, is what the council needs to see.

Statutory credits as a last resort

Statutory biodiversity credits are sold by the government and are designed to be the option nobody chooses. They are priced above the market rate for equivalent off-site units, deliberately, so that private supply is used first. Prices are published and revised, so quote the current published schedule rather than a figure from an older document.

Credits also require evidence that off-site units were not reasonably available. A plan that reaches for credits without a documented search will be questioned, and the search record should name the habitat type sought, the brokers or gain sites approached and the outcome.

There is one situation where credits are the sensible answer despite the price: a small residual shortfall of a habitat type with no local supply, where waiting for a gain site to come forward would delay the scheme by a year. Say that plainly in the plan.

The 30 year commitment

Every off-site unit, and every significant on-site enhancement, carries a habitat management and monitoring commitment running for at least 30 years. That is a legal obligation and a cost line, not a paragraph of intent.

The plan should set out who holds the land, who carries out the management, how it is funded, what monitoring is done and at what intervals, and what happens if the habitat fails to reach its target condition. Where the land is retained by the developer and later sold, the obligation runs with it, and the sale documents have to reflect that.

For on-site provision within the shared open space of a housing scheme, name the body that will hold and manage it. A management company that does not yet exist is the weakest part of many gain plans, and a green infrastructure plan that shows maintenance responsibilities parcel by parcel makes the commitment credible.

What the authority checks before discharging the condition

The biodiversity gain condition requires the plan to be approved before development begins. An officer working through it looks for the pre-development biodiversity value with its baseline metric, the post-development value on site, the off-site allocation shown on the register, any credits purchased, and the arithmetic showing the required percentage is met.

They also check consistency against the biodiversity net gain assessment itself. The habitat parcels in the metric should match the landscape drawings, the retained trees should match the tree survey categories, and the areas should match the site plan. Mismatches between documents are the most common reason a plan goes back for revision, and each round costs weeks.

Programme: securing units before or after permission

There is a real tension here. Buying units before permission is granted commits money to a scheme that may be refused. Waiting until after permission risks the units for your habitat type being gone, and leaves the gain plan, and therefore the start on site, hanging on a market you do not control.

The middle course most developers take is to identify supply and agree terms conditional on permission, then complete the allocation once the decision is issued and before the gain plan is submitted. Build the lead time into the programme deliberately. A biodiversity gain plan submitted the week the contractor mobilises is a plan submitted too late.

For specialist technical reports, qualified professional input may be required. The statutory metric has to be completed by a competent person under the Environment Act 2021 regime.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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