Off-site biodiversity gain and the statutory register

Off-site biodiversity gain becomes necessary when the site cannot deliver a ten per cent uplift within its own boundary, which on a dense urban scheme is most of the time. The mechanism is not a private contract between developer and landowner. Units only count once the land is entered on the biodiversity gain sites register kept by Natural England and the units are formally allocated to your development, and that allocation is what the local planning authority checks before approving the biodiversity gain plan.

When off-site units become necessary

The biodiversity gain hierarchy runs on-site first, then off-site, then statutory biodiversity credits as a last resort. Authorities apply it in that order and will ask what on-site delivery was tested before you moved down.

The honest answer on many schemes is that the layout was fixed before the ecologist saw it. Running the metric at concept stage rather than at validation stage frequently finds a percentage or two on site, in roof habitat, in the drainage basin margins or in the open space that was going to be amenity grass anyway.

Where a genuine shortfall remains, quantify it precisely and by module. Area habitat units, hedgerow units and watercourse units are separate currencies. A deficit in hedgerow units cannot be met by buying area units, because the trading rules require like for like or better within the same module.

That single point causes more late problems than any other, and the errors councils spot in submitted metrics are worth reading before you commission a gain site search. The list of biodiversity metric errors councils find covers most of them.

How a gain site is registered

Natural England operates the biodiversity gain sites register under regulations made through the Environment Act 2021 regime. Registration is a separate act from selling units and it comes first.

To register, the applicant must show:

• who owns the land and that they are entitled to apply
• the habitat works to be carried out, with a baseline and a post-works value from the statutory metric
• a legal agreement securing the habitat for at least 30 years, either a conservation covenant with a responsible body or a planning obligation
• a habitat management and monitoring plan covering that period

A registration fee is payable, and the figures are published and revised, so check the current schedule rather than a number from a previous transaction.

Registration does not attach the units to anything. It creates a pool of allocable units on a named parcel, and the entry is public.

Allocating units to a specific development

Allocation is the step developers most often discover late. The gain site operator applies to Natural England to allocate a stated number of units, of stated habitat types and distinctiveness, to a named planning permission.

Until that allocation appears on the register, the units are not yours in any sense the local planning authority will accept. A signed unit purchase agreement is not an allocation, and an authority reviewing a biodiversity gain plan will look for the register entry.

Sequence it deliberately:

1. Agree the shortfall by module and habitat type from the final metric.
2. Identify a gain site whose registered units match that profile.
3. Contract, with the allocation as a condition of completion rather than a promise to be honoured afterwards.
4. Obtain evidence of the allocation on the register.
5. Submit the biodiversity gain plan for approval before development starts.

The biodiversity gain condition is deemed to apply to most planning permissions, and development must not begin until the authority has approved the plan. Building the allocation lead time into the programme is not optional.

Contract terms to check before buying

Read the securing agreement, not just the sales particulars. Three questions decide whether the units are safe.

First, what happens if the habitat fails. A well-drafted agreement puts remediation obligations on the gain site owner with a mechanism that does not depend on the developer having any continuing interest. Habitat creation does fail, and the reasons are usually mundane, as the account of planting seasons and habitat creation failure sets out.

Second, who monitors and reports, at what intervals, and to whom. The 30 year obligation is worthless without a reporting regime that the responsible body or authority can act on.

Third, what the price includes. Some offers cover registration, allocation and the full 30 year management. Others price the units and leave management costs to be recovered later. Compare like with like before comparing rates.

Also check whether the site is already selling units to other developments and how many remain unallocated. A registered site is a finite pool.

Spatial risk multipliers and location

The statutory metric discounts off-site units according to where they are delivered. Units created in the same local planning authority area or the same National Character Area as the development carry their full value. Units in a neighbouring area are discounted, and units further afield are discounted more heavily.

The arithmetic consequence is straightforward. A distant gain site needs to supply substantially more units to close the same deficit, so a lower headline price per unit can work out dearer once the multiplier is applied. Do the comparison on units delivered after discount, not on units purchased.

Check the multiplier values in the current published version of the metric, since these can change between versions and the version you use must match the one the authority expects.

Local plan policy sometimes goes further, with authorities expressing a preference for gain sites within the district or within a named strategic network. That preference is not in the metric, but it shows up in officer reports.

An illustrative example

A 60 unit scheme on a former industrial yard achieves 4 per cent on site through a green roof and a naturalised attenuation basin. The residual requirement is 3.1 area habitat units and 0.4 hedgerow units. A registered gain site in the adjoining district offers grassland units at an attractive rate, but the spatial multiplier means 4.1 units must be purchased to deliver 3.1, and the hedgerow deficit has to be met from a different site entirely. This example is illustrative.

Evidence the authority will want

The biodiversity gain plan is the document that carries all of this, and it stands or falls on its attachments.

Expect to supply the completed statutory metric in its published file format, the pre-development baseline with survey dates and the ecologist's competency statement, the register entry reference for each gain site, the allocation evidence, and a copy or summary of the securing agreement. Much of the underlying survey work comes from the preliminary ecological appraisal and any follow-on survey, which must be carried out at an appropriate time of year.

The statutory metric must be completed by a competent person, and for protected species the survey work requires a suitably licensed ecologist. An AI-drafted biodiversity net gain assessment can structure the document and flag what is missing, but it is a starting point for that professional rather than a substitute for them.

Risks if a gain site fails

The 30 year horizon is longer than most development companies. Ask what happens at year twelve when the grassland has reverted and the original owner has sold.

Mitigate it in three ways. Prefer gain sites secured by a conservation covenant with an established responsible body over sites relying solely on a planning obligation with a small authority. Prefer agreements with funded remediation rather than best endeavours. And keep your own record of the allocation, the plan and the agreement, because the party who needs to produce it in fifteen years will be a purchaser's solicitor, not an ecologist.

Enter your postcode on PlanScope to see which ecology reports a site is likely to need before you scope the metric.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.