Biodiversity gain plan: the document you file after permission

A biodiversity gain plan is the document you submit after planning permission is granted and before you are allowed to start work. It is not part of the application, it is not the same thing as the metric calculation, and it is the point at which a lot of small residential schemes stall for a month or two because nobody budgeted for it.

The plan sets out how the development will deliver at least a 10 per cent increase in biodiversity value, measured with the statutory metric, and it has to be approved in writing by the local planning authority before development begins.

What the plan is and where it comes from

Schedule 7A of the Town and Country Planning Act 1990, inserted by the Environment Act 2021, created a general biodiversity gain condition. Most planning permissions granted in England are treated as carrying it whether or not the decision notice mentions it.

The condition says development may not begin until a biodiversity gain plan has been submitted to and approved by the planning authority. So the sequence is: permission, then plan, then approval, then site work. Anyone who reads the decision notice, sees no biodiversity wording and sends the diggers in has breached a condition that was deemed into their permission.

Two dates set the regime. Mandatory gain applied to major development from 12 February 2024 and to small sites from 2 April 2024. Applications validated before those dates are generally outside it, which is why older permissions in the same street may carry no obligation at all.

The deemed condition and what it actually blocks

The condition blocks the start of development, which in planning terms means any material operation. Site clearance, demolition and digging foundations all count. Preparatory work that does not amount to a material operation does not, but that is a fine line and not one worth testing.

Some conditions can be discharged in parallel with early works. This one cannot, and its effect is absolute. If your funding schedule assumes a spring start, the gain plan has to be with the council in the winter.

A second trap sits in phased development. Where permission is phased, the regime allows an overall plan plus plans for each phase, and the phasing has to be reflected in the plan itself rather than assumed. Get that wrong on a small site divided into two build phases and you have to go back and redo the approval.

What the plan must contain

The content is prescribed rather than left to judgement. A gain plan sets out:

1. Information about the steps taken or to be taken to avoid or reduce the adverse effect of the development on the onsite habitat, following the mitigation hierarchy.
2. The pre-development biodiversity value of the onsite habitat, and the date to which that value is calculated.
3. The post-development biodiversity value of the onsite habitat.
4. Any registered offsite biodiversity gain allocated to the development, and who registered it.
5. Any statutory biodiversity credits bought from the government.
6. Any other matters the regulations require.

The arithmetic behind the second and third points comes from the statutory biodiversity metric, and the metric is where errors are expensive. Habitat type, distinctiveness, condition and strategic significance all feed the habitat units that the calculation trades in, and a condition assessment recorded optimistically at survey stage produces a shortfall that only appears when the council checks the workings.

Where onsite enhancement is doing the work, the maintenance obligation runs for 30 years and has to be secured, usually by a planning condition, a section 106 obligation or a conservation covenant. Thirty years of management on a garden-scale plot is a real commitment, and it is worth pricing before you design the scheme around onsite delivery.

Who approves it and how long that takes

The local planning authority determines the plan. Approval is a formal decision, not a courtesy, and the council can refuse it if the metric does not stack up or the mitigation hierarchy has plainly been skipped.

Councils work to a determination period set by the regulations rather than a fixed national eight-week clock, and practice varies on how quickly ecology advice comes back. The practical answer is to ask your case officer at the point of grant what the authority's process is and who reviews gain plans, then submit as soon as the design is fixed. Waiting until the contractor is booked is how a fortnight becomes six weeks.

Bring the ecology evidence with the plan. A Preliminary Ecological Appraisal that already records habitat parcels and condition scores means the plan is a summary of work already done rather than a new survey exercise, and it removes the risk of a habitat baseline being challenged after permission.

Exemptions and the small sites route

Not every permission carries the condition. Householder applications are exempt, as are developments below the de minimis threshold, which turns on impacts to less than 25 m² of onsite habitat and less than 5 m of onsite linear habitat such as hedgerow. Self-build and custom build schemes meeting the size and use tests are exempt, as are biodiversity gain sites themselves and certain urgent Crown development.

Small sites that are caught can use the small sites metric, a shortened version of the statutory metric intended for developments below the major thresholds that do not affect priority habitat. It asks for less data. It does not lower the 10 per cent requirement.

Check the exemption carefully rather than assuming it. A single new dwelling in a garden with a hedgerow along the boundary usually fails the de minimis test on the linear habitat alone, and the exemption for self-build is narrower than the phrase suggests.

An illustrative case makes the point. Take a plot of former lawn with a 12 m native hedge along one side, and a proposal for one house with a driveway that clips 4 m of that hedge. The hedgerow impact alone takes the scheme past the linear threshold, so the de minimis exemption falls away even though the habitat lost is modest. The owner then needs a baseline, a metric run and a gain plan for what began as a straightforward single plot. That is the moment most self-builders discover the requirement, and by then the design is fixed and the simplest fix, moving the drive by two metres, is no longer on the table.

Why the competent person rule shapes your programme

Biodiversity net gain sits in the category of work where a licensed or competent professional is legally required. The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and the planning authority approves the resulting plan. A draft is a starting point for that professional, never a substitute for them.

That has a scheduling consequence. Habitat condition assessment is a field exercise best done in the botanical survey season, roughly April to September, when the sward can actually be identified. A January baseline recorded from aerial imagery will be picked apart. If you are working out which evidence a scheme needs before you commit to a programme, the Biodiversity Net Gain Assessment page sets out what the report covers and who is expected to prepare it.

Enter your postcode on PlanScope to see which reports your site is likely to need, then take the ecology work to a competent person early enough to survey in season.

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.