What goes in a biodiversity gain plan and when it is approved
A biodiversity gain plan is not part of the planning application. It comes after permission, before any work starts, and the council has to approve it.
A biodiversity gain plan is the document the local planning authority has to approve before development starts on a site subject to the mandatory net gain regime. It is not part of the planning application. The permission is granted first, with a biodiversity gain condition deemed to be imposed on it under Schedule 7A to the Town and Country Planning Act 1990, and the plan then has to be submitted and approved before any work can lawfully begin. For a self-builder on a single plot, that means a second approval stage after the one everybody plans for.
The regime came in under the Environment Act 2021, applying to major development from 12 February 2024 and to small sites from 2 April 2024.
The biodiversity gain condition and how it works
The condition is not written on the decision notice by the case officer. It is deemed to be imposed by statute on planning permissions granted in England, subject to the exemptions and transitional provisions.
Its effect is straightforward. Development may not begin until the authority has approved a biodiversity gain plan for the development. Starting work without that approval is a breach of condition, and on a site where the permission is close to expiry it can also mean the permission was never lawfully implemented.
The gain itself is 10 per cent, measured using the statutory biodiversity metric published by Defra, with the habitat maintained for at least 30 years.
Contents of the plan the authority must approve
The plan has to set out, in substance, the following.
- Information about the development and the permission it relates to.
- The pre-development biodiversity value of the onsite habitat, with the date it was assessed.
- The post-development biodiversity value of the onsite habitat.
- Any registered off-site biodiversity gain allocated to the development.
- Any statutory biodiversity credits purchased.
- A statement covering irreplaceable habitat where any is present.
Behind each of those sits the metric calculation itself, the habitat survey data it was built from, and the plans showing the habitat parcels before and after. Authorities routinely ask for the metric file rather than a summary table, because a summary cannot be checked.
Irreplaceable habitat is treated separately. Ancient woodland, ancient and veteran trees, lowland fens, blanket bog and other listed habitats cannot be dealt with through the metric, and the plan has to explain the arrangements made instead.
Pre-development and post-development baselines
The baseline is the value of the onsite habitat on the date of the application, assessed through a habitat survey to the condition assessment criteria in the metric. Survey quality is where most plans fail, because habitat condition cannot be assessed from an aerial photograph or a winter site visit.
A Preliminary Ecological Appraisal usually provides the underlying survey, carried out in the growing season with the habitats classified to the required standard. Where the survey was done outside the season, expect the authority to question the condition scores.
The degradation provision matters. Where onsite habitat has been degraded after 30 January 2020 without planning permission, the authority can take the baseline as it was immediately before the degrading activity. Clearing a field of scrub before applying does not lower the starting figure, it simply makes the baseline harder to evidence and invites a higher assessment.
Post-development value comes from the proposed landscape scheme, with habitat types, areas, target condition and the time to reach that condition all feeding the calculation. The landscape drawings and the metric have to agree, and they frequently do not on the first submission.
Why the plan comes after permission
The sequencing looks odd until you consider what it avoids. Putting the gain plan inside the application would mean fixing the detailed landscape scheme before the principle of development is settled, and would mean recalculating it every time the layout changed during determination.
The practical consequence is that the applicant carries risk between permission and approval. A scheme that cannot achieve 10 per cent on site has to buy off-site units or statutory credits, and the cost of those is discovered after the permission is in hand.
Which is an argument for doing the metric work early even though the plan is submitted late. A Biodiversity Net Gain Assessment is a report type where a competent person is legally required under the Environment Act 2021 regime, and an AI-assisted draft is a starting point for that person rather than a substitute for the metric calculation or the survey behind it.
If the authority refuses the plan
Refusals cluster around three problems. A baseline the authority does not accept, usually because the survey is thin. A post-development calculation that does not match the approved drawings. Or an off-site arrangement that is not properly secured or registered.
| Problem | What the authority wants | Usual fix |
|---|---|---|
| Weak baseline survey | Season-appropriate habitat survey with condition assessment | Re-survey, which can mean waiting months |
| Metric does not match drawings | Consistency between landscape plan and metric areas | Revise one or both, then resubmit |
| Off-site units not secured | Register entry, covenant or obligation in place | Complete the legal documentation first |
| Irreplaceable habitat present | Separate treatment and justification | Redesign to avoid the habitat |
Fix the substance rather than arguing about it. The regulations provide a route to challenge a refusal, and the current guidance sets out how it operates, but an appeal takes longer than correcting a metric.
Exemptions and what they do not cover
The exempt categories are narrow. Householder development, certain self-build and custom build schemes, development below the de minimis thresholds for habitat area and linear features, and a small number of other cases set out in the legislation.
Self-builders read the self-build exemption optimistically and are often disappointed. It carries its own conditions about the number of dwellings, the site area and the nature of the build, and a plot bought with an outline permission does not automatically qualify.
The de minimis route is the other one people reach for. It applies where the development affects less than a stated area of onsite habitat and less than a stated length of linear habitat, and the figures are low enough that a single hedgerow removal can take a site outside it. Hardstanding counts as habitat in the metric, at a value of zero, which sometimes helps and sometimes complicates the calculation.
Where an exemption is claimed, say so expressly in the application and explain which one and why. Authorities are checking.
Keeping the plan consistent with approved drawings
The most avoidable failure is drift between documents. The landscape architect revises a planting plan, the ecologist's metric still reflects the earlier version, and the two arrive at the authority disagreeing with each other.
Lock the versions together. Name the drawing numbers and revisions in the gain plan, and check the habitat areas in the metric against the areas on those drawings before submission.
Then think about the 30 years. Whoever ends up holding the land holds the management obligation, and the monitoring reports required by the securing mechanism are a real commitment with a real cost. Set out in the plan who that will be.
Where the site also supports protected species, the ecology work runs on two tracks, because a Protected Species Survey answers a legal question about licensing that the metric does not touch. You can enter your postcode on PlanScope to see which ecology documents your site is likely to need.
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.
Frequently asked questions
When does the biodiversity gain plan have to be submitted?
After permission is granted and before development begins. The gain condition is deemed to be imposed on the permission, and the plan has to be submitted to and approved by the local planning authority before any work starts. Most applicants prepare the substance of it during the application so that the post-permission stage is a formality rather than a delay.
Can I submit the gain plan with the application?
The plan itself is a post-permission document, but nothing stops you providing the metric calculations, the habitat baseline and the proposed approach with the application. Doing so lets the case officer and the county ecologist review the approach before the permission is granted, which reduces the risk of an argument at the point where the programme is tightest.
What is the pre-development baseline?
It is the biodiversity value of the onsite habitat, normally measured on the date of the application. Where habitat has been degraded or cleared after 30 January 2020 without permission, the authority can assess the baseline as it was before that activity. That provision exists to stop sites being cleared to lower the starting figure, and authorities look for it.
What happens if the authority refuses to approve the plan?
Development cannot begin. The usual reasons are an incorrect baseline, a metric completed without adequate survey data, or an off-site arrangement that is not properly secured. There is a route to challenge a refusal, and the regulations set out how, so take the current position from the guidance and correct the substantive problem in parallel.
Which developments are exempt?
The exemptions are narrow and specific, covering householder development, some self-build and custom build schemes, development below the de minimis habitat thresholds, and certain other categories set out in the legislation. Do not assume an exemption applies because a site looks like hardstanding, because small areas of habitat still count and the thresholds are low.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.