Biodiversity gain plan or a BNG assessment: what differs
The metric assessment you commission to design a scheme and the biodiversity gain plan you submit to discharge the deemed condition are different documents with different deadlines. Mixing them up stops a site from starting lawfully.
The difference between a biodiversity gain plan and a BNG assessment is timing and purpose. The assessment is the metric work you commission before you apply, to find out what habitat you have, what the scheme destroys and how you reach the 10 per cent uplift. The gain plan is a separate document submitted after permission is granted, which the local planning authority has to approve before you may lawfully start on site. One shapes the design. The other is what lets you break ground.
Self-builders get caught by this more than anyone, because the two sit either side of the decision notice and only one of them is obvious at application stage.
What the metric assessment does
The assessment is the evidence base. An ecologist surveys the site, classifies habitat using the UK Habitat Classification, scores condition against the published criteria, and runs the areas through the statutory biodiversity metric to produce a pre-development baseline in biodiversity units. The same tool then models the post-development habitat and tells you whether the design clears the 10 per cent requirement.
That output changes the layout. Losing a species-rich hedgerow costs far more units than losing amenity grassland, and creating units on a small site is hard because the land simply is not there. Finding that out while the drawings are still editable is the whole point. A Biodiversity Net Gain Assessment prepared at design stage lets you move a parking court off a hedge line or widen a retained margin before the application is fixed.
On most sites the assessment follows a Preliminary Ecological Appraisal, which identifies habitats and any protected species constraints. The two are often commissioned together, and councils increasingly ask for both on the validation list for a new dwelling.
What the gain plan is for
The biodiversity gain plan is a statutory document. It records the pre-development baseline, the post-development onsite value, any registered offsite units, any statutory credits bought, the steps taken to minimise habitat loss, and how the gain will be secured and managed. It also has to reconcile with the approved drawings, which is where plans most often fail.
It is submitted to the authority that granted permission, not to Natural England, and it is approved or refused as a discrete decision. Until it is approved, the condition is unmet and starting work is a breach.
The deemed condition on every permission
Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, imposes the biodiversity gain condition on every grant of planning permission in England unless an exemption applies. The condition is deemed. It does not appear as a numbered condition that someone forgot to attach, and you cannot negotiate it away.
The regime came into force for major development on 12 February 2024 and for small sites on 2 April 2024. Exemptions cover householder applications, developments affecting less than 25 m² of onsite habitat and less than 5 m of linear habitat, biodiversity gain sites themselves, and self-build and custom-build schemes of nine dwellings or fewer on sites no larger than 0.5 ha where every dwelling is self-build or custom-build. Check the exemption wording against your scheme carefully, because the self-build route has conditions attached to it that a part-speculative plot will not meet.
| Stage | Document | When |
|---|---|---|
| Design | Metric assessment and habitat plans | Before and with the application |
| Decision | Deemed biodiversity gain condition | Attached automatically on grant |
| Pre-commencement | Biodiversity gain plan | After permission, before any work |
| Delivery | Habitat management and monitoring plan | Secured for at least 30 years |
Baseline date, degradation and irreplaceable habitat
The baseline is normally the habitat present on the date of the application. That rule exists to stop a site being cleared to lower the target, and it is reinforced by the degradation provision: where onsite habitat was degraded on or after 30 January 2020 without planning permission, the baseline is taken as the condition before the degradation. Bulldozing scrub in the spring before you apply buys nothing and costs credibility with the case officer.
Irreplaceable habitat sits outside the percentage calculation entirely. Ancient woodland, ancient and veteran trees, blanket bog, limestone pavement and the other listed types cannot be traded in units, and development affecting them needs bespoke compensation agreed with the authority alongside the policy tests in the National Planning Policy Framework (August 2026 edition). If your site has a veteran tree on it, resolve that question before you touch the metric.
Watercourses and hedgerows are measured as linear units and reported separately. A scheme can clear the area target and still fail on hedgerow units, which is a common outcome for frontage development on a rural plot.
Approval before you can begin
Submitting the gain plan is not the same as discharging the condition. The authority has to approve it, and the approval is what permits commencement. Build the time into the programme, because the clock does not start until the plan goes in and the plan cannot be finalised until the detailed landscape proposals match the drawings.
Practical sequencing that works:
- Agree the baseline with the authority during the application, in writing where you can.
- Fix the landscape layout before you submit the gain plan, so habitat areas reconcile with the approved drawings.
- Register any offsite units or allocate statutory credits before the plan goes in.
- Submit the gain plan with the habitat management and monitoring plan attached.
- Hold off commencement, including site clearance, until the approval arrives.
Where a hybrid or phased permission is in play the rules differ, and a phased development uses an overall gain plan plus a plan for each phase. Flag phasing early.
Thirty years and who carries it
Significant onsite habitat enhancement has to be secured for at least 30 years. Three mechanisms do this: a planning condition, a section 106 obligation, or a conservation covenant with a responsible body. Offsite gains are recorded on the biodiversity gain sites register for the same duration.
For a small developer the practical question is who holds the obligation after the houses are sold. A management company funded by estate charges is the usual answer, and the management plan needs to name the receptor, the works, the monitoring intervals and the reporting route. Councils have become sharper about refusing plans that leave this vague. In London, authorities may also apply the Urban Greening Factor used in London alongside the national requirement, and the two are measured differently.
Who must complete the metric
The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and a licensed or suitably qualified ecologist is legally required for the survey and condition assessment that feed it. This is not a box-ticking exercise an applicant can self-certify. Habitat condition scoring is judgement-based, and a metric built on a misclassified parcel will be rejected by the authority's own ecologist.
PlanScope can draft the narrative and structure of a Biodiversity Net Gain Assessment from your site details and flag where survey data is missing, and the draft is a starting point for the competent person rather than a substitute for them. You can send it to a reviewer who can sign the work off once the survey data exists. For specialist technical reports, qualified professional input may be required.
Before you commission anything, it is worth seeing the full set of report types a small site usually needs, because the ecology work rarely travels alone.
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
Is a biodiversity gain plan submitted with the planning application?
No. The gain plan is submitted after permission is granted and must be approved by the local planning authority before development may lawfully begin. What goes in with the application is the metric assessment, habitat plans and usually a statement of how the 10 per cent will be achieved, so the officer can judge whether the scheme is deliverable.
Which small sites are exempt from mandatory biodiversity net gain?
Householder applications are exempt, as are developments affecting less than 25 square metres of onsite habitat and less than five metres of linear habitat such as hedgerow. There is also a self-build and custom-build exemption for schemes of nine dwellings or fewer on a site no larger than 0.5 hectares where every dwelling is self-build or custom-build.
What baseline date does the metric use?
The default is the date of the planning application, measured on the habitat present then. Where habitat was degraded on or after 30 January 2020 without permission, the baseline is the pre-degradation condition, so clearing a site early does not lower the target. An earlier date can apply where the parties agree one in writing.
Can I complete the statutory metric myself?
The regime expects a competent person to complete the statutory biodiversity metric, because habitat classification and condition assessment follow the UK Habitat Classification and published condition sheets. In practice councils reject metrics completed by someone without ecological training. Commission a suitably qualified ecologist, and treat any draft as a starting point for that person.
How long does the 30 year obligation last and who holds it?
Significant onsite habitat enhancement must be secured for at least 30 years, through a planning condition, a section 106 agreement or a conservation covenant. Offsite units are registered on the biodiversity gain sites register with the same duration. The landowner or a management body carries the obligation, and the gain plan has to name who that is.
What happens if the gain plan is refused?
Development cannot begin, because the deemed condition is unmet. The authority has to decide the plan within the period set by the regulations, and refusal can be appealed. In most cases a refusal reflects a metric that does not reconcile with the approved drawings, which is fixable by resubmitting with corrected habitat areas.
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.