Biodiversity net gain dates and transitional cases
Whether a scheme carries the biodiversity gain condition turns on when the application was made, not when it was decided. The two commencement dates, the exemptions and the section 73 problem.
Two dates decide whether mandatory biodiversity net gain applies to a scheme: 12 February 2024 for major development and 2 April 2024 for small sites. Everything else in the transitional analysis follows from when the application was made, not from when it was determined or when work starts. Getting the biodiversity net gain dates right is the difference between a permission that can be implemented and one held up by a gain plan nobody budgeted for.
The two commencement dates
Mandatory net gain was brought in by Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021. It operates as a deemed pre-commencement condition on most planning permissions, requiring an approved biodiversity gain plan showing at least a 10 per cent increase in biodiversity value against the pre-development baseline, secured for 30 years.
Major development came into the regime on 12 February 2024. Major means 10 or more dwellings, or a site of 0.5 hectares or more where the number of dwellings is not known, or non-residential development with 1,000 square metres or more of floorspace or a site of one hectare or more.
Small sites followed on 2 April 2024. A small site is residential development of one to nine dwellings on a site under one hectare, or where the number is unknown a site under 0.5 hectares, and non-residential development below 1,000 square metres of floorspace on a site under one hectare.
Nationally significant infrastructure projects sit under a separate commencement arrangement. Check the current position on gov.uk rather than assuming the same dates apply.
The distinction between major and small matters twice over. It sets the commencement date, and it sets which version of the metric applies, because small sites can use the small sites metric where the qualifying conditions are met. A scheme sitting just above a threshold on site area is worth remeasuring before the application goes in.
What the transitional arrangements protect
The regime does not apply to a planning permission granted on an application made before the relevant commencement date. An application for a major scheme validated in January 2024 and determined in the autumn is outside the regime, even though the decision came long after 12 February.
Three practical consequences follow. An old, unimplemented permission stays clean and can be built out without a gain plan. A permission granted on a pre-commencement application keeps that status for as long as it remains extant. And the date that matters is the date the application was made, so keep the submission receipt rather than relying on the validation date, which can differ by weeks.
Outline permissions raise their own question, because reserved matters approvals sit under the original outline. Where the outline application predates commencement, the reserved matters that follow are generally in the same position, but the point has been litigated in other contexts and the safe course is to confirm it with the authority before relying on it.
Section 73 applications and the gain condition
Varying a condition under section 73 creates a new planning permission, which is why section 73 is the part of the transitional picture that has caused the most trouble. The government amended the arrangements after commencement to address how the gain condition applies to section 73 permissions where the original was granted outside the regime.
Because that position has moved, do not rely on a summary written in 2024. Check the current guidance before advising that a section 73 application on an old permission escapes the requirement. And where the answer is unclear, the cheaper route is often a non-material amendment under section 96A, which does not create a new permission, provided the change really is non-material.
Exemptions and how they are claimed
Several categories sit outside the regime. The de minimis exemption covers development that does not impact a priority habitat and impacts less than 25 square metres of on-site habitat or less than five metres of on-site linear habitat such as hedgerow. Householder applications are exempt. So are self-build and custom build schemes meeting the stated conditions on dwelling numbers and site area, and biodiversity gain sites themselves.
Exemptions are claimed on the application form with supporting information. They are not automatic and they are not assumed by the case officer. A scheme that qualifies but does not say so on the form will receive the deemed condition anyway, and unpicking that afterwards is slow.
Measure the de minimis figures properly. Twenty-five square metres is a small area, and a driveway widening plus a bin store plus a service trench can exceed it between them.
Evidence of the pre-development baseline
The baseline is normally the habitat present on the date of the application. There is an anti-degradation rule: where on-site habitat has been degraded on or after 30 January 2020, the baseline is taken as the habitat immediately before that degradation, provided the degradation can be evidenced.
That rule has a practical consequence for anyone acquiring land. Get dated aerial imagery and a habitat record for the site going back several years before exchange, because a previous owner's clearance becomes your baseline problem. The work is part of a Preliminary Ecological Appraisal and should be commissioned early rather than at application stage.
Habitat condition assessment for most habitat types has to be done in the growing season, which is a programme constraint as much as a cost. A scheme that needs a survey in March and a decision in June is already behind.
Where an old permission is still clean
Extant permissions granted on pre-commencement applications are valuable, and they can be lost carelessly. Three things put one at risk.
- Letting it lapse. A permission not implemented within its time limit falls away, and any fresh application is in the regime.
- Varying it under section 73 without checking the current position on the gain condition.
- Implementing it defectively, so that what was done does not amount to a lawful start and the permission expires.
Where a pre-commencement permission is central to a land value, take advice on a lawful implementation before the expiry date and document what was done with dated photographs, invoices and a site inspection record.
Checking a scheme against the dates
Run four questions in order, and write the answers into the covering letter rather than leaving the case officer to work them out. When was the application made, as opposed to validated? Is the scheme major or small by the definitions above? Does an exemption apply, and has it been claimed on the form? And has anything happened since, such as a section 73 variation, that created a new permission?
Illustrative example: a five-unit scheme applied for on 20 March 2024 and granted in July. Small sites did not enter the regime until 2 April, so the permission carries no gain condition. The same scheme resubmitted after a design change in 2025 would be fully within the regime, which is why the design change was made by non-material amendment instead.
Keep the analysis on file with the permission. Purchasers, funders and the council's monitoring officer all ask the same question later, and a one-page note recording the application date, the site area, the classification and any exemption claimed saves the work being redone from scratch three years on.
Where the regime does apply, the metric work and the gain plan sit with a competent person. The Environment Act 2021 expects the statutory metric to be completed by someone competent to do it, and a Biodiversity Net Gain Assessment drafted in advance is a starting point for that ecologist rather than a substitute for them.
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. To see which ecology documents a site is likely to need, enter the postcode on PlanScope.
Frequently asked questions
Does net gain apply if my application was made before the commencement date?
No. The regime applies to permissions granted on applications made on or after the relevant date, which is 12 February 2024 for major development and 2 April 2024 for small sites. A major application made in January 2024 and determined in the autumn carries no gain condition. Keep the submission receipt, because the validation date can differ.
What counts as a small site?
Residential development of one to nine dwellings on a site under one hectare, or where the dwelling number is unknown a site under 0.5 hectares. For non-residential development it is floorspace below 1,000 square metres on a site under one hectare. Anything above those figures is major development and came into the regime earlier.
Does a section 73 application bring an old permission into the regime?
A section 73 permission is a new planning permission, which is why the point has been the most troublesome part of the transitional picture. The arrangements were amended after commencement, so check the current guidance rather than a summary written in 2024. Where the change is genuinely minor, section 96A may avoid the question.
How small is the de minimis exemption?
It covers development that does not impact a priority habitat and impacts less than 25 square metres of on-site habitat, or less than five metres of on-site linear habitat such as hedgerow. That is a small area, so measure it properly. A driveway widening, a bin store and a service trench can exceed it between them.
What is the anti-degradation rule?
Where on-site habitat was degraded on or after 30 January 2020, the baseline is taken as the habitat present immediately before that degradation, where it can be evidenced. Anyone buying land should obtain dated aerial imagery and habitat records going back several years, because a previous owner's clearance becomes the purchaser's baseline.
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.