Biodiversity net gain: the phased commencement dates
Which biodiversity net gain regime applies depends on when the application was made, not when it was decided. The two commencement dates, the exemptions and the transitional rules.
Mandatory biodiversity net gain started on two different dates. The biodiversity gain condition applied to most major development in England from 12 February 2024, and to small sites from 2 April 2024. Which date governs your permission depends on when the application was made, not on when it was decided, and that transitional rule is the single most useful thing a self-builder can know about the regime. A permission from an application submitted before the relevant date is outside the requirement altogether, even if the decision arrived long afterwards.
Major infrastructure consented through development consent orders follows a separate timetable that has been revised more than once, so that route needs checking against current guidance rather than assumed.
Where the duty comes from
The Environment Act 2021 inserted Schedule 7A into the Town and Country Planning Act 1990. Rather than requiring a separate application, it works by deeming a condition onto planning permissions in England.
The condition prevents development from starting until the local planning authority has approved a biodiversity gain plan. That plan has to show a gain of at least 10 per cent against the pre-development baseline, measured with the statutory biodiversity metric, with habitat secured and managed for at least 30 years. The mechanism is a condition, so the consequence of ignoring it is that you have no lawful start.
Commencement was staged through separate regulations rather than switched on in one go, which is why the dates matter so much.
12 February 2024 and major development
From 12 February 2024 the condition applied to major development. Major means ten 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 m2 or more of floorspace or a site area of one hectare or more.
For most self-builders that threshold is irrelevant. It matters if you are building out a small paddock at density, or if your site area crosses half a hectare even though the unit count is low, which happens more often than people expect on rural plots with large curtilages.
Where the scheme is major, the Biodiversity Net Gain Assessment work has to be done properly, and the Environment Act regime expects the statutory metric to be completed by a competent person. An AI-assisted draft can structure the narrative and identify what the plan has to cover, but the metric itself and any habitat condition assessment need a suitably qualified ecologist. That is a legal requirement rather than a recommendation.
2 April 2024 and small sites
Small sites came into the regime on 2 April 2024. A small site is residential development of between one and nine dwellings on a site of less than one hectare, or where the number of dwellings is unknown, a site of less than 0.5 hectares. For non-residential development it is less than 1,000 m2 of floorspace on a site of less than one hectare.
That date is when the regime started to bite on ordinary plots. A single new dwelling on a garden plot is caught, unless an exemption applies, and the baseline assessment has to be done before you disturb the site rather than after.
The practical sequence for a small site is worth stating plainly:
- Survey the existing habitats on site and record the baseline before any clearance.
- Design the scheme with retained and created habitat in mind, since on-site gain is far simpler than buying units.
- Complete the metric and produce the gain plan.
- Submit the plan for approval and wait for it, because approval is a precondition of a lawful start.
- Secure the 30-year management, normally by a planning condition or a section 106 agreement.
The step people skip is the first. Clearing a site of scrub before the baseline is recorded does not reduce the baseline. Degradation of habitat after 30 January 2020 is treated as if it had not happened, so the calculation still uses the earlier condition.
Development outside the requirement
Several categories sit outside the regime, set out in exemption regulations made in 2024.
- Development below the de minimis threshold, meaning it does not affect an on-site priority habitat and affects less than 25 m2 of habitat or less than five metres of linear habitat such as a hedgerow.
- Householder applications, which covers most extensions, loft conversions and outbuildings within a domestic curtilage.
- Self-build and custom housebuilding, subject to conditions on the number of dwellings and the site area.
- Development on a designated biodiversity gain site, where the land is already being managed for habitat.
- Certain urgent Crown development and specified infrastructure projects.
- Development permitted by the General Permitted Development Order rather than by a planning permission granted on application.
Read the self-build exemption in full if it applies to you, because it carries conditions on the number of dwellings and on whether each dwelling will be occupied by a person who commissioned it. It is not a general exemption for anyone building their own house on a large plot.
The householder exemption is broader than people assume, and it is why most extension projects never encounter the metric. The detail of who is exempt from the biodiversity gain condition repays a careful read before you commission ecology work you may not need.
How earlier applications and permissions are treated
The transitional provisions turn on the date the application was made. An application made before 12 February 2024, for major development, is outside the regime. An application made before 2 April 2024, for a small site, is likewise outside it.
That produces a long tail. Permissions granted in late 2024 and through 2025 from applications submitted before the relevant date carry no biodiversity gain condition, and some of those permissions still have time to run. If you hold an older consent, check the application submission date on the statutory register entry before assuming you need a gain plan.
Outline permissions add a wrinkle. Where an outline permission predates the relevant date, reserved matters submissions under it are generally treated as following the original permission rather than starting afresh, but the position depends on the detail of the permission, so take advice rather than reading across.
Section 73 and variations of older permissions
A section 73 application to vary a condition creates a new planning permission, which raises the obvious question of whether the new permission picks up the biodiversity gain condition.
The transitional regulations address this. Broadly, where the original permission was granted on an application made before the relevant commencement date, a later section 73 permission is not brought into the regime by the variation alone. That treatment is specific and conditional, so confirm it against the current regulations for your circumstances before relying on it.
The practical advice is simple. If you are considering a section 73 on a pre-2024 permission, check the biodiversity position before you apply, because discovering afterwards that the varied permission requires a gain plan is a poor moment to find out.
Development consent orders and their own timetable
Development consent orders for nationally significant infrastructure were always intended to come into the regime later than town and country planning permissions, and the implementation date has been revised.
Because that timetable has moved, do not take a date from an article or a memory. Check the current position on gov.uk and record the date you checked it. As at 24 September 2026, the safe course for anyone working on a development consent order is to confirm the commencement position directly with the current statutory guidance. Reform news of this kind is tracked on the planning law timeline, which is the place to follow announcements as they are made.
Working out which regime applies
Answer four questions in order. When was the application made, and does that date fall before the relevant commencement date for the scale of development. If not, is the development major or small by the definitions above. Does an exemption apply, and can you evidence it. And is this a fresh permission or a variation of an older one.
Get those four answers written down at the start of a project. A Preliminary Ecological Appraisal commissioned early will tell you what habitat you actually have, which is the input every other answer depends on.
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 did biodiversity net gain become mandatory?
The biodiversity gain condition applied to most major development in England from 12 February 2024 and to small sites from 2 April 2024. The duty comes from Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, and was brought into force in stages by separate regulations rather than all at once.
Does the requirement depend on when my permission was granted?
No, it turns on when the application was made. An application submitted before the relevant commencement date for the scale of development falls outside the regime even if the decision arrived much later. That produces a long tail of permissions granted well after 2024 that carry no biodiversity gain condition.
Is a single self-build house exempt?
Possibly, under the self-build and custom housebuilding exemption, but that exemption carries conditions on the number of dwellings, the site area and whether each dwelling will be occupied by the person who commissioned it. It is not a blanket exemption for anyone building their own home, so check the regulations against your circumstances.
Can I clear the site before the baseline survey?
It will not help you. Degradation of habitat after 30 January 2020 is treated as if it had not occurred, so the baseline calculation uses the earlier habitat condition. Survey and record the site before any clearance, both because the calculation demands it and because retained habitat is far easier than buying off-site units.
Does a section 73 application bring an older permission into the regime?
Generally not by itself. The transitional regulations provide that where the original permission came from an application made before the relevant commencement date, a later section 73 permission is not drawn in by the variation alone. That treatment is conditional, so confirm it against the current regulations before applying.
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.