Biodiversity net gain start dates and transitional rules
Whether the ten per cent gain requirement applies turns on the date the application was made, not the date of the decision. The two start dates, the transitional rules and the exemptions that matter most.
Mandatory biodiversity net gain did not arrive in one piece. Major development came into the regime on 12 February 2024 and small sites followed on 2 April 2024, and those two biodiversity net gain start dates still decide whether a live scheme has to deliver ten per cent gain or not. Plenty of permissions in the system predate both, and a surprising number of applicants assume the requirement applies when it does not, or that it does not when it plainly does.
The duty itself sits in Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021. It works as a condition deemed to be attached to most planning permissions, so it is not something a council chooses to impose.
How the condition actually works
Every permission caught by the regime carries a biodiversity gain condition. Development may not begin until the local planning authority has approved a biodiversity gain plan for the site.
The plan has to show that the post-development biodiversity value exceeds the pre-development value by at least ten per cent, measured with the statutory biodiversity metric published by Defra. Gains delivered on site or off site must be secured and maintained for at least 30 years, through a planning obligation or a conservation covenant. Statutory biodiversity credits bought from the government are the last resort in the hierarchy, priced to be less attractive than delivering real habitat.
The baseline date matters as much as the target. The pre-development value is normally taken immediately before development, but where the authority is satisfied that activities carried out after 30 January 2020 reduced the biodiversity value of the site, the earlier value applies instead. Clearing a site to improve its metric score is a strategy that the Act anticipated.
12 February 2024 for major development
Major development entered the regime first. For these purposes major means ten or more dwellings, 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.
Applications made on or after that date are caught. Applications made before it are not, and that is the test rather than the date of the decision, which means permissions granted well into 2024 and 2025 on earlier applications sit outside the regime entirely.
2 April 2024 for small sites
Small sites followed seven weeks later. And the definition is not the one most people expect. A small site is residential development of fewer than ten dwellings on a site under one hectare, or where the number is unknown a site under half a hectare, and non-residential development below 1,000 square metres of floorspace on a site under one hectare.
Small sites can use the small sites metric, a simplified version of the statutory tool intended for schemes without complex habitats. It is not available where the site contains or is close to certain priority habitats, or where protected species or designated sites are involved, and using it inappropriately is a quick route to a refused gain plan. A Preliminary Ecological Appraisal is usually the sensible first step, because it tells you which metric you may use before you have paid for the wrong one.
Which permissions the transitional rules protect
Three points cover most queries.
- The trigger is the date the application was made, not the date of the decision or the date development starts.
- Permissions granted on applications made before the relevant date remain outside the regime for their lifetime, including reserved matters flowing from an outline permission granted on such an application.
- Section 73 permissions generally take their status from the original permission, though this part of the regulations has been amended since 2024, so check the current wording before relying on it for a variation.
That third point is where advisers get caught. A section 73 application in 2026 on a permission granted in 2023 is not automatically a new entry point into the regime, but the detail has moved and it is not safe to assume either way.
Development that is exempt
Several categories sit outside the requirement altogether.
| Exemption | Scope |
|---|---|
| De minimis | Development affecting less than 25 square metres of onsite habitat and less than 5 metres of onsite linear habitat |
| Householder | Extensions, outbuildings and other householder applications |
| Self-build and custom build | No more than nine dwellings, site no larger than 0.5 hectares, all dwellings self-build or custom build |
| Biodiversity gain sites | Development whose purpose is to deliver the gain itself |
| Permitted development | Development granted permission by a development order, including prior approval routes |
The self-build exemption is the one self-builders most often miss. It has to be claimed and evidenced, and the conditions are cumulative rather than alternatives, so a tenth dwelling or a site of 0.6 hectares takes the whole scheme back into the regime.
Irreplaceable habitats are treated differently again. Ancient woodland, ancient and veteran trees, limestone pavement and certain other habitats cannot be traded through the metric, and a scheme affecting them needs bespoke compensation agreed with the authority alongside the ordinary policy tests on harm.
Outline permissions and phased sites
Outline permissions raise an obvious problem. The gain plan has to be approved before development begins, but at outline stage nobody knows the layout.
The regime deals with this by allowing the condition to be modified for phased development. A site can have an overall biodiversity gain plan setting the headline position, with phase-specific plans approved before each phase starts. Where an outline permission is granted for a phased scheme, ask the authority at application stage how it intends to apply the condition, because retrofitting a phasing arrangement onto a standard condition is slow.
Reserved matters applications on pre-regime outline permissions do not bring the site into the regime. The relevant application is the one that produced the outline permission.
Checking which regime applies to your site
Work through it in order, and write the answer down for the file.
- Find the date the application was made, from the validation record rather than from memory.
- Classify the development as major or small using the definitions above.
- Test the exemptions, with particular attention to de minimis and self-build.
- If the regime applies, establish the baseline date and whether anything happened on the site after 30 January 2020 that might move it.
- Commission the metric baseline from a competent person before the layout is finalised.
That last step is a legal point rather than a preference. The biodiversity gain plan under the Environment Act 2021 regime expects the statutory metric to be completed by a competent person, so a Biodiversity Net Gain Assessment is work to commission rather than to attempt in house. An AI-assisted draft can frame the policy position, the site description and the structure around the metric output, and the ecologist supplies the numbers.
Commencement dates in this area have moved before. Nationally significant infrastructure was always intended to join under a separate arrangement and its start date has shifted more than once, so treat any timetable you were given in 2024 as needing confirmation. The planning law timeline is a place to follow the position, and the primary legislation and Defra guidance should be checked at the point you rely on them.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, and you should check the position with your local planning authority or a qualified professional before relying on it. Browse the PlanScope report types to see what else a small residential scheme usually needs.
Frequently asked questions
When did biodiversity net gain become mandatory?
Major development came into the regime on 12 February 2024 and small sites followed on 2 April 2024. The duty sits in Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, and operates as a condition deemed to be attached to most planning permissions.
Does the requirement depend on the application date or the decision date?
The application date. Applications made before the relevant start date stay outside the regime even if the permission was granted much later, and reserved matters flowing from a pre-regime outline permission are also outside it. Take the date from the council's validation record rather than from memory.
What counts as a small site for biodiversity net gain?
Residential development of fewer than ten dwellings on a site under one hectare, or a site under half a hectare where the number of dwellings is unknown. For non-residential development it is under 1,000 square metres of floorspace or a site under one hectare. Small sites can often use the simplified small sites metric.
Which developments are exempt from biodiversity net gain?
Householder applications, development below the de minimis threshold of 25 square metres of onsite habitat and 5 metres of linear habitat, self-build and custom build schemes of up to nine dwellings on no more than 0.5 hectares, biodiversity gain sites themselves, and development permitted by a development order.
Can the baseline be taken after a site has been cleared?
Not if the clearance reduced the biodiversity value. Where the authority is satisfied that activities carried out after 30 January 2020 degraded the habitat, the pre-development value is taken at that earlier date instead. The Act anticipated site clearance as a way of improving a metric score.
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.