Biodiversity net gain exemptions and how to claim them

Four exemptions from mandatory biodiversity net gain sit in the Biodiversity Gain Requirements (Exemptions) Regulations 2024: development below a de minimis threshold, householder development, self-build and custom housebuilding, and development of a biodiversity gain site itself. If your scheme fits one of them, you still have to say so on the application form and explain why, because the council validates that claim before it validates anything else.

Getting the claim wrong costs weeks. A small site that wrongly claims de minimis is either invalidated at the front desk or, worse, granted permission with a biodiversity gain condition the applicant cannot discharge.

What the exemptions actually are

Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, deems a biodiversity gain condition into planning permissions granted on application in England. The condition prevents development from beginning until the authority has approved a biodiversity gain plan showing at least a 10 per cent uplift, maintained for 30 years. Mandatory net gain applied to major development from 12 February 2024 and to small sites from 2 April 2024.

The 2024 Exemptions Regulations carve out four categories:

Each is narrow. None of them is a judgement call dressed up as a threshold, and all four are checked against the submitted drawings.

The de minimis threshold and how it is measured

Two conditions have to hold together. The development must not impact any onsite priority habitat, and it must impact less than 25 square metres of onsite habitat that has a biodiversity value greater than zero, or less than 5 metres of linear habitat such as a hedgerow.

Priority habitat is the first thing to check, because it is an absolute bar. A pond, a patch of lowland meadow or a stretch of hedgerow classified as a priority habitat removes the exemption regardless of how small the footprint is. Check the Priority Habitats Inventory on MAGIC before assuming anything, and remember that mapped data is indicative and a site walkover may find habitat the inventory missed.

The 25 square metre figure counts habitat with a value above zero. Sealed surfaces such as tarmac and concrete score zero in the statutory metric, so an infill plot that is entirely hardstanding may fall under the threshold. Amenity grassland does not score zero, and a scheme taking 40 square metres of scruffy lawn is caught even though nobody would call it valuable.

Three practical traps. Temporary works such as site compounds and access tracks count as impact. Habitat lost to a soakaway, a bin store or a driveway counts, even where the main building sits on hardstanding. And the measurement is the area impacted rather than the area of the red line, so a large plot with a small intervention can still qualify.

Where the figures are close, the honest answer is to have an ecologist measure it. The errors councils see most often are the subject of the note on biodiversity metric errors, and habitat classification sits at the top of that list.

Householder applications

A householder application is an application for works to an existing dwellinghouse or within its curtilage, and those are exempt. Extensions, loft conversions, garden rooms, outbuildings, driveways and boundary treatments to a single existing house therefore fall outside the regime.

The boundary of this exemption is the definition itself. An application for a new dwelling in the garden of an existing house is not a householder application, even though it is submitted by a homeowner on a domestic plot, and it is caught by mandatory net gain unless another exemption applies. The same is true of a change of use, and of works to a block of flats, which are not householder applications either.

Councils check this by looking at the application type selected on the form. Choosing householder when the proposal is a new dwelling does not create an exemption, and it will be corrected at validation.

Self-build and custom housebuilding

This is the exemption most commonly claimed wrongly. Three conditions apply at once: the development consists of no more than nine dwellings, the site area is no greater than 0.5 hectares, and every dwelling is self-build or custom housebuilding within the meaning of the Self-build and Custom Housebuilding Act 2015.

That third limb is the hard one. The statutory definition turns on the prospective occupier having primary input into the final design and layout of the home, and a speculative plot sold with a finished house on it does not meet it. A developer building nine houses on a small site and calling them custom homes will be asked for evidence: build contracts, plot passports, marketing material showing purchasers choosing layouts, and registration on the council's self-build register.

Mixed schemes fail. If eight plots are self-build and the ninth is a market house, the exemption does not apply to the application, because the requirement is that the development consists exclusively of self-build or custom dwellings.

Development outside the regime

Some development is not exempt so much as never caught in the first place, which matters because the form asks a different question.

Permitted development, including prior approval schemes under Part 3 and Part 20 of the GPDO, does not attract the biodiversity gain condition, because the condition attaches to permissions granted on application. That is a real difference for anyone converting a barn or an office. The timetable for extending mandatory net gain to nationally significant infrastructure projects has moved more than once, so check the current position rather than relying on an earlier announcement.

Section 73 applications are their own puzzle. Where the original permission predates mandatory net gain or was itself exempt, a section 73 variation is generally treated as carrying the original position, though the detail depends on the date and the terms of the original permission. Take advice on this one rather than assuming.

Reserved matters applications do not carry a separate condition either, because the gain condition attaches to the outline permission, and the gain plan is dealt with at that level.

Evidencing the exemption on the form

Since February 2024 every application for planning permission in England has had to include a statement as to whether the applicant believes the biodiversity gain condition applies. Where an exemption is claimed, say which one and give the reason in a sentence. Where it is not, the application must include the pre-development biodiversity value of the onsite habitat, the date it was measured, and the completed statutory metric calculation.

For a de minimis claim, attach:

1. A plan showing the red line, the development footprint and the extent of habitat impacted, with the area annotated in square metres.
2. A short habitat description, ideally from an ecologist, identifying each habitat parcel and confirming none is priority habitat.
3. A note on temporary works, confirming that compounds, storage and access do not push the figure over the threshold.
4. Photographs dated close to submission.

For a self-build claim, attach the site area calculation, the dwelling count, and evidence that each plot meets the statutory definition. For a householder claim, nothing beyond the correct application type is usually needed.

A Preliminary Ecological Appraisal is the usual route to a defensible habitat description, and councils generally expect a suitably qualified author for it.

If the council disagrees

The claim is tested at validation. If the officer thinks the scheme is caught, the application will be held as invalid until the metric and pre-development value are supplied, and the determination clock does not start. That is the recoverable failure.

The costly one is a permission granted on a mistaken exemption. The gain condition is deemed into the permission by statute, so an incorrect exemption does not remove it, and development started without an approved gain plan is a breach of condition. Resolving that after the fact means retrofitting a metric to a site whose baseline habitat has already gone, which usually produces a far larger obligation than a proper assessment would have done.

Where the assessment is needed, note that the statutory metric is expected to be completed by a competent person under the Environment Act 2021 regime, and the biodiversity gain plan has to be approved by the local planning authority. PlanScope's Biodiversity Net Gain Assessment draft is a starting point for that professional rather than a substitute for them. Timing also matters, since habitat creation has planting seasons and failure rates, as set out in the piece on habitat creation.

Enter your postcode on PlanScope to see which ecology documents a small site is likely to need.

This is general guidance, not legal advice. For specialist technical reports, qualified professional input may be required, and a licensed or competent professional is legally required for the statutory biodiversity metric. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.