Biodiversity net gain exemptions and the de minimis test
The biodiversity gain condition applies to most planning permissions in England, but householder development, small habitat losses and qualifying self-build schemes sit outside it. The evidence for a claim is the part people underestimate.
Biodiversity net gain exemptions matter more than the ten per cent figure for anyone building at a small scale, because an exempt scheme skips the metric, the gain plan and the thirty-year commitment entirely. The regime sits in Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, and it works by deeming a biodiversity gain condition onto planning permissions in England. Exemptions are made by regulations, and they are specific.
Four routes cover almost every small case: householder development, the de minimis habitat test, self-build and custom housebuilding, and development on a biodiversity gain site. The wording of each one repays reading, because the label rarely matches what people assume.
The householder development exemption
Householder development is exempt. That means an application for development within the curtilage of a dwellinghouse, of the kind that goes in on a householder application form: extensions, loft conversions with external alterations, outbuildings, garden rooms, driveways and boundary works.
The exemption falls away where the application is not a householder application. A change of use, a conversion of a house into flats, a new dwelling in a garden or a scheme that includes land outside the residential curtilage all leave the householder category, and the gain condition applies. So does anything requiring a full application because of its scale or because it is not incidental to the enjoyment of the dwelling.
Nothing in the exemption affects other ecological duties. Bats in a roof space, nesting birds in a hedge and protected species on site are controlled by the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017 regardless of whether net gain applies, and a Preliminary Ecological Appraisal is still the sensible first step on any site with vegetation or an older roof.
The de minimis exemption and how area is measured
This is the one that gets misapplied. The test looks at the area of on-site habitat the development affects, measured against the threshold in the regulations, together with a separate length threshold for linear habitat such as hedgerow. It also excludes any development affecting a priority habitat, at any area.
Three consequences follow. Site area is irrelevant, so a large yard with a small footprint of works can pass while a small garden plot with a wildflower lawn can fail. Sealed hardstanding counts as developed land in the metric and so barely registers, which is why infill on a concrete apron is often exempt. And colonising vegetation is habitat, so a disused yard that has been left for five years may have crossed the line without anyone noticing.
To claim it credibly you need a plan showing habitat parcels with areas, a note confirming no priority habitat is present, and a statement of the habitat area affected by the works. That is a short piece of ecological work rather than a full metric run.
The self-build and custom build exemption
Narrower than it sounds. The development has to consist of self-build or custom housebuilding, the site has to be within the maximum area stated in the regulations, and the number of dwellings has to be within the stated limit. All three conditions apply together.
Two failure patterns recur. A developer builds four houses and sells them on, describing the scheme as custom build because purchasers choose kitchens. And a genuine self-builder acquires a plot that forms part of a larger site being brought forward in phases, where the site area test bites. Where either is arguable, get the exemption confirmed before the application is determined rather than after work starts.
Biodiversity gain sites and other exclusions
Development on a registered biodiversity gain site is exempt, which stops the regime eating its own supply. Urgent Crown development and certain high speed rail works are also outside it, and development that does not require planning permission at all, including permitted development, is outside by definition, though prior approval schemes should be checked individually.
Nationally significant infrastructure sits under a separate consenting regime with its own arrangements, so do not reason from the town and country planning position. And the position for retrospective applications and for applications varying an earlier permission depends on the date and nature of the original consent, which is a technical question worth asking early. The planning law timeline is the place to follow how the regime is being adjusted.
Evidence needed to rely on an exemption
Councils validate exemption claims, and the better ones ask for evidence at validation. What to supply:
| Exemption claimed | Evidence that usually satisfies an officer |
|---|---|
| Householder | Application type and a site plan showing the curtilage |
| De minimis | Habitat plan with areas, confirmation no priority habitat is present, area affected stated |
| Self-build or custom build | Evidence of the self-build arrangement, site area and unit numbers |
| Biodiversity gain site | The register entry reference |
An ecologist's short letter costs far less than a metric run and settles the question. A statement on the form that "the site contains no habitat" does not, because bare ground, amenity grassland and scrub are all habitat with values in the metric.
Illustrative example: a barn conversion to a single dwelling on a farmyard. The barn footprint is sealed, the access track is compacted hardcore, and the works touch a two-metre strip of species-poor grass along one elevation plus four metres of a mixed native hedge for visibility. The hedge length is the sensitive figure, and on those facts the scheme may sit just inside or just outside the linear threshold depending on how the splay is drawn. Redesigning the access to save the hedge costs less than a metric, a gain plan and a thirty-year management commitment. Illustrative only, and the thresholds should be checked against the current regulations.
Where an exemption is wrongly claimed
The condition is deemed by statute, not granted by the officer. If the scheme was never exempt, the condition was attached to the permission from the day it was issued, and development that starts without an approved biodiversity gain plan breaches it.
The practical consequences: a breach of condition notice can be served, with no appeal against it, the development is unlawful in the meantime, and a purchaser's solicitor will find the problem at the worst possible moment. Resolution means preparing the baseline retrospectively, which is harder and more expensive once the habitat has been cleared, because the metric then has to work from whatever evidence survives.
Checking the current list before you rely on it
The exemptions have been amended since the regime began in 2024, and further adjustments have been consulted on. Treat any list, including this one, as a prompt to check the current regulations and the gov.uk guidance rather than as the answer. Small sites carry their own procedural differences too, and the small sites rules that differ covers those.
Where the exemption does not apply, the work is ordinary: baseline survey, metric, design response, gain plan. A Biodiversity Net Gain Assessment draft structures the reporting and flags the site data it cannot supply, and the statutory metric expects a competent person, so the draft is the starting point for that person rather than a substitute. The wider evidence base for a small scheme is set out across all 66 report types.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
Is a house extension exempt from biodiversity net gain?
Householder development is exempt, which covers extensions, outbuildings, hard surfacing and similar works within the curtilage of a dwelling where the application is a householder application. Converting a house into flats is not householder development, so that exemption does not apply even where the building footprint does not change.
How is the de minimis threshold measured?
By the area of on-site habitat the development affects, not by the size of the site or the floorspace. The test turns on the development affecting less than the stated area of on-site habitat and less than the stated length of linear habitat such as hedgerow, and on no priority habitat being affected at all.
Does a hard-surfaced yard count as habitat?
Sealed surfaces are recorded in the metric as developed land with minimal value, which is why schemes entirely on hardstanding often fall within the de minimis test. Gravel, bare ground and self-seeded vegetation are not sealed surfaces, and a yard with colonising scrub around its edges can exceed the area threshold.
What does the self-build exemption require?
It is narrower than the phrase suggests. The development must consist of self-build or custom housebuilding, the site must be no larger than the stated maximum, and the number of dwellings must be within the stated limit. A developer-led scheme marketed to individual purchasers does not qualify merely because buyers choose finishes.
What happens if an exemption is claimed wrongly?
The gain condition applies by operation of law, so if the development was never exempt the condition was on the permission all along. Starting work without an approved biodiversity gain plan is then a breach of condition, which is enforceable and can make the development unlawful until it is resolved.
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.