The de minimis BNG exemption and how it is measured

The de minimis exemption from biodiversity net gain applies where development impacts less than 25 m² of on-site habitat and less than 5 metres of on-site linear habitat such as a hedgerow. That is the whole of the de minimis exemption from the biodiversity gain condition, and the two limits are cumulative rather than alternative. Twenty-four square metres of lawn plus six metres of hedge is not exempt. The measurement is of habitat impacted, not habitat lost, and that distinction decides a surprising number of small schemes.

What the threshold covers

The exemption sits in the 2024 exemptions regulations made under the Environment Act 2021 regime, alongside the householder exemption, the self-build exemption and a handful of others. It removes the deemed biodiversity gain condition, which would otherwise prevent development beginning until a biodiversity gain plan had been approved.

Two points about its scope.

It applies to habitat with a biodiversity value greater than zero. In the statutory metric, sealed surfaces such as buildings and intact hardstanding score zero distinctiveness and so have no biodiversity value, which means their loss does not count towards the 25 m². A development entirely on a concrete yard may therefore have no on-site habitat impact at all, and the exemption applies for that reason rather than because of the threshold.

And it is an exemption from one procedural requirement. Protected species law, tree protection, local plan biodiversity policy and the Framework's protection of irreplaceable habitats all continue to apply, so a small scheme next to a pond or under a mature oak still needs ecological thought.

Householder development is separately exempt, so the de minimis exemption matters most for the schemes in between: a single infill plot, a modest commercial unit, a change of use with a small amount of external work, a new access, or an annexe that is not householder development because it creates a separate dwelling.

Measuring on-site habitat impact

The measurement is taken from a habitat plan, drawn to the UK Habitat Classification and dimensioned. Councils increasingly ask to see it rather than accepting a statement.

Three measurement rules matter more than the rest.

The area measured is the habitat affected by the development, which includes anything outside the building footprint that the development touches. Site compounds, material storage, scaffold zones, temporary access, root protection area surfacing and spoil heaps all sit on habitat and all count. A 20 m² extension with a 30 m² compound on the lawn beside it is over the threshold.

Habitat that is retained but degraded counts as impacted. Where a hard surface is laid over the rooting zone of a shrub bed, or where grassland is compacted by plant movement and then reseeded, the metric records a change and the impact is real.

The parcel is measured as habitat, not as ownership. A lawn running across the whole garden is one parcel of modified grassland, and only the part affected is measured, so the plan needs the affected area hatched and dimensioned rather than the whole garden coloured in.

A preliminary ecological appraisal is the usual source for the habitat plan on anything other than the simplest site, and it carries the survey date, the classification and the condition assessment that make the measurement defensible.

Hedgerows measured in metres

Linear habitat is measured by length, and the limit is below 5 metres. That is short. A single field gate widening, a new vehicle crossover, or a visibility splay requiring a hedge to be set back will usually exceed it.

Watercourses are also linear habitat in the metric and are measured in the same way, so a culverted crossing of a ditch is a linear impact even where the area taken is tiny.

The separate protection under the Hedgerows Regulations runs alongside this and is not displaced by any biodiversity exemption. A hedge that qualifies as important under those regulations needs notification before removal regardless of the metric, and the relationship between the two regimes is set out in hedgerow units and the Hedgerows Regulations overlap.

Why hardstanding is not automatically outside it

Intact sealed surfaces score zero, so their loss is outside the threshold. The trap is that not all hard surfaces are intact or sealed.

Cracked concrete colonised by vegetation, gravel with ruderal plants established in it, a long-disused yard with scrub coming through, and bare compacted ground with sparse vegetation are all classified differently from a sound sealed surface, and several of those categories carry a biodiversity value above zero. A site that looks like a derelict car park may hold a surprising area of habitat once it is classified properly.

Open mosaic habitat on previously developed land is the case to watch. It is a priority habitat, it occurs on brownfield sites, and it has high distinctiveness in the metric. A scheme on a long-abandoned industrial site is one of the least likely candidates for the de minimis exemption, which is the opposite of most people's intuition.

The practical answer is to have the surfaces classified rather than assumed. An afternoon of survey on a brownfield plot is low-cost insurance against discovering a priority habitat after permission.

Evidence to support the claim

Councils query bare assertions, and the evidence needed is modest.

1. A site plan with the red line and the development footprint.
2. A habitat plan identifying each parcel by UK Habitat Classification type, with the affected areas hatched and dimensioned.
3. A measured schedule: area in square metres for each affected parcel, length in metres for each affected linear feature, with the totals shown against the thresholds.
4. Dated photographs of the surfaces and features in question.
5. A short statement explaining the construction arrangements, specifically where the compound, storage and access will go.
6. The survey date and who carried out the classification.

Item five is the one that wins the argument. An applicant who shows the compound on existing hardstanding rather than on the lawn has both reduced the impact and shown the council that the figure is real.

Where the figures are close to the thresholds, record the pre-development biodiversity value anyway. If the authority disagrees with the exemption, the condition applies and a gain plan is needed, and having the baseline already assessed saves a season.

Where landscaping pushes you over

The irony of this exemption is that good landscaping can defeat it.

Creating a wildflower meadow where there was lawn, planting a new hedge along a boundary or digging a pond are all changes to on-site habitat, and the metric records changes rather than improvements alone. Where the proposal involves more than a token amount of soft landscaping, the area of habitat affected is likely to exceed 25 m² even if the building footprint sits entirely on hardstanding.

That is not an argument for doing less. It is an argument for knowing which side of the line you are on before the application is submitted, because a scheme that will exceed the threshold is better off running the metric properly and claiming the gain than claiming an exemption the council rejects.

Small measures that improve a site without triggering a measurement are worth knowing: nest and bat boxes on buildings, hedgehog gaps in fences, and native planting in existing beds. The way councils condition those measures is covered in hedgehog gaps and connectivity conditions.

Recording the position on the application

The application form carries a biodiversity net gain section in which the applicant states whether they believe the gain condition applies and, if not, which exemption they rely on. That statement is a belief, not a determination.

Three things to do with it.

Name the exemption precisely and give the figures. "De minimis: 18 m² of modified grassland and 2.5 m of species-poor hedgerow affected" is a statement an officer can check. "Below the threshold" is not.

Ask for the decision notice to record the exemption and the basis for it, because that is the document a purchaser, a lender or a building control body will look at later.

And re-check the position if the scheme changes. A section 73 application produces a new permission, and a minor amendment that extends the footprint onto grassland can take a previously exempt scheme over the line. Where that happens, a biodiversity net gain assessment draft gives the structure, and a competent person must complete the statutory metric.

See which ecology documents a small site needs by browsing the report categories and starting a draft.

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.