The 25 square metre biodiversity net gain exemption
Below 25 square metres of habitat and 5 metres of hedgerow, the biodiversity gain condition does not apply. The measurement is where the argument happens.
The biodiversity net gain de minimis exemption is the one most small schemes rely on, and it is tighter than people expect. It applies where development affects less than 25 square metres of on-site habitat with a biodiversity value greater than zero, and less than 5 metres of on-site linear habitat such as a hedgerow. Both limits have to be satisfied. Affect any on-site priority habitat and the exemption does not apply at all, whatever the area.
That wording is doing a lot of work, and the phrase to hold on to is "biodiversity value greater than zero". A building footprint and an existing tarmac yard are recorded in the statutory metric as sealed surface with no value, so they do not count towards the 25 square metres. Grass does.
What the de minimis exemption covers
The biodiversity gain condition comes from Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021. It is treated as a condition on most planning permissions in England, and it requires a 10 per cent gain in biodiversity value maintained for 30 years, with a biodiversity gain plan approved by the local planning authority before development begins.
The exemptions are set out in regulations made in 2024. The de minimis exemption is one of several, alongside householder development, self-build and custom build housebuilding of up to nine dwellings on a site of half a hectare or less, development on a biodiversity gain site, and a handful of others.
So the first question is not whether the site is small. It is which exemption, if any, applies on the facts, and the answer has to be stated on the application form.
How the area is measured and what counts as habitat
Measure the habitat affected, not the site area and not the building footprint. That means everything the development touches during construction and after it.
Work through it like this:
- Draw the development footprint on a plan, including the building, hardstanding, drainage runs and any regrading.
- Add the temporary works: the compound, the crane or scaffold zone, the materials laydown, the spoil heap, the access route the machines will use.
- Classify what is currently there. Sealed surfaces and buildings have no value. Lawn, amenity grassland, scrub, planted beds, bare ground with vegetation, ponds and trees all do.
- Measure the area of each valued habitat affected, in square metres, and total it.
- Measure separately the length of any linear habitat affected, in metres.
- Check whether any of the habitat is priority habitat.
Step two is where claims fall over. A garage on an existing concrete base is a nil area project until you notice that the mixer will stand on the lawn for three weeks and the spoil will go on the border. Authorities do ask about this.
Step six is the one that ends the exercise. Priority habitat on site removes the de minimis exemption completely, and priority habitats are not all rare or obvious: a species-rich native hedgerow, an area of lowland meadow, a pond meeting the priority habitat description. Where there is any doubt, a short Preliminary Ecological Appraisal settles it and costs far less than a metric exercise.
Linear habitat and the hedgerow limit
The 5 metre limit applies to linear habitats: hedgerows, lines of trees and watercourses. Five metres is short. It is roughly the width of a domestic vehicular access, which is why so many otherwise tiny schemes fail the test at the gate.
Measure the length affected rather than the length removed. Where an access is formed through a hedge, the affected length includes the gap plus any section where the hedge is cut back, where roots within the rooting zone are severed, or where the canopy is reduced. A plan with a dimensioned annotation is the evidence.
Watercourses deserve a mention because they surprise people. A ditch running along a boundary is linear habitat, and culverting a short length of it to form an access engages the same limit.
Householder development and its separate exemption
Householder development has its own exemption, and it is broader than de minimis. It covers development within the curtilage of a dwellinghouse, which takes in extensions, loft conversions with external alterations, outbuildings, garden rooms, garages, driveways and boundary works.
Two boundaries to that exemption. It does not cover a new dwelling, so subdividing a garden and building a house in it is not householder development even though the site is a garden. And it does not cover flats, since the definition turns on a dwellinghouse.
The practical effect for a homeowner is that most extension projects need no metric, no gain plan and no ecology work for net gain purposes. The application form still asks the question, so answer it by naming the householder exemption rather than leaving it blank.
Change of use and demolition cases
A change of use with no external works affects no habitat, so the de minimis exemption applies cleanly. Record the position in one sentence and move on.
Demolition is more interesting. A building is sealed surface with no biodiversity value, so demolishing it and rebuilding on the same footprint can fall within the exemption even at a substantial scale. What usually takes such a scheme outside it is the working area around the building, the hardstanding replaced by landscaping, or a boundary hedge that has to come out for access.
Illustrative example. A rear extension of 28 square metres on an existing patio and a strip of lawn. The patio contributes nothing, and the lawn area affected measures 9 square metres, with a further 12 square metres of lawn used as a mixing and storage area during the build. Total affected valued habitat is 21 square metres, no hedge is touched, and there is no priority habitat. That is inside de minimis, and in any event the householder exemption applies. The point of doing the measurement is that the same project with a 2.5 metre wide access cut through a native hedge would not be exempt on the linear test.
Evidencing the exemption on the application form
Keep it short and keep it on file:
- The exemption claimed, named on the form.
- A plan showing habitat areas and any linear habitat, with the affected extents hatched and dimensioned.
- A one paragraph note recording the measurement, the habitat classifications used and the date of the site inspection.
- Photographs of the habitat affected, dated.
- Where priority habitat was considered and ruled out, a sentence saying who looked and on what basis.
That pack is proportionate for a small scheme and it answers the only questions an authority can properly ask. Where the claim is marginal, running the statutory metric anyway gives you a fallback position, and a Biodiversity Net Gain Assessment has to be prepared by a competent person under the Environment Act 2021 regime, so factor that in before relying on a borderline exemption.
Where authorities push back
Four arguments come up repeatedly. Whether a mown lawn counts as habitat, and it does, as modified grassland with a value above zero. Whether temporary works count, and they do. Whether a hedge is priority habitat, which turns on species composition and needs someone competent to look. And whether the development is really householder development, where a garden plot subdivision is the usual dispute.
None of these is resolved by assertion. The way through is a dimensioned plan, dated photographs and a short note from someone who has looked at the habitat.
One thing the exemption never does is displace protected species law. A Protected Species Survey and, where mitigation is needed, a licence from a suitably licensed ecologist are required by the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017 regardless of net gain, and bats in a roof or nesting birds in a hedge are the commonest issues on exactly the small projects that are exempt.
PlanScope drafts the ecology narrative for a small application and flags where habitat measurements have to be inserted. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and the statutory biodiversity metric must be completed by a competent person.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Enter your postcode on PlanScope to see which of the reports a project needs apply to your site.
Frequently asked questions
What is the de minimis exemption for biodiversity net gain?
An exemption from the biodiversity gain condition for development that affects less than 25 square metres of on-site habitat with a biodiversity value greater than zero, and less than 5 metres of on-site linear habitat. Both limits have to be met, and the exemption does not apply at all where any on-site priority habitat is affected.
Does a house extension need biodiversity net gain?
No, because householder development has its own exemption under the 2024 exemption regulations. That covers extensions, outbuildings, driveways and other works within the curtilage of a house. Building a new dwelling in a garden is not householder development, so it needs a gain plan unless another exemption applies.
Does hardstanding count towards the 25 square metres?
Not where it has no biodiversity value under the statutory metric. Sealed surfaces such as buildings and existing tarmac are recorded with a value of zero, so redeveloping a yard can fall within the de minimis threshold even if it is large. Grass, scrub, planting beds and bare ground with vegetation all do count.
How is 5 metres of hedgerow measured?
As the length of linear habitat affected by the development, including where a hedge is removed for an access, thinned, or where roots and canopy are affected by works. Measure it on a plan and record the method. If the hedge is a species-rich native hedgerow it may be priority habitat, which removes the exemption regardless of length.
Does the exemption remove all ecology obligations?
No. Protected species law under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017 applies whatever the size of the works, and local plan policies on biodiversity and trees still apply. An exemption from the gain condition is narrow, and it is not permission to clear a hedge in the nesting season.
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.