Biodiversity net gain on small sites: what applies and when

Biodiversity net gain small sites rules took effect on 2 April 2024, seven weeks after the regime started for major development on 12 February 2024. If your application was made on or after that April date and the site is not exempt, a biodiversity gain condition is deemed attached to any permission, and you cannot start work until the council has approved a biodiversity gain plan showing a 10 per cent uplift maintained for 30 years.

Self-builders and small developers get caught out in two ways. They assume small means exempt, and they leave the baseline survey until after the design is fixed.

Two commencement dates and which applies

The Environment Act 2021 inserted Schedule 7A into the Town and Country Planning Act 1990, and the regime was switched on in stages. Major development came first on 12 February 2024. Small sites followed on 2 April 2024. Nationally significant infrastructure was scheduled to follow later, and the commencement position for that should be checked on gov.uk rather than assumed.

What matters for an individual application is the date the application was made rather than the date permission was granted. Applications made before the relevant commencement date are outside the regime, which is why a few older consents are still being implemented without a gain plan.

Section 73 applications sit in a more awkward place, since the treatment depends on the date of the original permission. Check the current guidance before assuming a variation carries the original position across.

What counts as a small site

The definition is set out in the regulations and it has two limbs.

For residential development, a small site is one where between one and nine dwellings are proposed on a site of less than one hectare, or, where the number of dwellings is not known, a site of less than 0.5 hectares.

For non-residential development, a small site is one where the floorspace created is less than 1,000 m² or the site area is less than one hectare.

Being a small site does not remove the 10 per cent requirement. It changes which metric you may use and, in some cases, the reporting expected with the gain plan.

The small sites metric

Defra publishes a simplified version of the statutory biodiversity metric for small sites. It uses the same underlying approach with fewer inputs, and it is intended to be usable without the depth of habitat condition assessment the full metric requires.

Its availability is conditional. Broadly, the small sites metric can be used where the development is a small site, where it does not affect priority habitats other than the limited exceptions the guidance allows, and where it does not affect statutory designated sites or European protected species. Where any of those conditions fails, you are back to the statutory metric.

Read the current user guide before choosing. The conditions have been refined since launch, and a gain plan calculated on the wrong tool is rejected at the approval stage rather than at the application stage, which is the worst point to find out.

Either way, the statutory metric must be completed by a competent person under the Environment Act 2021 regime, and the biodiversity gain plan is approved by the local planning authority. Commission a Biodiversity Net Gain Assessment from someone with the relevant ecological competence, and treat any AI-assisted draft as a starting point for that person.

Exemptions to check before spending anything

The de minimis exemption is narrower than it sounds. Twenty-five square metres is a small patch, and the linear limit of five metres of hedgerow is easily exceeded by a single access opening. Both limbs have to be satisfied.

The self-build exemption is the one most misunderstood. It requires the scheme to consist exclusively of self-build or custom build dwellings, so a plot sold on with a house already built does not qualify, and a mixed scheme of one self-build plot and two open market houses falls outside it.

Where you think an exemption applies, say so on the application form with the reasoning. The council has to be satisfied, and an unexplained claim generates a validation query.

Getting the baseline right

The baseline is the pre-development biodiversity value of the site, and it is normally assessed at the date of the application.

The exception matters. Where activities carried out on or after 30 January 2020 reduced the biodiversity value of the land without planning permission, the baseline is taken as the value immediately before those activities. Clearing a site does not reduce the requirement, it just makes the baseline harder to prove.

Survey in the right season. Habitat classification and condition assessment need vegetation to be identifiable, which broadly means April to September for grassland and other botanical work. A December survey produces a low confidence baseline that an ecologist will caveat and a council may query.

Where the site has features that might trigger species work, run a Preliminary Ecological Appraisal at the same time. Doing both in one visit saves a fee and, more usefully, saves a second seasonal window.

The biodiversity gain plan and the condition

The condition is deemed attached rather than written into the decision notice, so it applies whether or not you can see it. Development cannot lawfully begin until the plan is approved.

The plan sets out the pre-development value, the post-development value, the steps taken to avoid and minimise habitat loss, the on-site and off-site provision, and any irreplaceable habitat information. It goes in after permission is granted and before commencement, and the authority has a period to determine it.

Two practical points. Build the layout around the habitats you intend to retain, because retained hedgerows and trees carry units that new planting takes decades to replace. And get the on-site habitat creation and enhancement drawn properly, with a habitat management and monitoring plan covering the 30 year period, since that is what the council will condition and what a future buyer will inherit.

A Landscape Strategy and, on larger small sites, a Green Infrastructure Plan turn the metric outputs into something a contractor can build and a management company can maintain.

Off-site units and statutory credits

The hierarchy runs on-site first, then off-site, then statutory credits.

Off-site units are bought from a registered biodiversity gain site, and the allocation is recorded on the biodiversity gain sites register. Prices vary by habitat type and by area, and units in the same national character area or local planning authority area are worth more in the metric than distant ones because of the spatial risk multiplier.

Statutory biodiversity credits sold by the government are the route of last resort and are deliberately priced above the market so that they are unattractive. Treat them as a backstop rather than a plan.

Whichever route you use, the 30 year commitment has to be secured legally, through a planning obligation or a conservation covenant, and the cost of that securing and of the monitoring belongs in the appraisal from the start.

This is general guidance, not legal advice. Check the current exemptions, metric versions and commencement position on gov.uk, and take advice from a qualified ecologist before relying on any of it.

To see which ecology documents a small site is likely to need, enter the postcode on PlanScope.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.