Biodiversity net gain for small sites: what differs

Small sites biodiversity net gain works to the same 10 per cent requirement as a major scheme, with the same 30-year commitment, but three things differ. A simplified metric is available on qualifying sites, a set of exemptions takes many small schemes out of the regime entirely, and self-build and custom build housing has its own carve-out.

The duty comes from the Environment Act 2021, which inserted a biodiversity gain condition into the planning permission regime. It applied to major development from 12 February 2024 and to small sites from 2 April 2024.

What counts as a small site

The definition is set out in the legislation and is easy to get wrong because it has two limbs.

For residential development, a small site is one where the number of dwellings is between one and nine on a site under one hectare, or, where the number of dwellings is not known, a site of less than half a hectare. For non-residential development, it is a site where the floorspace created is under 1,000 m2 or the site area is under one hectare.

Measure the site area rather than the developable area. Applications are routinely submitted with a red line that includes an access strip or a garden, and that line is what counts.

Note what small does not mean. It does not reduce the gain requirement, it does not shorten the management period, and it does not remove the need for the authority to approve a biodiversity gain plan before development begins.

The small sites metric and when it can be used

The statutory metric is a detailed tool that expects habitat condition assessment by someone competent to carry it out. A simplified version exists for small sites, designed to be usable with less survey input.

Eligibility is restricted, and the restrictions are the point. The simplified route is not available where the site contains priority habitat, subject to limited exceptions, where protected species licensing is engaged, or where certain watercourse and habitat features are present. The current user guide sets out the criteria, and they have been refined since the regime began, so check the published version rather than a summary.

Where the simplified route is available it saves survey effort and cost. Where it is not, the full statutory metric applies and the habitat baseline has to be assessed properly, which usually means an ecological survey in the right season. When the small sites metric can be used sets out the eligibility criteria in detail. The eligibility question is worth answering before anything else, because it sets the cost of the whole exercise.

The exemptions that apply to small schemes

Several exemptions take development outside the biodiversity gain condition altogether, and three of them matter on small sites:

• householder applications, so an extension, a loft conversion or a garden building attached to an existing dwelling is outside the regime
• de minimis development, where the works affect less than 25 m2 of on-site habitat with a biodiversity value above zero and less than 5 metres of on-site linear habitat such as hedgerow
• self-build and custom build housing meeting the statutory conditions

Permitted development, including prior approvals, is also outside the regime, which is a meaningful difference between a Class Q conversion and an equivalent full application.

The de minimis threshold is the one most often misapplied. It is measured against habitat with a biodiversity value greater than zero, so a plot of hard standing and buildings can exceed 25 m2 of development and still be exempt, while a small area of species-poor grassland can take a scheme into the regime. Have the areas measured rather than estimated, and record the measurement in the application.

Self build and custom build

The exemption covers development consisting exclusively of self-build or custom build housing, of no more than nine dwellings, on a site no larger than half a hectare.

Exclusively is the word that catches people. A scheme of five self-build plots and one speculative house for sale does not qualify, and nor does a mixed scheme with a commercial element. The nine dwelling cap and the half hectare limit are both hard.

Where the exemption is claimed, the application should state it, identify which limb is relied on, and evidence it. Authorities have been asking for that evidence more consistently as the regime has bedded in, and an unevidenced claim tends to be met with a request for a biodiversity gain plan instead.

Securing gain on a constrained plot

Small sites are usually tight, and on-site gain competes with parking, drainage, amenity space and the building itself. The hierarchy still applies, so on-site enhancement comes first, then off-site units, then statutory credits as a last resort.

What works on a small plot tends to be modest and specific. Native species-rich hedgerow along a boundary, which scores in linear units and needs little footprint. Grassland created and managed as diverse meadow rather than amenity lawn, which depends on the management plan rather than the plan drawing. Retained trees, which are worth far more in the metric than new planting. Green roofs on outbuildings, where the structure allows.

What does not work is a strip of shrub planting drawn on the landscape plan and priced at nothing. The metric rewards habitat distinctiveness and condition, and condition has to be achievable and then delivered.

Where on-site gain is not possible, off-site units can be bought from a habitat bank or delivered on other land in the applicant's control. Statutory credits are deliberately priced to be the least attractive option. Price both before fixing the layout, because the comparison sometimes justifies losing a plot. A Biodiversity Net Gain Assessment should set out that comparison rather than presenting a single route.

The gain plan and when it has to be approved

The biodiversity gain condition is deemed to be attached to most planning permissions, and it prevents development from beginning until the authority has approved a biodiversity gain plan. That is a pre-commencement requirement in substance, whatever the decision notice looks like.

Two timing points follow. The plan is submitted after permission is granted, not with the application, though the metric work has to be done beforehand because the application itself needs to show the scheme can deliver the gain. And the approval takes time, so a self-builder planning to start on site the month after permission should submit the gain plan immediately and allow for the authority's determination period.

The baseline date also matters. The habitat baseline is normally taken at the date of the application, with earlier dates applying in specified circumstances, and there are rules addressing habitat that was degraded before the application. Clearing a site in the hope of reducing the baseline does not work and creates a worse position than leaving it alone.

Keep the metric file. Authorities ask for the calculation file rather than a summary, and a small site application that supplies a two-page note instead usually receives a request for further information.

The evidence the authority will check

A small site application needs the metric calculation file, a baseline habitat plan, a post-development habitat plan, a habitat management and monitoring plan proportionate to the gain proposed, and the biodiversity gain plan itself, which the authority must approve before development starts.

Three things get checked hardest. Whether the baseline was assessed at the right date and in a condition-assessable season, whether the proposed habitats are achievable given the layout and the drainage design, and whether the management arrangements are secured for 30 years.

The statutory metric expects a competent person, and the Environment Act regime requires the biodiversity gain plan to be approved by the local planning authority. Treat the metric as work for an ecologist rather than as a spreadsheet exercise.

The regime is still settling, with guidance updated periodically and further changes signalled. Follow developments on the planning law timeline, and check the current guidance on the day you prepare the application. Browse all 66 report types to see how the ecology documents fit with the rest of a small site submission.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.