The statutory biodiversity metric and the small sites version

The statutory biodiversity metric small sites version is a simplified calculator, not a lower standard. A self-builder using it still has to deliver a 10 per cent gain, still has to secure it for 30 years, and still has to have the calculation completed by a competent person. What the small sites metric removes is the need to handle high distinctiveness habitats, off-site units and the full condition assessment framework, and the price of that simplification is a tight set of eligibility conditions that disqualify a surprising number of sites.

Get the eligibility question wrong and the gain plan is refused, which on a small site usually means months lost.

What the metric measures, and what it does not

The metric converts habitat into biodiversity units. Three separate accounts, each with its own 10 per cent requirement:

The accounts do not trade. A surplus of area units cannot make up a shortfall in hedgerow units, which is the single most common misunderstanding on small sites, because it means removing 40 metres of hedge has to be fixed with hedge rather than with a wildflower lawn.

For an area habitat the baseline score is habitat area multiplied by distinctiveness, condition and strategic significance. Post-development habitats carry two further multipliers, for the time it takes to reach target condition and for the difficulty of creating or enhancing that habitat, and off-site units carry a spatial risk multiplier that reduces their value the further they are from the development.

What the metric does not measure is also worth naming. It is a proxy for habitat value, not a survey of species. It says nothing about protected species, which remain governed by the Wildlife and Countryside Act 1981 and the habitats regulations and need their own survey work, and nothing about irreplaceable habitats, which sit outside the metric entirely as covered in irreplaceable habitats and the biodiversity gain exception.

Eligibility rules for the small sites version

Two sets of conditions, and a site has to clear both.

Size. Residential development of between one and nine dwellings on a site of one hectare or less, or, where the number of dwellings is not known, a site of less than 0.5 hectares. Non-residential development with a floorspace of less than 1,000 square metres or a site area of less than one hectare. Minerals and waste development is excluded, and nationally significant infrastructure projects cannot use it either.

Content of the site. The small sites metric cannot be used where the site includes:

1. Priority habitat, other than the specific hedgerow and arable field margin types the tool covers.
2. A statutory protected site, or anything triggering an assessment of effects on a European site.
3. European protected species.
4. Ancient or veteran trees.
5. Any habitat type the tool does not list, including riparian zones.
6. Any habitat of high or very high distinctiveness, since the tool only handles very low, low and medium distinctiveness.

Off-site units cannot be calculated within the small sites metric either, so a site that cannot achieve its gain on site has to move to the full metric. And you cannot mix the two tools for the on-site calculation: one development, one tool.

Check for protected sites and priority habitat within 500 metres of the boundary as part of that screening. Finding one does not automatically disqualify the site, but it is the point at which ecological advice earns its fee rather than being optional.

Habitat condition assessment and who may do it

Condition is assessed against published criteria sheets, with each habitat scored good, moderate, poor or, for some types, a fixed condition. The scoring is rule-based rather than a matter of impression, and the sheets are specific: a grassland failing a stated number of criteria drops a condition band.

Two practical constraints. The baseline has to be surveyed on the ground, not derived from aerial imagery, and the survey has to happen when the habitat can be identified. Botanical identification on grassland is realistic between April and September, and a February walkover will not support a defensible condition score for a species-rich sward.

On who may complete it, the position is narrower than it sounds. The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and the biodiversity gain plan to be approved by the local planning authority before development begins. For the small sites metric specifically, government guidance states that an ecologist is not required, so a competent person who is not an ecologist may complete it. Competence still has to be real, and on any site where the habitat identification is arguable, a Preliminary Ecological Appraisal by an ecologist is the sensible first step. A Biodiversity Net Gain Assessment drafted with PlanScope sets out the structure, the policy framing and the evidence the gain plan needs, and the metric itself remains the competent person's work. For specialist technical reports, qualified professional input may be required.

Strategic significance and the spatial risk multiplier

Strategic significance asks whether the habitat sits somewhere the area's own ecological strategy identifies as a priority. High significance applies where a habitat lies within an area identified in a published strategy such as a Local Nature Recovery Strategy or a relevant local plan policy. Otherwise the multiplier sits at the lower end.

Two things follow for a small site. First, check whether a strategy covers your site before assuming the lower figure, because a site within a priority area carries a higher baseline score and therefore a higher gain requirement in absolute units. Second, the same multiplier applies to the post-development habitat, so creating habitat in a priority location is worth more than creating the same habitat elsewhere.

Spatial risk only bites off site. Units delivered within the same local planning authority or national character area count at full value, with reductions applied as the delivery site moves further away. On a small site that mainly matters as a reason to design the gain in rather than buy it off site, since the reduction makes off-site delivery cost more units than the shortfall suggests.

Trading rules and the habitat hierarchy

The trading rules stop a developer replacing a valuable habitat with an easy one. They work from distinctiveness.

Very high distinctiveness habitats require bespoke compensation, meaning the same habitat type in the same or better condition, and they take the site outside the small sites metric in any case. High distinctiveness requires the same broad habitat type at the same or better distinctiveness. Medium distinctiveness requires the same broad habitat group. Low and very low distinctiveness can be replaced by habitat of the same or higher distinctiveness.

For a typical self-build plot the practical consequence is narrow and useful. Modified grassland and amenity grassland are low distinctiveness, so they can be replaced with almost anything better. A species-rich hedgerow is not, and it has to be replaced with hedgerow.

Where small site users most often make errors

The recurring ones, in the order they cause trouble:

• Surveying the baseline in winter and recording a condition score the habitat cannot be shown to hold.
• Mislabelling the baseline, usually calling a species-rich sward modified grassland, which inflates the apparent gain and gets picked up by the council's ecologist.
• Forgetting that hedgerow units need their own 10 per cent, separately from the area account.
• Treating the landscaping plan as the gain without condition targets, a management regime or a monitoring commitment attached to it.
• Missing the 30-year obligation, which is a legal commitment secured by condition, planning obligation or conservation covenant, not a statement of intent.
• Assuming an exemption applies. The de minimis exemption covers development affecting less than 25 square metres of on-site habitat and less than five metres of on-site linear habitat. Householder development is exempt, and so is qualifying self-build and custom build of up to nine dwellings on a site of no more than 0.5 hectares. Those are narrow, and a plot with a hedge along one boundary usually fails the de minimis test on the linear figure alone.

The gain plan itself has to be approved before development begins, because the biodiversity gain condition is deemed to be attached to almost every permission, as set out in the biodiversity gain condition on every permission.

Keeping the metric version consistent

Defra publishes updated versions of both tools, with user guides that change. Use the version current when you submit, record the version number and publication date in the submission, and use the same version through to the gain plan.

Where a new version is published mid-application, check the guidance for transitional arrangements rather than switching tools, and tell the case officer which version you used. A recalculation in a different version produces different unit figures for the same site, and an unexplained change between the application and the gain plan reads as an attempt to improve the numbers.

Keep the completed calculation file, the condition assessment sheets and the dated survey notes together. Those three documents are what a council's ecologist asks for, and assembling them two years later from memory is not realistic.

This is general guidance, not legal advice; check the current metric version and guidance on gov.uk and confirm 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.