A worked biodiversity net gain calculation on a small site

A small residential plot rarely gives you neat numbers. The statutory biodiversity metric wants each habitat parcel measured in hectares, put in a distinctiveness band and given a condition score, and the arithmetic that follows decides whether your scheme clears the 10 per cent requirement or lands a fraction short. This is an illustrative biodiversity net gain small site worked example for a plot of roughly 0.2 hectares with a pair of new dwellings on it, using the published small sites metric method. The numbers below are illustrative. They are here to show where the units come from, not to be copied into your own gain plan.

Where the 10 per cent obligation comes from

Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, attaches a biodiversity gain condition to most planning permissions in England. Post-development biodiversity value has to exceed the pre-development value of the onsite habitat by at least 10 per cent, and the gain must be secured for at least 30 years. The regime applied to major development from 12 February 2024 and to small sites from 2 April 2024.

Several exemptions matter at this scale. Development that affects less than 25 m2 of onsite habitat and less than 5 m of linear habitat falls under the de minimis threshold. Householder applications are out. So is self-build and custom housebuilding of no more than nine dwellings on a site no larger than 0.5 hectares, provided the other conditions in the regulations are met. If you are relying on an exemption, say which one and why in the application, because a validation officer will look for it. The wider mechanics are set out in our explainer on the statutory framework behind mandatory net gain.

Drawing the baseline parcels

Start with a plan, not a spreadsheet. Walk the site, mark every distinct habitat and measure it. Our illustrative plot divides into three parcels: 0.11 hectares of modified grassland (a mown lawn and a rough back corner), 0.06 hectares of developed land and sealed surface (the existing bungalow footprint, a concrete drive and a patio), and 0.03 hectares of a second modified grassland parcel in noticeably better condition at the rear boundary. A 22 m native hedgerow runs along one side.

Two things trip people up here. Hedgerows, watercourses and area habitats are scored in separate modules of the metric, and units cannot be traded between them. A 10 per cent gain in area units does nothing for a hedgerow shortfall. And the baseline is the habitat as it exists, unless the land was cleared after 30 January 2020 without permission, in which case the metric asks you to assess the habitat as it was before clearance.

Condition assessment and who can do it

Each area parcel needs a condition assessment against the criteria in the metric's own habitat condition sheets. Modified grassland, for instance, is assessed on sward height variation, the proportion of wildflower cover, bare ground and the presence of undesirable species. Get the condition wrong and every number downstream is wrong.

The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and the biodiversity gain plan has to be approved by the local planning authority. For most small sites that means an ecologist who can defend the condition calls. A Biodiversity Net Gain Assessment drafted from site information is a starting point for that professional, not a replacement for the site visit.

An illustrative baseline figure

Area habitat units in the metric are the product of four things: area in hectares, the distinctiveness score of the habitat type, the condition score and the strategic significance multiplier. Distinctiveness runs in bands from very low through low, medium and high to very high, scored 0, 2, 4, 6 and 8. Condition runs from poor at 1 through moderate at 2 to good at 3, with half steps between.

That gives a baseline of 0.34 area habitat units, assuming strategic significance of 1.0 because neither parcel sits in an area identified in the local nature recovery strategy. The 10 per cent target is therefore 0.374 units. Small numbers, and that is the point: on a plot this size, a single parcel moved one condition step changes the answer.

The hedgerow sits in its own column and needs its own 10 per cent uplift, calculated from length in kilometres rather than area.

The post-development side of the sheet

Proposed habitats are scored the same way, then discounted twice. The metric applies a time-to-target-condition multiplier, because a created meadow is not a meadow on the day of handover, and a difficulty multiplier reflecting how reliably that habitat can be created or enhanced. Both are built into the published spreadsheet. Do not attempt them by hand, and do not quote a multiplier from an older metric version.

Trading rules bite hardest on small sites. Habitat of medium distinctiveness or above must be replaced with the same habitat type, or something of higher distinctiveness, at the same or better condition. Losing a patch of priority habitat and offering amenity grassland in exchange does not work, whatever the unit total says. Our illustrative scheme keeps the better grassland parcel, converts 0.04 hectares of former sealed surface into a species-rich lawn managed on a two-cut regime, plants 14 m of new native hedgerow and retains the existing hedge with a 1 m protected root zone. Retained habitat carries forward at its existing value, so retention buys units at the lowest cost on the sheet.

When onsite gain falls short

Most small urban plots cannot reach the target on site, because the developable footprint eats the only habitat there is. The statutory hierarchy then applies: avoid losses first, then deliver gains on site, then off site, and only then buy statutory biodiversity credits from the government scheme as a last resort. Credits are priced to be unattractive by design and the price schedule is published, so check the current figures before you build them into an appraisal.

Off-site units bring a spatial risk multiplier that reduces their value the further they sit from the development, and they need registering on the biodiversity gain sites register with a 30-year legal agreement behind them. The practical consequences of that choice are set out in our note on trading down and the off-site sections of a gain plan. Budget the time as well as the money. Finding a habitat bank with the right habitat type in the right catchment has delayed more than a few small schemes.

Recording the result

The metric output goes into the application as evidence, usually alongside a short ecology statement explaining the condition calls and the management regime. But the biodiversity gain plan itself is a post-permission document, submitted for approval before development can lawfully begin, and it has to show the pre-development value, the post-development value, the percentage change and how the gain will be maintained. Our guide to the gain plan as a post-permission document sets out what goes in it.

Keep the metric file, the condition sheets, the habitat plan and the photographs together from day one. When a case officer queries a condition score eight months later, the site as it was has usually gone under a slab.

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 metric must be completed by a competent person.