Biodiversity units explained

A biodiversity unit is the output of the statutory biodiversity metric: a number expressing the value of a habitat parcel, calculated from its size, its distinctiveness, its condition and its strategic significance. Units are what the mandatory 10 per cent gain is measured in, under the biodiversity gain condition introduced by Schedule 7A to the Town and Country Planning Act 1990. They are not a measure of area, they cannot be compared across habitat types, and two sites of identical size can differ by an order of magnitude in the units they hold.

That last point is what surprises developers who costed biodiversity net gain by the hectare.

What a unit measures

For area habitats, the calculation multiplies the parcel size in hectares by a score for distinctiveness, a score for condition and a multiplier for strategic significance. The result is the habitat's baseline value in biodiversity units.

Two parallel calculations exist for linear features. Hedgerows are measured in hedgerow units based on length rather than area, and watercourses in watercourse units. A site with all three has three separate baselines and three separate gain requirements.

The metric itself is a published spreadsheet with a user guide, and the version in force at the time of the application is the one that matters. It is updated, so check which version the authority expects before the baseline survey is commissioned.

Distinctiveness, condition and strategic significance

Distinctiveness reflects how valuable the habitat type is, with bands running from very low through to very high, each carrying a published score. Modified grassland scores low. Lowland meadow scores high. The band is assigned by the habitat classification recorded in the survey, which is why the accuracy of the survey drives everything downstream.

Condition is assessed against published criteria for each habitat type, typically as poor, moderate or good, with a score attached. A poor condition example of a high distinctiveness habitat can be worth fewer units than a good condition example of a medium one.

Strategic significance applies a multiplier where the parcel lies in a location identified in a relevant strategy, such as a local nature recovery strategy. It rewards habitat in the places where habitat has been identified as needed rather than wherever it happens to be convenient.

Difficulty and time to target condition

Created and enhanced habitats are discounted in two ways, and this is where the arithmetic turns against optimistic schemes.

Difficulty of creation or enhancement applies a risk multiplier reflecting how reliably that habitat can be produced. Some habitats are straightforward to create. Others, particularly species rich grassland on the wrong substrate, are not.

Time to target condition applies a further discount for every year the habitat takes to reach the condition claimed. A habitat that will take 30 years to mature delivers far fewer units today than one reaching target condition in five, even though both end up in the same place.

The practical effect is consistent: replacing a habitat like for like never breaks even. Losing units is easy and regaining them is slow, which is why retaining existing habitat is almost always cheaper than recreating it.

Why units are not interchangeable with area

A hectare of arable land and a hectare of lowland heath are the same area and nothing like the same value. Scheme budgets built on a rate per hectare collapse on contact with a baseline survey that finds a higher distinctiveness habitat than assumed.

The same applies across the three modules. Hedgerow units cannot be used to compensate for lost area habitat units, and watercourse units are their own category again. Removing 400 m of hedgerow creates a hedgerow unit deficit that planting a wildflower meadow does not fix.

Get the baseline survey done in the survey season, before the layout is fixed. A baseline taken in February on grassland that turns out to be species rich in June is not a baseline, and habitat classification in winter is unreliable.

Trading rules between habitat types

The metric enforces trading rules that stop losses of valuable habitat being offset with low value replacements. In outline, habitat lost must be compensated by habitat of the same broad type and the same or higher distinctiveness. The higher the distinctiveness of what is lost, the narrower the range of acceptable compensation.

Irreplaceable habitats sit outside the system entirely. Ancient woodland, blanket bog and other listed irreplaceable habitats cannot be traded through the metric, and any loss requires bespoke compensation arrangements agreed with the authority, in addition to the policy tests that make such losses difficult to justify.

Onsite, offsite and statutory credits

The regime works through a hierarchy. Deliver the gain on site where possible. Where the site cannot accommodate it, buy units from an off-site gain site registered on the biodiversity gain sites register, with a spatial risk multiplier discounting units delivered further away. Where neither is possible, buy statutory biodiversity credits from the government, priced on a published schedule that is revised, so check the current figures.

Credits are deliberately priced to be the least attractive option. They are the backstop, not the plan.

Off-site units are a market, and the availability of the right habitat type in the right area varies enormously. A scheme needing high distinctiveness units in a particular catchment may find very few sellers, which is a procurement risk worth testing early rather than at the gain plan stage.

Reading a metric output

The output is a spreadsheet, and there are four numbers to look for. The baseline units by module. The post-development units. The percentage change. And the units of shortfall, if any.

Then check the assumptions behind them: which condition was assigned to each baseline parcel, what target condition is claimed for created habitat, how long it is assumed to take, and whether the strategic significance multiplier has been applied consistently.

An illustrative example: a 2 hectare site recorded as modified grassland in a winter walkover, scoring modestly. A June survey finds a third of it meets the criteria for a higher distinctiveness grassland in moderate condition. The baseline jumps, the 10 per cent gain requirement jumps with it, and the layout no longer accommodates the compensation. Nothing changed on the ground. The survey date did.

The statutory metric expects completion by a competent person under the Environment Act 2021 regime, and the biodiversity gain plan must be approved by the local planning authority before development begins. An AI-assisted draft is a starting point for that professional rather than a substitute for the survey and the calculation. Where a European site is also in play, the separate process in habitats regulations assessment applies alongside.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Browse the report types on PlanScope to see which ecology documents a scheme needs, or enter a postcode on the home page to start. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.