Biodiversity net gain or a preliminary ecological appraisal

These two pieces of ecology work answer different questions, and on a small site you may need one, both or neither. A preliminary ecological appraisal asks what lives here and what the law says about it. Biodiversity net gain asks what habitat is here, in what condition, and how much of it you have to put back with ten per cent on top. Comparing biodiversity net gain against an ecological appraisal as alternatives is the mistake, because the appraisal is about protected species and designated sites while the metric is about habitat area and condition.

Two different questions about the same site

A preliminary ecological appraisal is the standard first step in ecology for a planning application. It combines a desk study of records and designations with a walkover habitat survey, identifies which protected species could reasonably be present and recommends what further survey work the authority will expect. Its conclusions drive your programme, because a recommendation for bat emergence surveys sets your submission date.

The biodiversity gain condition works from different inputs. Schedule 7A to the Town and Country Planning Act 1990 requires most planning permissions in England to deliver at least a ten per cent gain in biodiversity value, measured with the statutory metric and maintained for thirty years. The metric does not care whether a badger walks across the site. It cares what habitat types are present, how large each parcel is, what condition each parcel is in and where it sits against local strategy.

So the overlap is the habitat survey. A single site visit at the right time of year can collect the data for both, and the surveyor should be told at the outset that the output needs to feed the metric as well as the appraisal. Commissioning them separately means paying twice for the same walkover.

What the statutory metric needs as an input

The metric runs on four things for every habitat parcel: the habitat type classified to the UK Habitat Classification, the area in hectares or the length in kilometres for linear features, the condition assessed against the habitat-specific criteria, and the strategic significance of the location. Get any one of them wrong and the baseline is wrong, which usually means the gain requirement is wrong too.

Area sounds trivial and is not. The metric measures actual habitat extent, so a garden is not one parcel but a mosaic of lawn, border, hardstanding and possibly a tree with a canopy area of its own. Hedgerows and watercourses are measured as lengths and have their own units, which cannot be traded against area units.

Condition is where most baselines fail scrutiny. Each habitat type has a published condition sheet with pass or fail criteria, and the assessor has to record the evidence for each one. A grassland recorded as good condition without a species list is an assertion.

Habitat condition assessment and who may do it

The regime expects the statutory metric to be completed by a competent person, and the condition criteria need botanical identification that most non-ecologists do not have. In practice that means an ecologist with current survey experience in the habitat types present, and most authorities will say so when they validate.

That competence requirement is not satisfied by software. A draft Biodiversity Net Gain Assessment prepared on PlanScope gives you the structure, the policy framing and a clear list of what is missing, and a competent person still has to carry out the condition assessment and complete the metric. For protected species work the position is firmer still, because surveys and any mitigation licence require a suitably licensed ecologist.

Where a site is simple, the assessment is short. Where it includes priority habitat, a watercourse, mature trees or anything that looks like semi-natural grassland, expect a proper survey and expect it to take a day.

Survey season and the baseline date

Habitat condition assessment needs plants to be identifiable, which puts the sensible window between April and September and the best of it between May and July. A February visit can map habitat types from structure, and it cannot assess grassland condition.

The baseline is taken as at the date of the planning application, or an earlier date where that is agreed. There is also a protection against clearing a site first: where habitat has been degraded by activity after 30 January 2020 without permission, the baseline reverts to the value before that activity. Clearing scrub in the hope of a lower baseline is a strategy that produces a higher one.

This is why the sequence matters more than the documents. Book the ecology visit before the design is fixed, inside the growing season, with a brief that covers both the appraisal and the metric. The full framework behind the duty is set out in mandatory biodiversity net gain and the statutory framework.

Small sites and the simplified route

Two things reduce the burden for small schemes, and they are often confused.

The first is exemption. Householder applications are exempt from the biodiversity gain condition. So are developments below the de minimis threshold, meaning those affecting less than 25 square metres of on-site habitat and less than five metres of hedgerow. And self-build or custom build schemes are exempt where the development is no more than nine dwellings on a site of no more than half a hectare and consists exclusively of self-build or custom housebuilding. For a single self-build plot that exemption is often the answer, and it is worth checking against the current regulations rather than assumed.

The second is the small sites metric, a simplified version of the statutory tool for smaller development. It is not available on every small site. Where priority habitat other than hedgerow or arable field margin is present, or where protected species or designated sites are involved, the full metric applies. Check the current user guide before scoping the fee on the basis of the simplified route.

Neither exemption nor the simplified metric removes the ecology work. Protected species law applies regardless of scheme size, and a Preliminary Ecological Appraisal is still the document that tells you whether you have a bat problem.

How the gain plan is approved

The gain plan is not submitted with the application. The biodiversity gain condition is deemed to be imposed on the permission, and the plan goes to the authority for approval after permission and before development starts. What accompanies the application is the metric itself, the habitat baseline and enough of the proposed habitat design to show that ten per cent is achievable.

That split catches people out. A permission can be granted while the gain plan remains unapproved, and development cannot lawfully begin until it is. Treat the plan as a pre-start consent with its own eight week determination risk, and the detail of what goes in a biodiversity gain plan becomes a programme item rather than paperwork.

Sequencing the two pieces of work

Run the desk study and the walkover as one commission, timed inside the growing season, with the brief asking for both the appraisal conclusions and metric-ready habitat data. Then design. Then complete the metric against the design, and only then price mitigation or off-site units.

Doing it the other way round, designing first and surveying afterwards, is how schemes end up with a layout that cannot reach ten per cent without buying units. Off-site units are a legitimate answer and they are the expensive answer.

One caveat worth holding on to. The exemptions and the small sites route have been amended since the regime began, so check the current regulations and guidance for your application date rather than relying on the position as it stood when the duty first took effect.

Enter your postcode on PlanScope to see which ecology and habitat reports a site is likely to need.

This is general guidance, not legal advice; check 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.