Biodiversity metric work or an ecological appraisal

Whether you need biodiversity metric work or an ecological appraisal is usually the wrong question, because most development sites need both and they answer different things. The statutory metric converts habitats into biodiversity units so the 10 per cent gain required by the Environment Act 2021 can be calculated. An ecological appraisal tells you what lives on the site, what is legally protected, what the designated sites nearby will object to, and which further surveys the season will force on your programme.

Commission one and assume it covers the other and you will find out at validation, or worse, at the discharge of the biodiversity gain condition.

What the statutory metric measures

The metric is an accounting tool. It takes the habitats present on the baseline site, classifies them, scores each parcel for distinctiveness, condition, strategic significance and area, and produces a figure in biodiversity units. It then does the same for the proposed post-development habitats, applying multipliers for difficulty of creation, time to target condition and any off-site delivery, and reports whether the scheme achieves the required uplift.

Three currencies run in parallel: area habitats, hedgerows and other linear features, and watercourses. Each has to balance on its own. A scheme that shows a large area surplus and a hedgerow deficit does not pass.

What the metric does not do is tell you anything about species. A bare arable field with a bat roost in the barn scores poorly on habitat and carries a significant legal constraint, and only one of those facts appears in the calculation.

What an ecological appraisal assesses

A Preliminary Ecological Appraisal combines a desk study of records and designations with a walkover by an ecologist. It maps habitats, identifies potential for protected species, checks distances to statutory and local designated sites, and recommends the further survey work needed.

That output drives the parts of the programme that cannot be compressed. A barn with bat potential means emergence surveys in the season. A pond within likely commuting distance means great crested newt work, whether by environmental DNA or by a district level licensing route. A hedgerow with a badger path through it means a sett survey. None of that appears in a metric calculation, and all of it can stop a scheme.

The appraisal is also where the legal exposure sits. Protected species work engages the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017, and any survey or mitigation licence requires a suitably licensed ecologist. PlanScope's draft is a starting point for that professional, never a substitute.

Habitat condition assessment and who can do it

The metric's condition assessment is the part that most often goes wrong. Each habitat type has criteria to be scored in the field, and moving a parcel from poor to moderate condition can change the unit figure materially, in either direction.

Under the Environment Act 2021 regime the statutory metric is expected to be completed by a competent person, and the biodiversity gain plan is approved by the local planning authority. A Biodiversity Net Gain Assessment is one of the report types where a competent person is legally required rather than merely advisable. Councils increasingly ask for the assessor's name, qualifications and the date of the field visit alongside the calculation file.

The practical test is whether the person scoring condition can defend the score. That means a botanist able to identify the indicator species the criteria list, in a season when those species are identifiable.

Where the two datasets have to agree

Officers and their ecology advisers cross-check, and mismatches are the most common reason a submission goes back.

• The habitat parcels in the metric must match the habitat plan in the appraisal, in extent and in classification.
• The areas in the metric must match the areas on the landscape drawing, not an earlier version of it.
• Habitat retained on the metric baseline must actually be retained on the general arrangement plan, including during construction.
• Enhancement measures recommended in the appraisal should appear as post-development habitat in the metric, or be explained as species measures that sit outside it.
• Irreplaceable habitats sit outside the metric altogether and need bespoke compensation agreed with the authority.

Where a site has trees, add a fourth drawing to the reconciliation. The root protection areas on the tree survey constrain what can be planted where, and a landscape plan that places new woodland inside a retained tree's root protection area will not survive review.

Programme implications of survey timing

Metric condition assessment and botanical survey both depend on the growing season, broadly April to September for most grassland and other vegetated habitats, and an assessment made in January will either be precautionary or be challenged.

Protected species windows are tighter and less forgiving. Bat emergence surveys run from May to September with the best results in the middle of that period, great crested newt environmental DNA sampling is restricted to a spring window, reptile surveys need warm but not hot conditions in spring and early autumn, and breeding bird survey work follows the March to August season. Miss a window and the wait is a year, not a month.

That asymmetry is why sequencing matters. Get the appraisal done first, early, because it tells you which windows you are about to miss. The metric baseline can follow within the same growing season.

The gain plan and the condition on starting work

Planning permissions granted subject to the statutory biodiversity gain condition cannot begin until a biodiversity gain plan has been submitted to and approved by the local planning authority. The plan sets out the pre-development and post-development values, the habitat it relies on, and any registered off-site gains or statutory credits.

Mandatory net gain applied to major development from 12 February 2024 and to small sites from 2 April 2024. Statutory exemptions exist, including householder development, development below the de minimis threshold, and qualifying self-build and custom housebuilding, and the detail of each matters because the exemptions are narrower than they sound. Check the current gov.uk guidance rather than a summary written when the regime started.

The point most developers underestimate is that the gain plan is a post-permission document that blocks a start on site. A scheme that reaches the funding stage with an unapprovable gain plan is a scheme that cannot start.

Commissioning both from one team

Split the work between two consultancies and you own the reconciliation risk. One team walks the site once, produces the habitat plan that both outputs use, scores condition in the field on the same visit, and answers for any inconsistency. That usually costs less as well as carrying less risk.

Brief them together. Give both the same drawing, dated and numbered, and insist that the metric file, the appraisal and the landscape plan all cite it. Then hold a single review before submission where the ecologist, the landscape architect and the planner look at the three documents side by side.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For biodiversity net gain and protected species work a licensed or competent professional is legally required, so use a draft as the structure a qualified ecologist then fills and signs. Enter your postcode on PlanScope to see which reports the site's constraints are likely to trigger.