Statutory metric versions and transitional application

The statutory biodiversity metric version used for a calculation is part of the answer, not a footnote to it. Two ecologists can survey the same field, agree the habitat type and condition, and still produce different unit totals because they ran different versions of the tool. Under the Environment Act 2021 regime, the biodiversity gain plan has to show a gain of at least 10 per cent calculated using the metric, and a reviewer who cannot tell which version produced the figures cannot check them. So the version number, the file, the date it was downloaded and the condition sheets all belong in the submission.

Why the version changes the answer

The metric is a spreadsheet model with published values behind it. Habitat distinctiveness scores, condition criteria, strategic significance multipliers, the time-to-target multipliers that discount delayed creation, and the difficulty multipliers that discount habitats which are hard to create all sit in lookup tables. When a version is published, those tables can change, as can the condition assessment criteria and the trading rules that govern what can substitute for what.

And the effect is rarely neutral. A change to the condition criteria for modified grassland can move a baseline by a material margin, which changes the deficit and therefore the area of creation needed. A change to a time-to-target multiplier changes how much credit a scheme gets for planting that takes fifteen years to reach target condition.

Three outputs have to be satisfied separately: habitat units for area habitats, hedgerow units and watercourse units. A version change that shifts hedgerow values can leave a scheme passing on area habitats and failing on hedgerows, which is a design problem rather than a rounding problem.

Which version applies to an application

The governing principle is simple to state and easy to get wrong. The metric to be used is the publication in force under the Environment Act 2021 regime at the relevant point, and when a new version is published Defra and Natural England issue a transitional note alongside it setting out which applications and gain plans may continue to use the previous version and for how long.

Read that note rather than assuming. Transitional arrangements have been framed by reference to the date an application was submitted or validated, the date a biodiversity gain plan is submitted, and in some cases a fixed cut-off after which only the new version is accepted. Those are different triggers and they produce different answers for a scheme that was submitted under one version and determined under another.

Two practical rules follow:

1. Check the publication page for the current version and its transitional note before you start, and again before you submit, because a long-running application can straddle a change.
2. Where a scheme may straddle a version change, run both versions and keep both files. The cost is an afternoon. The alternative is re-doing the design work under time pressure once the authority asks.

Separate from this sits eligibility for the small sites metric, which is a simplified tool for qualifying small developments. Eligibility depends on the scale of the development and on the absence of certain features, including priority habitat and the need for a protected species licence. A site that fails one of those tests must use the full metric, and switching tools mid-application is a bigger change than switching versions.

Surveys done before a version change

Field data does not expire when a version is published, but its usefulness depends on what changed. Habitat classification to the UK Habitat Classification survives most version changes. Condition assessments do not, if the condition criteria themselves have been revised.

So the question to ask of an older survey is narrower than "is it out of date". It is whether the recorded evidence lets you re-score condition under the new criteria without going back to site. That depends on whether the surveyor recorded the underlying attributes, such as sward height, species frequency, bare ground and scrub cover, or only the resulting condition category. Surveys that record only the outcome have to be repeated, and if the survey season has closed that means a delay of months.

This is the strongest argument for insisting on full condition assessment sheets in the deliverables rather than a summary table. The guidance published by the Chartered Institute of Ecology and Environmental Management on ecological assessment supports that level of recording, and it costs nothing extra at the time.

Recording the version in the submission

A Biodiversity Net Gain Assessment that a reviewer can audit states, on its face:

• The name and version of the metric used, and whether the full or small sites tool was applied.
• The date the tool was obtained and the transitional basis on which that version applies.
• The completed metric file itself, submitted as a spreadsheet rather than as a printed summary.
• The habitat classification system used and the survey dates.
• The condition assessment sheets for every parcel, including the parcels scored as poor.
• The name and competency of the person who completed the metric, because the Environment Act 2021 regime expects the statutory metric to be completed by a competent person.

Authorities increasingly refuse to validate without the native spreadsheet. A printed table cannot be checked, cannot be re-run and cannot be reconciled against a design change.

Re-running a metric after a design change

Any change to the red line, the building footprint, the levels or the landscape proposals changes the metric. The ones people forget are the small ones: a wider access to satisfy the highway authority, a swale moved to clear a drainage easement, a root protection area that shrinks a planting bed, a sub-station the distribution network operator insists on.

Treat the metric as a live document under version control of your own. Name each file with the metric version and the scheme revision, keep a short change log recording what moved and what the resulting net change was, and re-issue the gain plan when the figure moves rather than at the end.

Where a design change pushes a scheme below the 10 per cent threshold, the options are on-site redesign, off-site units through a registered gain site, or statutory credits as a last resort. Each has a different lead time, and discovering the shortfall at the gain plan stage removes the least costly of the three.

Auditing a calculation prepared by someone else

A reviewer working through another consultant's metric should go in this order. It takes about an hour on a small scheme.

Start with the version and the transitional basis. Then reconcile the baseline area against the site plan, because the commonest error is a baseline that does not add up to the red line area. Check whether habitats on land outside the applicant's control have been included. Look at the condition scores for anything scored as good, and ask what evidence supports it, since an optimistic baseline inflates the deficit while an optimistic post-development condition inflates the gain.

Then test the multipliers. Strategic significance should match the local nature recovery strategy or other published mapping rather than being set to the default. Time-to-target should reflect the habitat actually proposed, not a faster-establishing substitute. Difficulty multipliers should not have been edited.

Finally check the trading rules. Habitats of medium or higher distinctiveness cannot be traded down, and a scheme that replaces species-rich grassland with amenity grassland plus a large area of new scrub may balance on units while breaching the rules. Where irreplaceable habitat is present the metric does not apply to it at all, and the separate route described for irreplaceable habitats and the biodiversity gain exception governs instead.

Errors a version change tends to expose

Version changes are useful diagnostics, because re-running an old calculation in a new tool surfaces problems that were always there.

Baselines recorded in hectares in one sheet and square metres in another. Hedgerows double counted as both hedgerow units and area habitat. Retained trees entered as new planting. Off-site units claimed without a registered gain site or a legal agreement. Post-development condition set to good for habitats that the management plan funds for five years rather than thirty.

The 30-year management and monitoring obligation is the one that bites after permission. The biodiversity gain condition is deemed to be attached to most permissions, and the practical consequences of that deeming are set out in the note on the deemed biodiversity gain condition. A metric that assumes target condition at year 30 commits someone to maintaining it, and the authority will want to see who and how.

Where a metric has been prepared in-house or by a non-specialist, have it checked. The professional directory is the route to a reviewer with the right competency, and a flawed metric is the single most common reason a biodiversity gain plan is rejected after permission has already been granted.

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. Reform dates and tool versions move, so follow them on the PlanScope planning law timeline and verify the current metric version on gov.uk before you rely on a number.