Baseline biodiversity value sections and their evidence

The baseline section of a biodiversity net gain assessment is the one a reviewer should read first and challenge hardest, because every unit of gain the scheme claims is measured against it. Get the baseline habitat value wrong and the 10 per cent uplift required by the Environment Act 2021 regime is being calculated from a false starting point. Overstate the baseline and the scheme looks worse than it is. Understate it, and the biodiversity gain plan will not survive the local planning authority's check.

What the section records is narrow and factual: what habitat was present on the site immediately before development, how much of it there was, what condition it was in, and how the surveyor reached those judgements.

What the baseline section sets out

A defensible baseline section contains four things in sequence.

First, the survey basis: who carried out the survey, when, to what standard, and in what weather and vegetation conditions. A UK Habitat Classification survey in February tells you much less about a grassland sward than one in June, and the section should say so rather than hoping nobody notices the date.

Second, the habitat parcels. Each parcel gets a habitat type from the classification used by the statutory metric, an area in hectares (or a length in kilometres for linear features), and a condition score.

Third, the condition assessment itself, parcel by parcel, against the published condition criteria for that habitat type.

Fourth, the resulting units, broken into area habitats, hedgerow units and watercourse units, which the metric keeps in separate accounts.

The section should also state the strategic significance applied to each parcel and the evidence for it, usually a local nature recovery strategy, a local plan designation or a published habitat map.

Habitat parcels, areas and condition

The commonest weakness in a baseline is parcel boundaries drawn for convenience rather than for ecology. A single polygon labelled "modified grassland" over an area that includes a damp hollow with rushes and a strip of species-rich margin is not a baseline, it is an average. Reviewers should expect to see parcels split where habitat type or condition changes, and a mapped plan at a scale where the splits can be read.

Areas must be measured, not estimated. A reviewer can check them: the plan is georeferenced, the areas are reported to a sensible number of decimal places, and the parcel areas sum to the site area within the red line. When they do not sum, something has been missed, usually hardstanding or buildings, which carry their own habitat type in the metric rather than being excluded.

Condition is where professional judgement enters, and it is where the section needs to show its working. Each condition criterion for the habitat type is either passed or failed, and the published rules convert the pattern of passes into good, moderate, poor or fairly poor condition. A condition sheet for each parcel, reproduced as an appendix, is the difference between an auditable assessment and an assertion.

Evidence a reviewer should expect

Ask for these, and treat their absence as a finding:

• a dated habitat plan with parcel references matching the metric input
• a completed condition assessment sheet per parcel, with the criteria scored individually
• species lists supporting any claim of species-rich or priority habitat
• photographs of each parcel, taken in the survey season
• the metric calculation file itself, rather than the summary table alone
• a statement of the surveyor's competence and the survey limitations

The Biodiversity Net Gain Assessment is a report type where a competent person is legally required under the Environment Act 2021 regime, and the local planning authority has to approve the biodiversity gain plan. A draft is a starting point for that professional, never a substitute for them. Where the site has protected species interest as well, that work sits in a separate Protected Species Survey carried out by a suitably licensed ecologist.

Degradation and the retrospective baseline

The regime anticipates that someone will clear a site before applying and then claim a bare-ground baseline. Where habitat has been degraded on or after 30 January 2020 without planning permission, the baseline is assessed as it was immediately before the degradation took place, not as it is found on the day of survey.

This is the provision most often handled badly, in both directions. An assessment that silently uses the current bare-ground state when the aerial imagery shows a hedgerow two years ago is unsound. An assessment that applies the rule where a farmer carried out ordinary agricultural operations within permitted practice may be over-correcting.

The evidence that resolves it is historic: dated aerial imagery, previous survey reports, agri-environment records, planning history. The baseline section should state which date it has taken as the assessment date and why, and should annex the imagery it relied on.

Watercourse and hedgerow units

Linear features are measured in kilometres and carried in their own modules, and they cannot be used to offset a shortfall in area habitat units. That makes their baseline treatment consequential rather than incidental.

For hedgerows, the baseline needs the hedgerow type, its length, whether it is associated with a bank or ditch, and its condition against the hedgerow criteria, which include gappiness, height, width, perennial weed cover and the presence of trees. A reviewer should check that gaps have been measured rather than eyeballed, because the gap threshold moves the condition score.

For watercourses, the baseline requires the watercourse type, length within and adjacent to the site, and condition assessed using the relevant published method. Culverted lengths and heavily modified channels still count, and a culvert within the red line should appear in the baseline rather than vanishing from it.

Where the baseline gets contested

Three disputes recur. The first is habitat type: whether a sward is modified grassland or other neutral grassland, which changes distinctiveness and therefore units. The second is condition: a parcel scored moderate where the criteria arguably support poor, or the reverse. The third is the red line itself, and whether land excluded from the application site was excluded for a planning reason or to remove an inconvenient habitat.

Honest uncertainty is better than a confident wrong answer. Where a survey was carried out outside the optimal season and the habitat type could not be confirmed, say that, state the precautionary assumption applied, and explain which way the assumption errs. Reviewers respond far better to a declared limitation than to one they discover.

Presenting the section so it can be audited

Write it so that another ecologist with the same site could reproduce the number. That means parcel references that are consistent across the plan, the condition sheets, the metric file and the text; areas quoted once and not restated differently later; and a short table at the end of the section giving the baseline units for area habitats, hedgerows and watercourses separately.

Illustrative example: a 1.4 ha paddock surveyed in late June splits into three parcels once condition is assessed properly. The bulk is modified grassland in moderate condition, a 0.2 ha wet corner is a separate parcel with rush cover, and a 180 m species-poor hedgerow with a ditch is recorded in the hedgerow module. One polygon would have produced a single misleading figure for all three. The split takes an extra hour on site and removes the main ground on which the baseline could be attacked.

If you are scoping which reports an ecologically constrained site will need alongside the metric, the 66 report types by category index is the quickest way to see the bundle. 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.