Trading rules in the statutory biodiversity metric

The biodiversity metric trading rules decide which habitats may compensate for which, and they operate before any question of unit totals. A calculation can show a comfortable surplus and still be non-compliant, because the surplus is made up of the wrong habitat. Reviewers check the trading position first for that reason: it is a pass or fail test, whereas the arithmetic is a matter of degree.

The rules exist because biodiversity units are not fungible. Two units of a species-rich lowland meadow and two units of amenity grassland are not the same thing, and a metric that let one substitute for the other would license exactly the trade the Environment Act 2021 regime was designed to prevent.

Why trading rules exist in the metric

The statutory metric assigns each habitat a distinctiveness band, reflecting its relative biodiversity value, alongside condition, strategic significance and area. Distinctiveness is what the trading rules key off.

Without them, the least costly route to a gain would always be to create large areas of low-distinctiveness habitat. Amenity grassland is quick to establish, easy to maintain and reliably delivers units per hectare. The rules block that route by requiring compensation to match or exceed the distinctiveness of what is lost, and in the higher bands to match the habitat type itself.

That is why the rules are sometimes described as preventing trading down. The shorthand is accurate as far as it goes, and it understates how specific the requirement becomes at the top of the scale.

Distinctiveness bands and what each requires

Read the bands as a ladder of specificity. At the top the requirement is effectively like for like, and the scope for design flexibility is close to nil. Lower down the requirement loosens to the broad habitat group, which gives a landscape designer room to work.

Very high distinctiveness deserves particular care. A scheme affecting such habitat cannot be resolved inside the metric by adding units elsewhere, and the compensation has to be agreed with the local planning authority, usually with input from Natural England. In practice, that is a reason to avoid the impact altogether rather than to plan for it.

Irreplaceable habitats sit outside this scheme entirely. Ancient woodland, ancient and veteran trees, blanket bog, lowland fen, limestone pavement and others are excluded from the metric calculation, and the biodiversity gain condition does not apply to them in the ordinary way. Where a proposal affects irreplaceable habitat, the answer is a bespoke compensation agreement and a much harder policy test, not a larger number in a spreadsheet.

Like for like and like for better in practice

The workable version of the rules for a design team is this. Losing a high distinctiveness habitat commits you to recreating that specific habitat. Losing a medium distinctiveness habitat gives you a choice within the same broad habitat type, or an upgrade. Losing low distinctiveness habitat gives you the broad habitat group or anything better.

Trading up is always permitted, and it is often the efficient answer. Replacing a medium distinctiveness grassland with a higher distinctiveness grassland satisfies the rule and generates more units per hectare, though it raises the difficulty multiplier and the time to target condition, which pull the other way.

The pitfall is the assumption that a broad habitat type is broader than it is. Grassland is not a single bucket in the metric, and neither is woodland. Check the actual classification of both the baseline parcel and the proposed parcel in the current metric rather than reasoning from the everyday meaning of the words.

Where trading down is not permitted

Three situations account for most breaches.

1. Higher distinctiveness lost, lower distinctiveness created, with the shortfall made up through area. The rule bites on habitat type, not on unit totals, so the extra area does not help.
2. Compensation proposed in the wrong module. The metric handles area habitats, hedgerows and lines of trees, and watercourses separately, and units do not move between them. Hedgerow loss needs hedgerow compensation, and river units cannot be made up with grassland.
3. Condition downgrade within the same habitat. For high distinctiveness habitat the replacement must be in the same or better condition, so recreating the habitat in poor condition does not discharge the requirement.

The second is the one reviewers see most. A scheme removes 400 metres of species-rich hedgerow, creates a large area of neutral grassland, reports an overall surplus, and has not compensated the hedgerow at all.

Effects on a scheme's landscape strategy

Trading rules land on the landscape architect rather than the ecologist, because they constrain what can be drawn. Three consequences worth planning for.

Habitat type is fixed before planting design begins. If the baseline includes high distinctiveness habitat, the replacement is specified by the rule, not by what suits the masterplan.

Areas get larger. Because created habitat starts in poor condition and takes years to reach target condition, the difficulty and temporal multipliers mean the created area usually exceeds the area lost, sometimes considerably.

Management horizons are long. Habitats have to be maintained for at least thirty years under the statutory regime, secured by planning obligation or conservation covenant, which changes what a Landscape Strategy and a Green Infrastructure Plan have to commit to. A planting schedule is not a management plan.

Illustrative example. A site with 0.4 hectares of high distinctiveness species-rich grassland in moderate condition, lost entirely, cannot be compensated by 1.2 hectares of other grassland types. It requires species-rich grassland of the same type in moderate or better condition, and because the created sward starts poor and takes years to establish, the on-site area needed is larger than 0.4 hectares. Where the site cannot accommodate that, off-site provision or statutory credits become the route. The figures are illustrative.

Common breaches a reviewer will catch

Reviewers work through a short list before looking at any totals. Whether the baseline habitat classifications are supported by the survey data rather than assumed from aerial imagery. Whether any very high distinctiveness or irreplaceable habitat is present, and if so whether it has been handled outside the metric. Whether each module balances in its own right. Whether high distinctiveness losses are matched by the same habitat type at equal or better condition. And whether condition assessments were carried out using the published criteria, with the evidence recorded.

Baseline misclassification is the root of most of it. Classifying a species-rich sward as modified grassland lowers the distinctiveness band, which relaxes the trading rule and reduces the units lost, and it is the single most consequential judgement in the whole exercise. That judgement belongs to a competent person who has walked the site in the right season, not to a desk exercise.

Documenting compliance in the calculation

The Biodiversity Net Gain Assessment should carry the trading position explicitly rather than leaving it implied by the spreadsheet. Set out, for each habitat lost, its distinctiveness band, the rule that applies, the habitat proposed as compensation, and how the rule is satisfied. Do the same for hedgerows and watercourses in their own tables.

Then state the survey basis: who carried out the habitat survey, when, in what season, and against which condition criteria. A Preliminary Ecological Appraisal carried out in February will not support a confident grassland classification, and saying so is better than having the authority discover it.

Biodiversity net gain assessment under the Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and the biodiversity gain plan is approved by the local planning authority. A drafted document is a starting point for that professional, not a substitute; the professional directory is the route to arranging review. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice: check the current metric and guidance before relying on it.