Biodiversity net gain · Measurement
The statutory biodiversity metric: how BNG units are scored
The statutory biodiversity metric is the only measure of biodiversity value that counts for BNG in England.
Schedule 7A requires the Secretary of State to publish it and lay it before Parliament, and Defra’s user guide, last updated June 2026, sets the rules for applying it.
This page explains how the BNG metric turns habitat into units, why the three unit types cannot be mixed, and how the trading rules and risk multipliers change the answer.
The instrument
A laid document, applied through a tool
Paragraph 3 of Schedule 7A says any reference to biodiversity value means value calculated in accordance with the biodiversity metric.
Paragraph 4 requires the Secretary of State to produce, publish and lay it before Parliament, and Defra laid the statutory metric Act paper on 29 November 2023.
In practice the metric is applied through a package: the calculation tools, the user guides and the condition assessments, all published on Defra’s metric tools and guides page.
The laid document carries no version number; only the tools do, and Defra says any version of the statutory tools may be used because updates do not change unit outputs.
The pre-statutory Biodiversity Metric 4.0 is a different product: the statutory tools began at version 1.0, and Defra published guidance on transferring data from 4.0 when BNG became mandatory.
| Item | Status |
|---|---|
| The metric document | Laid before Parliament; no version number |
| Main calculation tool | v1.0.4 (2 June 2026) |
| Small sites tool | v1.2.3 (2 June 2026) |
| Which tool version to use | Any version of the statutory tools |
| Biodiversity Metric 4.0 | Pre-statutory; a different product |
Rule 2
Three unit types, three separate 10% tests
The metric produces three kinds of unit: area habitat units, hedgerow units and watercourse units.
Rule 2 of the user guide says unit outputs must not be summed, traded or converted between types, and the requirement to deliver at least a 10% gain applies to each type.
So a scheme can pass on area units and still need to buy hedgerow units, and the metric tool’s headline results show a separate target and deficit for each.
Watercourses carry two extra quality inputs, riparian zone encroachment and watercourse encroachment, and a river condition assessment should be done by a qualified assessor.
A watercourse’s footprint can be recorded in the area module, but all its units are reported in the watercourse module.
The formula
Size × distinctiveness × condition × strategic significance
At baseline, each habitat parcel’s units are its size multiplied by three quality scores from the laid metric.
| Input | Categories and scores | Set by |
|---|---|---|
| Size | Hectares (area); kilometres (hedgerow, watercourse) | Survey |
| Distinctiveness | Very high 8 · High 6 · Medium 4 · Low 2 · Very low 1 (hedgerow) or 0 (area) | The habitat type (User Guide Table 5) |
| Condition | Good 3 · Fairly good 2.5 · Moderate 2 · Fairly poor 1.5 · Poor 1 · Assessment N/A 1 · N/A – other 0 | A condition assessment (User Guide Table 6) |
| Strategic significance | High 1.15 · Medium 1.10 · Low 1 | Location and local strategy (User Guide Table 7) |
Distinctiveness is set by the habitat type, and the tool assigns it automatically once a professional survey has identified the habitat.
Condition comes from a condition assessment for each parcel, using Defra’s condition sheets; the intermediate “fairly good” and “fairly poor” categories are limited to a one-step adjustment with ecological reasoning.
Strategic significance depends on location: where a local nature recovery strategy has been published, baseline parcels are always scored low, and high is available only for interventions consistent with a measure mapped in its Local Habitat Map.
Medium cannot be used once an LNRS is published.
Habitat of distinctiveness score four or more also matters outside the arithmetic: the first step of the statutory hierarchy in DMPO article 37A is to avoid, then mitigate, harm to it.
Try it
A baseline unit calculator, for one parcel
The calculator multiplies one parcel’s size by the three quality scores, which is the baseline arithmetic the laid metric applies.
A worked example: one hectare of a low distinctiveness habitat (2) in moderate condition (2) with low strategic significance (1) is worth 4 area habitat units.
The 10% target for that parcel alone is 4.4 units after development.
Real schemes have many parcels, and created or enhanced habitat is discounted by risk multipliers, so Rule 3 requires the official calculation tool for the actual figure.
What that figure feeds is the biodiversity gain plan, which must attach the completed tool with its publication date.
Baseline unit calculator — area habitat
One habitat parcel at baseline. Add parcels together in the official tool.
1 × 2 × 2 × 1
Baseline: 4 units
10% target for this parcel alone: 4.4 units
Illustrative arithmetic, not a metric calculation. The statutory tool assigns distinctiveness from the habitat type, needs a condition assessment for each parcel, and applies difficulty, temporal and spatial risk multipliers to created and enhanced habitat. Nothing you entered was stored or sent.
Risk multipliers
Why created habitat is worth less on paper
The metric applies three risk multipliers to every post-development enhancement and creation, across all three modules.
Difficulty reflects uncertainty in how well a habitat can be created or enhanced, and the tool assigns it by habitat type.
Temporal risk reflects the years a habitat takes to reach its target condition, and can be adjusted for habitat created in advance of clearance or delayed after it.
Spatial risk discounts off-site habitat the further it lies from the impact site.
Statutory credits are treated as distant habitat: Defra’s credits guidance says a credit is worth 0.5 units, so two credits are needed for each unit of shortfall.
Temporary impacts need not be recorded as a loss where the habitat is restored to its baseline type and condition within two years of the impact.
| Spatial risk | Area and hedgerow | Score |
|---|---|---|
| Within | Same LPA or National Character Area as the impact | 1.0 |
| Neighbouring | Outside, but in a neighbouring LPA or NCA | 0.75 |
| Outside | Beyond the neighbouring LPA or NCA | 0.5 |
Rule 1
The trading rules: like for like, up to no net loss
| Lost habitat | Area habitats | Hedgerows | Watercourses |
|---|---|---|---|
| Very high | Priority to the same habitat type; otherwise the same distinctiveness, then high | Same habitat type | Priority to the same habitat type; otherwise the same distinctiveness, then high |
| High | Same habitat type | Same habitat type or higher distinctiveness | Same habitat type |
| Medium | Medium habitat of the same broad type, or any higher distinctiveness | Same or higher distinctiveness | Higher distinctiveness |
| Low | Same or higher distinctiveness | Same or higher distinctiveness | Higher distinctiveness |
| Very low | Not applicable | Same or higher distinctiveness | Not applicable |
The trading rules set minimum creation and enhancement requirements to replace specific losses, and they apply only up to the point of no net loss.
Once they are met, the rest of the 10% can come from creating or enhancing any habitat within the same module.
The user guide gives the example: losing one area unit of high distinctiveness habitat means providing at least one area unit of the same habitat type.
The tool’s headline results show whether the trading rules are met, and the gain plan template has a section for the trading summary.
Rule 4 allows the planning authority to permit a departure from the methodology in exceptional ecological circumstances, and the user guide says most projects should not use it.
Who runs it
Competent persons and the small sites metric
Principle 1 of the user guide says the metric assessment should be completed by a competent person, with the knowledge and skills gained through training, qualifications, experience or a combination.
The guide aligns competency with the British Standard for designing and implementing biodiversity net gain, BS 8683.
Principle 4 is a useful reminder: the metric is not a comprehensive ecological model and is not a substitute for expert ecological advice.
The small sites metric is for development that is not major in the DMPO sense, and the Act paper rules it out where priority habitats, protected sites or European protected species are present.
It sets all habitats at moderate condition, so no condition assessments are needed, and it offers only very low, low and medium distinctiveness habitats.
Whether a scheme needs the metric at all depends on the exemption regulations, covered on the BNG exemptions page.
How a deficit is closed, and what statutory credits cost at Defra’s published prices, is on the units, credits and register page.
The metric output is the evidence behind the biodiversity values in the gain plan, and behind the gains a planning authority reports in its biodiversity report.
For the full sequence from survey to 30-year maintenance, start at the biodiversity net gain hub.
Frequently asked
Questions people ask
What is the statutory biodiversity metric?
It is the document Schedule 7A to the Town and Country Planning Act 1990 requires the Secretary of State to publish and lay before Parliament for measuring biodiversity value for BNG. It converts habitat into biodiversity units using habitat type, size, distinctiveness, condition and strategic significance, and it is applied through Defra's official calculation tool.
What version of the BNG metric should I use?
Any version of the statutory metric tools. Defra says updates do not affect biodiversity unit outputs. The tools are versioned (the main calculation tool is at v1.0.4 and the small sites tool at v1.2.3, as last updated on 2 June 2026), but the laid metric document itself carries no version number. The pre-statutory Biodiversity Metric 4.0 is a different product: the statutory tools began at version 1.0.
What are the three types of biodiversity unit?
Area habitat units, hedgerow units and watercourse units. Rule 2 of the metric says they must not be summed, traded or converted between types, and the 10% gain applies to each type separately. A surplus of area units cannot make up a hedgerow shortfall.
How are biodiversity units calculated?
For habitat at baseline, the laid metric multiplies the size of each parcel by its distinctiveness score (from 0 to 8), its condition score (from 1 to 3) and its strategic significance score (1, 1.10 or 1.15). Created and enhanced habitat also carries multipliers for the difficulty of creation, the time to reach target condition and, for off-site habitat, the distance from the impact.
Is there a BNG calculator?
Yes. Defra publishes the statutory biodiversity metric calculation tool and a small sites tool, and Rule 3 of the metric requires one of them to be used. The calculator on this page shows the baseline arithmetic for a single parcel only and is not a substitute for the official tool.
Who can complete a BNG metric calculation?
The user guide says the assessment should be completed by a competent person, meaning someone with the knowledge and skills to complete and review the calculation through training, qualifications, experience or a combination. It aligns competency with BS 8683. The small sites metric can be used on development that is not major, where no priority habitats, protected sites or European protected species are present.
What are the BNG trading rules?
Minimum requirements for replacing lost habitat, set by the distinctiveness of what is lost, which apply up to the point of no net loss. High distinctiveness losses must be replaced with the same habitat type; medium losses with medium habitat of the same broad type or anything of higher distinctiveness; low losses with habitat of the same or higher distinctiveness. Once no net loss is reached, the rest of the 10% can come from any habitat in the same module.
What is the spatial risk multiplier?
A multiplier that reduces the value of off-site habitat the further it is from the impact site: 1.0 within the same local planning authority or National Character Area, 0.75 in a neighbouring one, and 0.5 further away. Statutory credits are treated as worth 0.5 units each, which is why two credits are needed for every unit of shortfall.
Sources
Primary sources
Every figure, date and status on this page traces to the instrument’s owner. Secondary commentary is never the source for a number.
- legislation.gov.ukTown and Country Planning Act 1990, Schedule 7A, paragraphs 3 and 4 — the biodiversity metric
The metric as the statutory measure of biodiversity value; publication and laying before Parliament.
- DefraThe Statutory Biodiversity Metric — User Guide (last updated June 2026)
Rules 1–4, principles, Tables 5 and 6 (distinctiveness and condition scores), strategic significance, risk multipliers, Table 9 (spatial risk), credit calculation.
- DefraStatutory biodiversity metric (including small sites) — the Act paper laid before Parliament, 29 November 2023
The laid metric: unit types, trading rules, the major and small sites versions, and the unit formulae.
- Defra / Natural EnglandStatutory biodiversity metric tools and guides (last updated 2 June 2026)
The calculation tools (v1.0.4, small sites v1.2.3), condition assessments, and "You can use any version of the statutory metric tools".
- Defra / Natural EnglandCalculate biodiversity value with the statutory biodiversity metric
Inputs the formula takes into account; the small sites metric and off-site units.
- legislation.gov.ukDevelopment Management Procedure Order 2015, article 37A
The hierarchy's first step: avoid or mitigate harm to habitat of distinctiveness score four or more.
- legislation.gov.ukDevelopment Management Procedure Order 2015, article 2(1) — "major development"
The line between the main metric and the small sites metric.
- legislation.gov.ukSI 2024/47 — Biodiversity Gain Requirements (Exemptions) Regulations 2024
Development that needs no metric calculation at all.
- DefraStatutory biodiversity credits
A credit is worth 0.5 units under the spatial risk multiplier.
- DefraStatutory biodiversity credit prices (last updated 2 June 2026)
Tier prices A1–A5, H and W.
- DefraLocal nature recovery strategies (updated 14 September 2026)
The 48 strategies whose local habitat maps set high strategic significance.
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