The statutory biodiversity metric is the Defra tool that measures biodiversity value for biodiversity net gain (BNG) in England. It turns habitats into "biodiversity units" by scoring their area, distinctiveness, condition and strategic significance, so that a development can show a gain of at least 10%. It is quick, consistent and enforceable, but it measures habitat rather than wildlife, and the gains it records are promises that have to be delivered over 30 years (Statutory Biodiversity Metric User Guide, June 2026).
This guide looks at the metric critically: what it does well, what it misses, what research on early BNG plans has found, how the statutory credit backstop works, and what is changing. If you need the formula, the scores and a worked example, see how the BNG metric calculates units. For the law, see BNG regulations and exemptions.
4
Scoring factors
Area, distinctiveness, condition, strategic significance
34%
Loss of non-urban habitat area
In early BNG plans studied
21%
Applications with a metric error
Six councils, 2020 to 2022
2 : 1
Credits per unit short
Statutory credit backstop
Sources: Metric user guide, June 2026; zu Ermgassen et al. 2021; Rampling et al. 2024; GOV.UK credit prices.
Key Takeaway
The metric gives England a single, auditable currency for nature in planning, and its rules stop the worst trades, such as swapping rare habitat for common habitat. But research on early BNG plans found that most units were promised on development sites themselves, often by improving the condition of smaller areas rather than keeping habitat, and that simple calculation errors were common. The metric is only as good as the surveys, the checking and the 30 years of management behind it.
In this guide
What is the statutory biodiversity metric?
The metric is a spreadsheet calculation tool, published with a user guide and habitat condition assessment sheets on GOV.UK. Ecologists record every habitat parcel on a site, and the tool calculates biodiversity units before and after development. Area habitats, hedgerows and watercourses are counted separately, and each must show its own 10% gain.
It replaced earlier voluntary versions, such as Biodiversity Metric 3.1 and 4.0, which some councils used before BNG became law. Mandatory BNG must use the statutory metric, or the simpler Small Sites Metric for eligible minor development without priority habitat (GOV.UK).
Biodiversity units = area × distinctiveness × condition × strategic significance
Created and enhanced habitats are then adjusted for how long they take to mature, how hard they are to create and, if off-site, how far away they are. The full scoring is in our BNG metric guide.
What does the metric do well?
- One currency. Every development in England is measured the same way, so plans can be checked, compared and audited, and units can be traded on a regulated market.
- Quality, not just quantity. Condition scores reward well-managed habitat, so restoring a degraded meadow counts, not just planting new ground.
- Trading rules. Losses of high-distinctiveness habitat must be replaced with the same habitat type, and hedgerow, watercourse and area units cannot be swapped for each other.
- Time and risk. Multipliers for time to target condition and difficulty of creation mean a new woodland earns fewer units than its eventual value, discouraging over-optimistic plans.
- Place. Strategic significance scores reward habitat in locations mapped by Local Nature Recovery Strategies, and the spatial risk multiplier favours gains close to the development.
- Irreplaceable habitat is ring-fenced. Ancient woodland, veteran trees and similar habitats are not given a unit value that could be traded away; losses need bespoke compensation (PPG).
What does the metric miss?
- Habitat is a proxy for wildlife. The metric does not count species. Two "lowland meadows in moderate condition" can support very different plants and insects, depending on soil, history and surroundings.
- Condition can be hard to score consistently. Condition assessments involve judgement, and small changes in a score can make a large difference to the units a site generates.
- Promises versus delivery. Post-development units are forecasts. A meadow sown in 2025 is scored on the condition it is expected to reach, which depends on decades of the right management.
- Area can fall while units rise. Because condition multiplies area, a scheme can lose habitat area overall and still show a net gain by promising better condition on what remains.
- On-site gains are hard to police. Habitat in gardens, verges and public open space on housing estates is managed by many people over time, and authorities have limited capacity to monitor it.
What has research found about BNG plans?
Two peer-reviewed studies by researchers who have tracked BNG since before it became mandatory give the clearest picture so far:
| Study | What it looked at | Key findings |
|---|---|---|
| zu Ermgassen et al., 2021, Conservation Letters | BNG assessments from six early-adopter councils, before BNG was mandatory | Developments were associated with a 34% loss of non-urban habitat area, generally compensated by commitments to smaller areas of higher-quality habitat years later. 95% of units came from habitat within or next to the development footprint, managed by developers. |
| Rampling et al., 2024, Conservation Biology | All new major developments in six early-adopter councils, 2020 to 2022 | 27% of units were in governance gaps: better-condition habitat promised on-site that was unlikely to be monitored or enforced. 21% of applications contained a simple recurring error in their calculations, and half of these had been approved. |
These studies examined plans, not outcomes on the ground, and mostly predate mandatory BNG and the 30-year legal agreements it introduced. They do not show that BNG is failing. They show where it is most likely to fail: on-site gains that nobody checks, and arithmetic that nobody re-runs. Official data on delivery is still thin; there is no national dataset yet on how much BNG habitat has reached its target condition.
For the bigger picture of how England is trying to reverse wildlife decline, and where development fits in, see our overview of the causes of biodiversity loss in the UK.
How does the statutory credit backstop work?
If a developer cannot reach 10% on-site or off-site, the metric works out a "unit shortfall", which is met by buying statutory biodiversity credits from Natural England. Credit prices are set at the cost of creating, maintaining and monitoring habitat plus a mark-up, and the metric applies a spatial risk multiplier so that two credits are needed for every unit (GOV.UK):
| Tier | Habitats (examples) | Price per credit | Cost per unit short |
|---|---|---|---|
| A1 | Low-distinctiveness habitat; medium grassland; reedbeds; traditional orchards | £42,000 | £84,000 |
| A2 | Medium woodland; lowland calcareous grassland; lowland heathland | £48,000 | £96,000 |
| A3 | Wet woodland; upland birchwoods; littoral mud | £66,000 | £132,000 |
| A4 | Ponds; saltmarsh; native pine and lowland mixed deciduous woodland | £125,000 | £250,000 |
| A5 | Lakes | £650,000 | £1,300,000 |
| H | Hedgerows | £44,000 | £88,000 |
| W | Watercourses | £230,000 | £460,000 |
Sales have been modest: £206,180 in the first year of mandatory BNG (Defra) and £426,100 in 2025 to 2026 (Defra). Defra used credit income to buy 44.6 units of floodplain wetland mosaic habitat in Lincolnshire, secured for at least 30 years under a conservation covenant. Low sales are what the policy intends, but they also mean almost all BNG depends on on-site and off-site delivery, where the evidence above points to the risks.
What is changing in 2026, and what is next?
From 6 August 2026, new planning applications on sites of 0.2 hectares or less, and temporary development of 5 years or less, are exempt unless they affect onsite priority habitat, and minor development can go straight to off-site gains (SI 2026/790). BNG reaches nationally significant infrastructure projects on applications from 2 November 2026 (GOV.UK).
For the metric itself, the government has confirmed changes to how open mosaic habitat on brownfield land is treated and to base spatial risk on LNRS areas rather than local authority boundaries. These need a new version of the metric, and timing has not been confirmed. Most of the proposed revisions to the Small Sites Metric were not taken forward (Defra, July 2026; government response, April 2026).
What would make the metric work better?
Researchers, ecologists and planners broadly agree that the weaknesses lie less in the formula than in what surrounds it. The main suggestions are:
- Secure on-site gains like off-site gains. Rampling and colleagues argue that applying the higher governance standards of off-site gains to on-site gains, and resourcing planning authorities to monitor them, would close the gap (Rampling et al., 2024).
- Check the arithmetic. Authorities need the ecological expertise, or access to it, to review metric calculations before approving gain plans.
- Report outcomes, not just plans. Public monitoring data on habitat condition at years 5, 10 and beyond would show whether the units promised are being delivered.
- Use the LNRS maps. Steering gains to places identified for nature recovery should give more wildlife value per unit than scattered on-site patches.
The metric is a measuring tool, not a guarantee. Whether BNG reverses habitat loss will be decided by the meadows, ponds and hedgerows still being managed in 2055. For the rules that govern that, see BNG regulations and compliance.
Frequently asked questions about the statutory biodiversity metric
What is the statutory biodiversity metric?
It is Defra's official tool for measuring biodiversity value in England for biodiversity net gain. It scores each habitat by area, distinctiveness, condition and strategic significance to give biodiversity units, so that a development can show a gain of at least 10%. It has been mandatory since February 2024.
Is the statutory biodiversity metric the same as Biodiversity Metric 4.0?
No. Biodiversity Metric 4.0 and earlier versions such as 3.1 were voluntary tools used before BNG became law. Mandatory BNG must use the statutory biodiversity metric, whose current user guide is dated June 2026, or the Small Sites Metric for eligible minor development.
Does the metric measure species?
No. It uses habitat type and condition as a proxy for biodiversity. Protected and priority species are dealt with through separate surveys, planning policy and wildlife law, not through biodiversity units.
What are the main criticisms of the metric?
That habitat is an imperfect proxy for wildlife; that losses of existing habitat area can be offset on paper by promised improvements in condition years later; that many gains are planned on development sites where monitoring and enforcement are weakest; and that calculation errors are common.
How common are errors in BNG metric calculations?
One study of major developments in six early-adopter councils from 2020 to 2022 found a simple recurring error in the BNG calculations of 21% of applications, and half of those applications had been approved (Rampling et al., 2024). The study predates mandatory BNG, so current rates may differ.
Why are statutory biodiversity credits so expensive?
Credits are priced at the cost of creating and maintaining habitat plus a mark-up, so they do not undercut the private off-site market. Developers must also buy two credits for every unit they are short, which makes credits a deliberate last resort.
Is the metric going to change?
Yes. The government has said it will update the metric, including how open mosaic habitat is treated and basing spatial risk on Local Nature Recovery Strategy areas. These changes need a new version of the metric and the timing has not been confirmed.
Understand biodiversity net gain
Plain-English guides to how BNG works, how the metric scores habitat and the law behind it.
Sources: Statutory Biodiversity Metric User Guide (June 2026); Statutory biodiversity metric tools and guides; GOV.UK, calculate biodiversity value; Planning Practice Guidance; GOV.UK, credit prices; Defra credits report 2024–25; Defra credits report 2025–26; SI 2026/790; Defra, July 2026; Government response, April 2026; zu Ermgassen et al. 2021; Rampling et al. 2024.
