The BNG metric, officially the statutory biodiversity metric, is the Defra tool used in England to measure a site's biodiversity value in "biodiversity units" before and after development, to show the development leaves nature at least 10% better off. Each habitat is scored on four things: its size, its distinctiveness (how rare and valuable the habitat type is), its condition and its strategic significance. Area habitats, hedgerows and watercourses are counted separately, and each must show its own 10% gain (Statutory Biodiversity Metric User Guide, June 2026).
This guide explains who has to use the metric, how the units are calculated with the actual scores, the trading rules, the Small Sites Metric and the most common mistakes. It also covers what changed in 2026: the new exemptions in force since 6 August 2026, and mandatory BNG for nationally significant infrastructure projects from 2 November 2026. For the wider policy, see our complete guide to biodiversity net gain.
10%
Minimum net gain
Environment Act 2021
3
Separate unit types
Area, hedgerow, watercourse
30 years
Minimum habitat commitment
Off-site and significant on-site gains
0.2 ha
New exemption threshold
Applications from 6 Aug 2026
Sources: Environment Act 2021; Statutory Biodiversity Metric User Guide (June 2026); SI 2026/790.
Key Takeaway
Biodiversity units = area × distinctiveness × condition × strategic significance, with extra risk multipliers for habitat that is created, enhanced or delivered off-site. From 6 August 2026, new planning applications on sites of 0.2 hectares or less are exempt unless they affect priority habitat. From 2 November 2026, BNG also applies to new nationally significant infrastructure project applications.
In this guide
What is the BNG metric?
Biodiversity net gain is a legal requirement under the Environment Act 2021: most new development in England must leave habitats at least 10% better than before. It became mandatory for major development from 12 February 2024 and for minor development from 2 April 2024 (Defra).
The statutory biodiversity metric is how that 10% is measured. It is a spreadsheet tool published by Defra and Natural England, with a user guide, condition assessment sheets and a simplified Small Sites Metric. The current user guide is dated June 2026. Older advisory versions, such as Biodiversity Metric 3.1 and 4.0, are not used for mandatory BNG.
The metric does not count species. It uses habitats as a proxy for biodiversity, scoring each habitat parcel on its type, quality and location. That makes it quick and consistent, but it is a deliberate simplification. For more on how ecologists measure biodiversity directly, see how to measure biodiversity.
Who has to use the BNG metric?
Anyone applying for planning permission in England must deliver 10% BNG, and use the metric to prove it, unless an exemption applies. The main exemptions are:
- Householder applications, such as extensions to existing homes;
- De minimis development: development that does not affect priority habitat and affects less than 25 m² of habitat or less than 5 m of hedgerow or other linear habitat;
- Sites of 0.2 hectares or less (new from 6 August 2026), measured by the red line boundary, unless onsite priority habitat is affected;
- Temporary development (new from 6 August 2026) where the land is reinstated within five years or less, unless onsite priority habitat is affected;
- Biodiversity gain sites themselves, and development forming part of the high-speed rail network.
The previous exemption for self-build and custom-build homes was removed for applications made from 6 August 2026, although many such schemes will now fall under the 0.2 hectare exemption instead (SI 2026/790; GOV.UK exempt developments guidance). Exempt development is still subject to normal planning policy and wildlife law.
How are biodiversity units calculated?
For area habitats at baseline, the metric uses this formula:
Biodiversity units = area (hectares) × distinctiveness × condition × strategic significance
The scores come from the statutory metric user guide:
| Factor | Categories and scores | What it reflects |
|---|---|---|
| Distinctiveness | Very high 8 · High 6 · Medium 4 · Low 2 · Very low 0 (1 for hedgerows) | How rare and valuable the habitat type is; fixed by habitat type |
| Condition | Good 3 · Fairly good 2.5 · Moderate 2 · Fairly poor 1.5 · Poor 1 | The habitat's ecological health, assessed against condition sheets |
| Strategic significance | High 1.15 · Medium 1.1 · Low 1 | Whether the location is identified for nature, for example in a Local Nature Recovery Strategy |
Habitat that will be created or enhanced after development is also multiplied by two risk factors: time to target condition, because a new meadow or woodland takes years to mature, and difficulty of creation or enhancement. Habitat delivered off-site is further multiplied by a spatial risk factor, which reduces its value the further it is from the development. These multipliers stop a plan from claiming full value for habitat that does not yet exist.
A quick illustration using the scores above: one hectare of lowland meadow (high distinctiveness, 6) in moderate condition (2) with no strategic significance (1) is worth 12 units. One hectare of modified grassland (low, 2) in poor condition (1) is worth 2 units. That six-fold difference is why losing good habitat is so hard to make up.
What are the three types of biodiversity unit?
The metric works in three separate modules:
- Area habitat units for habitats measured in hectares, such as woodland, grassland, heathland, wetland and urban habitats;
- Hedgerow units for hedgerows and lines of trees, measured in kilometres;
- Watercourse units for rivers, streams, canals and ditches, measured in kilometres.
The three must not be added together, traded or converted. Each must achieve its own 10% gain where it is present on the site (metric user guide). Planting a large new woodland cannot make up for removing a hedgerow.
How is the baseline habitat survey done?
Every calculation starts with a baseline: a survey of the habitats on the site before development. A competent person, usually an ecologist:
- divides the site into habitat parcels;
- classifies each one using the UK Habitat Classification (UKHab);
- assesses its condition against the criteria in the official condition assessment sheets;
- maps hedgerows and watercourses and assesses them separately;
- records evidence, such as photographs and species lists, to support each judgement.
Surveys must be done at a suitable time of year, because a grassland surveyed in midwinter can look far poorer than it really is.
What are the BNG trading rules?
The trading rules stop a developer from destroying valuable habitat and replacing it with a larger area of something common. They set the minimum type of habitat that must replace each loss:
- Very high distinctiveness (score 8): like-for-like replacement of the same habitat type, or bespoke compensation agreed with the authority;
- High (6): the same habitat type, like for like or better;
- Medium (4): habitat of medium distinctiveness or higher;
- Low (2): habitat of low distinctiveness or higher.
The metric tool flags any trading-rule breach automatically, and an application that breaches the rules cannot claim net gain however many units it shows.
Irreplaceable habitats sit outside the metric
Ancient woodland, ancient and veteran trees, blanket bog and limestone pavement cannot be recreated within a human lifetime, so they cannot be "traded" for units. Losses need bespoke compensation agreed with the planning authority, and are very hard to justify at all (GOV.UK).
What is the Small Sites Metric?
The Small Sites Metric (SSM) is a simplified version of the statutory metric for minor development on sites without priority habitat. It uses a shorter list of habitats and simpler condition assessment, but the same 10% target, the same three unit types and the same trading rules (Small Sites Metric User Guide). Use one tool or the other for a site, never both.
Two 2026 changes matter for small sites. First, new applications on sites of 0.2 hectares or less are now exempt altogether, unless they affect priority habitat. Second, minor developments that are not exempt can now go straight to off-site gains, rather than having to show they tried on-site first. The government decided not to make most of the other proposed changes to the SSM (government response, April 2026).
How does a BNG calculation work in practice?
A simplified example on a 3-hectare site shows the four stages:
- Calculate the baseline. The survey finds 1.5 ha of modified grassland (low distinctiveness, poor condition), 1 ha of lowland mixed deciduous woodland (high, moderate), 300 m of hedgerow and a 200 m ditch. Each parcel is scored with the formula; hedgerow and ditch units are calculated separately by length.
- Map the impacts. Mark which habitats will be lost to buildings and roads, which will be retained and which will be enhanced or newly created.
- Calculate post-development units. Created and enhanced habitats are reduced by the time and difficulty multipliers. Each unit type must reach at least 110% of its baseline value.
- Close any shortfall. If on-site provision falls short, buy units from a registered off-site habitat bank. As a last resort, statutory biodiversity credits can be bought from the government. Off-site gains must be on the biodiversity gain sites register and maintained for at least 30 years (GOV.UK).
The developer then submits a biodiversity gain plan, which the planning authority must approve before work begins.
What are the most common BNG metric mistakes?
- Over-optimistic condition scores: rating a habitat "good" when it fails key criteria, which inflates the baseline and the target.
- Misclassifying habitats: calling modified grassland a species-rich meadow without the species evidence to back it up, or the reverse.
- Ignoring time to target condition: assuming new habitat counts at full value from day one.
- Breaching trading rules: replacing high-distinctiveness habitat with low-value habitat.
- Missing a unit type: leaving out hedgerows or ditches that are on the site.
- Surveying in the wrong season, which undervalues grassland and wetland.
- Using an out-of-date metric version, or mixing the Small Sites Metric with the full metric.
What changed for BNG in 2026, and what's next?
| Change | Status | What it means |
|---|---|---|
| 0.2 hectare exemption | In force, 6 Aug 2026 | New applications on sites up to 0.2 ha are exempt unless onsite priority habitat is affected |
| Temporary development exemption | In force, 6 Aug 2026 | Exempt if the land is reinstated within five years and no priority habitat is affected |
| Self-build exemption removed | In force, 6 Aug 2026 | Self- and custom-build schemes over 0.2 ha now need BNG |
| Minor development hierarchy | In force, 6 Aug 2026 | Non-exempt minor sites can go straight to off-site gains |
| BNG for NSIPs | From 2 Nov 2026 | New development consent order applications must deliver at least 10% gain on the habitats they affect |
| Metric changes (open mosaic habitat; spatial risk based on LNRS areas only) | Announced; timing to be confirmed | Needs the metric to be re-laid before Parliament, with transitional arrangements |
| Further exemptions (conservation projects; parks and playing fields) | In preparation | Second phase of reforms, subject to parliamentary time |
| Brownfield residential exemption | Consulted on; response pending | Consultation closed 10 June 2026 |
Sources: Defra, July 2026; Defra, August 2026; GOV.UK NSIP guidance.
The changes apply to new applications. Applications made before 6 August 2026, and permissions already granted with a BNG condition, stay under the earlier rules. For NSIPs, BNG applies only to the habitats a project affects rather than the whole site, gains can be delivered on- or off-site from the outset, and an outline biodiversity gain plan is needed at application stage.
BNG is one of several tools for nature recovery in England, alongside Local Nature Recovery Strategies and farm schemes. For the rules in full, see our guide to BNG regulations and compliance. For what the research says about the metric's strengths and weaknesses, see the statutory biodiversity metric: strengths and criticisms.
Frequently asked questions about the BNG metric
What is the BNG metric?
The BNG metric, officially the statutory biodiversity metric, is Defra's tool for measuring the biodiversity value of habitats in England in biodiversity units. It is used to show that a development delivers at least a 10% biodiversity net gain.
How do you calculate biodiversity units?
For area habitats, biodiversity units = area in hectares × distinctiveness score × condition score × strategic significance. Created and enhanced habitats are also multiplied by time-to-target-condition and difficulty factors, and off-site habitat by a spatial risk factor.
What are the distinctiveness and condition scores?
Distinctiveness scores are 8 (very high), 6 (high), 4 (medium), 2 (low) and 0 (very low; 1 for hedgerows). Condition scores are 3 (good), 2.5 (fairly good), 2 (moderate), 1.5 (fairly poor) and 1 (poor).
Can hedgerow units make up for lost grassland?
No. Area, hedgerow and watercourse units are calculated separately and cannot be added together or traded. Each unit type present on the site must achieve its own 10% gain.
Is my small development exempt from BNG?
For planning applications made on or after 6 August 2026, sites of 0.2 hectares or less (by red line boundary) are exempt unless the development affects onsite priority habitat. Householder applications and de minimis development are also exempt.
When does BNG apply to nationally significant infrastructure projects?
BNG becomes mandatory for NSIP applications for a development consent order made on or after 2 November 2026. Such projects must deliver at least 10% net gain on the habitats they affect.
What is the Small Sites Metric?
The Small Sites Metric is a simplified version of the statutory metric for minor development on sites without priority habitat. It has the same 10% target and trading rules, with a shorter habitat list and simpler condition assessment.
How long must BNG habitat be maintained?
Habitat created or enhanced for off-site gains, or for significant on-site gains, must be maintained for at least 30 years. Off-site gains are secured by a conservation covenant or a section 106 planning obligation and registered on the biodiversity gain sites register (GOV.UK).
Understand the law behind UK nature
Plain-English guides to biodiversity net gain, protected species and the policies shaping nature recovery in England.
Sources: Environment Act 2021; Statutory Biodiversity Metric User Guide (June 2026); Statutory biodiversity metric tools and guides; Small Sites Metric User Guide; SI 2026/790; GOV.UK, exempt developments; Government response (April 2026); Defra blog, July 2026; Defra blog, August 2026; GOV.UK, BNG for NSIPs; GOV.UK, irreplaceable habitat.
