What is the biodiversity metric?
The biodiversity metric is a standardised calculation tool published by DEFRA that converts habitat information into a single number: biodiversity units. It provides a consistent, repeatable way to measure the biodiversity value of any piece of land — before development, after development, and on any proposed off-site compensation land.
The current version is the Statutory Biodiversity Metric 4.0, published in November 2023. It is the only metric that can legally be used for mandatory BNG calculations in England. DEFRA distributes it as an Excel spreadsheet (.xlsm file), though the underlying calculations can also be performed by third-party software.
There is also a Small Sites Metric — a simplified version designed for smaller developments. Both tools and their user guides are available from GOV.UK.
The core formula
At its heart, the metric multiplies several factors together to produce a biodiversity unit value for each parcel of habitat. The formula for area-based habitats is:
For hedgerows and watercourses, the formula is identical except that length (km) replaces area as the base measure. This means the metric produces three separate types of biodiversity unit:
- Area habitat units — for all land-based habitats
- Hedgerow units — for hedgerow habitats (measured in km)
- Watercourse units — for rivers, streams, and ditches (measured in km)
These three unit types are not interchangeable. A development that destroys hedgerow must compensate with hedgerow units — it cannot substitute area habitat units.
The six input factors
Each factor in the formula captures a different aspect of the habitat's ecological value or the difficulty of replacing it.
1. Area or length
The physical size of the habitat parcel: measured in hectares for area habitats, or kilometres for hedgerows and watercourses. This is the base measure that everything else multiplies against.
2. Distinctiveness
A measure of how ecologically rare, important, or irreplaceable the habitat is. Each habitat type has a fixed distinctiveness score assigned by DEFRA:
| Tier | Score | Example habitats | Trading tier |
|---|---|---|---|
| Very High | 8 | Blanket bog, limestone pavement, ancient woodland | A5 |
| High | 6 | Reedbeds, wet woodland, lowland meadows, native pine | A4 |
| Medium | 4 | Mixed scrub, traditional orchards, most grasslands | A3 |
| Low | 2 | Modified grassland, introduced shrub, intensive gardens | A2 |
| Very Low | 0 | Developed land, sealed surfaces, bare ground | A1 |
The distinctiveness score determines the trading tier — which controls what kind of habitat can compensate for its loss.
3. Condition
The current ecological quality of the habitat. An ecologist assesses this during the baseline survey using DEFRA's condition assessment criteria for each habitat type.
| Condition | Score | What it means |
|---|---|---|
| Good | 3 | The habitat is in excellent ecological condition, meeting all assessment criteria |
| Moderate | 2 | The habitat meets some but not all criteria — there is room for improvement |
| Poor | 1 | The habitat is degraded and meets few or no criteria |
Condition matters commercially: a poorly-maintained grassland scores fewer units than the same grassland in good condition. This means landowners who improve habitat condition before selling units can generate higher returns.
4. Strategic significance
Whether the habitat is located in an area identified as a priority for nature recovery. This is increasingly linked to Local Nature Recovery Strategies (LNRS), which are being adopted across England during 2025–2026.
| Category | Multiplier | When it applies |
|---|---|---|
| High | 1.15 | Within an area identified in an LNRS or other strategy as a priority for the habitat type |
| Medium | 1.10 | Within a strategy area but not specifically identified for this habitat type |
| Low | 1.00 | Not within any identified priority area |
5. Difficulty multiplier
Reflects how hard it is to create or restore the habitat type. Some habitats are straightforward to establish (e.g., mixed scrub), while others are extremely difficult and uncertain (e.g., lowland raised bog). The difficulty multiplier only applies to proposed/created habitats, not to baseline.
Values range from 1.0 (easy — no discount) down to 0.1 (very difficult — heavy discount on the units you can claim). Each habitat type has a fixed difficulty score in the DEFRA lookup tables.
6. Temporal multiplier
Accounts for the time it takes for a newly created habitat to reach its target condition. A habitat that takes 30 years to mature is worth less today than one that reaches target condition in 3 years.
The temporal multiplier only applies to proposed/created habitats and is calculated from the years to target condition value for each habitat type. Values range from 1.0 (immediate) down to approximately 0.33 (32+ years). This uses a discounting approach that reflects the ecological principle that delayed habitat delivery is less valuable than immediate delivery.
A worked example
Imagine a development site with 2 hectares of modified grassland in moderate condition:
To achieve 10% BNG, the post-development habitat value must be at least 17.60 units (16.00 × 1.10).
If the development covers 1.5 hectares and retains 0.5 hectares of grassland enhanced to good condition, plus creates 0.3 hectares of new mixed scrub:
The on-site delivery produces 7.34 units against a 17.60 target — a shortfall of 10.26 units that must be sourced off-site or via statutory credits.
On-site vs off-site calculations
The metric has separate tabs for on-site and off-site calculations:
- On-site baseline — what's on the development site now
- On-site post-intervention — what will remain or be created on the development site
- Off-site baseline — what's on the compensation site now
- Off-site post-intervention — what habitats will be created on the compensation site
Off-site calculations include an additional factor: the Spatial Risk Multiplier (SRM). This discounts units delivered further away from the development site to reflect the greater ecological risk of distant compensation. For statutory credits, the SRM is a fixed 2x — you must buy two credits for every unit of shortfall.
Headline results
The metric's Headline Results sheet aggregates everything into a summary showing:
- Total on-site baseline units vs post-intervention units
- Total off-site baseline units vs post-intervention units
- Combined net unit change across all habitats
- Overall percentage change (must be ≥ 10% to pass)
- Unit shortfall summary by tier — showing exactly how many off-site units or credits are needed, broken down by distinctiveness tier
Common mistakes to avoid
- Clearing the site before survey: If you clear habitat before your baseline assessment, the LPA may require you to demonstrate what was there before — and the pre-clearance value will be used as the baseline, not the cleared site.
- Ignoring the trading rules: You cannot offset loss of high-distinctiveness habitat with low-distinctiveness creation. The metric enforces like-for-like or better matching.
- Forgetting hedgerows and watercourses: These are assessed separately and their units cannot be substituted with area habitat units.
- Underestimating temporal discounting: Habitats that take decades to establish are heavily discounted. This can create much larger shortfalls than expected.
- Not engaging an ecologist early enough: The baseline survey drives everything. Starting late means less time to optimise your BNG strategy.