DEFRA Statutory Biodiversity Metric 4.0: How to Calculate BNG
The DEFRA Statutory Biodiversity Metric 4.0 is the mandatory tool for calculating Biodiversity Net Gain in England. Since February 2024, most developments must deliver at least 10% net gain in biodiversity units, calculated using this tool and maintained for 30 years.
How to Use the DEFRA Biodiversity Metric 4.0
The Statutory Biodiversity Metric 4.0 is an Excel-based tool available on GOV.UK. It uses three input modules:
| Module | What It Calculates | | --- | --- | | Area module | Biodiversity units for area habitats | | Linear module | Biodiversity units for hedgerows and watercourses | | Watercourse module | Biodiversity units for watercourses (updated in 4.0) |
Step-by-Step Process
| Step | Action | | --- | --- | | 1 | Baseline assessment — survey existing habitats (size, type, condition) before development | | 2 | Input data into the Excel calculation tool | | 3 | Post-development assessment — estimate biodiversity value after development | | 4 | Calculate net gain: post-development units minus baseline units; must be ≥10% | | 5 | Apply spatial risk multipliers (off-site only) and temporal multipliers | | 6 | Submit Biodiversity Gain Plan with planning application |
Habitat Categories in Metric 4.0
Metric 4.0 introduced new habitat classifications aligned with UKHab:
| New in Metric 4.0 | Description | | --- | --- | | Willow scrub | New area habitat | | Tall forbs | New area habitat | | Rural trees | New area habitat (separate from urban) | | Watercourse footprint | New linear habitat |
Habitat Changes from 3.1 to 4.0
| Old (3.1) | New (4.0) | | --- | --- | | Vacant/derelict land | Split into Vacant or derelict land + Bare ground | | Winter stubble | Winter stubble (cereal crops) | | Sustainable urban drainage feature | Sustainable drainage system | | Urban trees | Individual trees (includes urban + rural) |
Distinctiveness Tiers
Habitats are scored by distinctiveness (low / medium / high) based on rarity and species richness:
| Distinctiveness | Examples | Unit Value | | --- | --- | --- | | Very high | Ancient woodland, species-rich grassland | Highest | | High | Lowland heathland, priority ponds, saltmarsh | High | | Medium | Woodland, hedgerows, ponds (non-priority), amenity grassland | Medium | | Low | Modified grassland, urban gardens, cropland | Lowest |
Condition Assessment
Condition is scored Poor / Moderate / Good based on habitat-specific criteria. Metric 4.0 updated condition sheets for 13 habitat types.
Grassland Condition Criteria
| Criterion | Poor | Moderate | Good | | --- | --- | --- | --- | | Species richness | <5 species/m² | 5–10 species/m² | >10 species/m² | | Invasive species | >30% cover | 10–30% | <10% | | Condition sheet compliance | Multiple criteria failing | Partial compliance | Full compliance |
Biodiversity Units Calculation
The biodiversity unit value depends on:
| Factor | Description | | --- | --- | | Area | Size of habitat in hectares | | Distinctiveness | Rarity and species richness tier | | Condition | Poor / Moderate / Good score | | Strategic significance | High / Medium / Low (set by LPA) | | Spatial risk multiplier | Applies to off-site BNG only | | Temporal multiplier | On-site: 1.0; Off-site: varies |
Spatial Risk Multipliers (Off-Site Only)
| Location | Multiplier | | --- | --- | | Same LPA and NCA | 1:1 | | Same NCA, different LPA | 1.08:1 | | Neighbouring NCA | 1.33:1 | | Further away | Up to 2:1 |
Temporal Multipliers
| BNG Type | Multiplier | | --- | --- | | On-site | 1.0 (no time delay) | | Off-site with immediate delivery | 1.0 | | Off-site with habitat creation delay | >1.0 |
Calculation Example
A development on a 0.5 ha site:
| Stage | Habitat | Area | Distinctiveness | Condition | Units | | --- | --- | --- | --- | --- | --- | | Baseline | Modified grassland | 0.50 ha | Low (4) | Poor | 6.00 | | Post-dev | Green roof (biodiverse) | 0.15 ha | Medium (6) | Good | 2.70 | | Post-dev | Native hedgerow | 0.05 ha | Medium (6) | Good | 0.60 | | Post-dev | Enhanced grassland | 0.20 ha | Low (4) | Moderate | 2.40 | | Net gain | | | | | +1.70 (28% net gain) |
Result: 28% net gain exceeds the 10% minimum requirement.
Strategic Significance
Strategic significance affects the biodiversity unit calculation. LPAs can set strategic significance (Very High / High / Medium / Low) for habitats based on local plan designations.
| Strategic Significance | Effect | | --- | --- | | Very high | Maximum unit value | | High | High unit value | | Medium | Standard unit value | | Low | Reduced unit value |
Check your Local Plan and LPA guidance to confirm strategic significance ratings for your area.
2025 BNG Assessment Costs
| Service | Cost (ex VAT) | | --- | --- | | Baseline BNG Metric calculation | £500–£2,000 | | Full BNG assessment + HMMP | £1,500–£10,000+ | | MRT completion per visit | £200–£500 |
Frequently Asked Questions
Q: Where do I download the DEFRA Statutory Biodiversity Metric?
The official Excel tool is available on GOV.UK. Search for "Biodiversity metric 4.0" or visit gov.uk/guidance/biodiversity-metric-4-0.
Q: Can I use the metric without an ecologist?
You can input data yourself, but habitat classification and condition assessment should be done by a competent ecologist. Incorrect classification can significantly affect the BNG calculation.
Q: What is the difference between Metric 3.1 and 4.0?
Metric 4.0 introduced new habitat categories aligned with UKHab, updated condition assessment sheets, a new watercourse module, and revised strategic significance guidance.
Q: How long is a BNG metric calculation valid?
Habitat data is valid for 6 months from the survey date. If planning submission is delayed, a new survey may be required.
Q: Does the metric calculate off-site BNG differently?
Yes — off-site BNG applies spatial risk multipliers that increase the number of units required based on the distance from the development site to the receptor site.
Q: What is strategic significance and who sets it?
Strategic significance reflects the local value of a habitat based on Local Plan designations and LPA priorities. It is set by the LPA when approving the Biodiversity Gain Plan.
Q: Can the metric handle phased developments?
Yes — the metric can be run multiple times for different phases. Each phase requires separate baseline and post-development assessments.