DEFRA Biodiversity Metric 4.0
The statutory Excel workbook for calculating biodiversity unit values before and after development — required for Biodiversity Net Gain compliance under the Environment Act 2021.
What it is
About DEFRA Biodiversity Metric 4.0
DEFRA Biodiversity Metric 4.0 is the statutory calculation tool used in England to quantify the biodiversity value of a site before and after development — and to demonstrate the legally required minimum 10% net gain. The tool is an Excel workbook with separate calculation tabs for habitat parcels, hedgerows, and watercourses. Each parcel is assessed on three dimensions: distinctiveness (how rare or ecologically significant the habitat type is), condition (measured against standardised field assessment criteria from Natural England), and strategic significance (informed by national and local habitat targeting maps). The product of these three factors, multiplied by parcel area, gives the biodiversity unit value. The pre-development total becomes the baseline; the post-development total — accounting for new habitat creation, enhancement, and a temporal discount for the time habitats take to establish — must exceed the baseline by at least 10% to satisfy the mandatory Biodiversity Net Gain condition attached to most planning permissions in England since February 2024. The metric is published and periodically updated by DEFRA and is freely available from gov.uk. A companion Small Sites Metric covers developments below the threshold for the full Biodiversity Metric. The outputs from the metric are submitted to local planning authorities as part of planning applications, biodiversity gain plans, and habitat management plans. Ecologists, environmental consultants, and in-house planning ecologists are the primary users — the tool requires field survey data (Phase 1 habitat surveys and, for condition assessment, more detailed Phase 2 surveys) to populate correctly. Errors in habitat distinctiveness assignment and condition scoring are the most common reasons metric calculations are challenged by local authority ecologists during the planning process.
What you can do with it
Capabilities
Open the Biodiversity Metric 4.0 workbook, navigate to the Habitat tab, and enter pre-development habitat parcels — recording habitat type, distinctiveness rating, condition score, strategic significance, and area in hectares for each distinct polygon from a Phase 1 habitat survey — to produce the baseline biodiversity unit value for a planning application
Input post-development habitat creation and enhancement areas into the metric — including new native woodland planting, wildflower grassland seeding, and enhanced neutral grassland management — and confirm the site achieves at least 10% net gain against the baseline before submitting to the local planning authority
Use the Hedgerow tab to calculate the biodiversity unit value of existing and proposed hedgerow lengths, entering hedgerow type, condition, and strategic significance scores derived from field assessment against the Natural England condition assessment criteria
Apply the metric's temporal discount multiplier to a proposed habitat creation scheme — selecting the appropriate creation or enhancement category, entering the target condition and years to target — and identify which habitat types deliver the strongest discounted unit return per hectare to optimise the biodiversity mitigation hierarchy
Complete the off-site tab to calculate how many biodiversity units a proposed habitat bank or off-site provision delivers, then cross-reference against a development site's deficit to confirm the off-site BNG arrangement closes the statutory 10% gap and document this in the Biodiversity Gain Plan
How to learn it
Learning Resources
DEFRA Biodiversity Metric 4.0 user guide — download free alongside the tool from gov.uk/guidance/biodiversity-metric-calculation-tool; the official guide covers every tab, all distinctiveness and condition criteria, and worked examples
Natural England's biodiversity metric training resources on the CIEEM website (cieem.net) — professional body for ecologists; member training events specifically on metric methodology and condition assessment
CIEEM (Chartered Institute of Ecology and Environmental Management) BNG guidance documents — free to CIEEM members; covers how to apply the metric correctly in different habitat contexts and common mistakes in condition scoring
DEFRA Biodiversity Net Gain practitioner resources at gov.uk/biodiversity-net-gain — includes guidance on habitat management plans, the biodiversity gain plan template, and how LPAs will assess submitted metrics
Pro Tip
Condition scoring is where most BNG calculations are challenged — not distinctiveness. Before going to site, read the full Natural England condition assessment criteria for every habitat type you expect to find. Many ecologists incorrectly assign "Moderate" condition to degraded grassland when "Poor" is correct, inflating the baseline and reducing the apparent net gain. LPA ecologists are increasingly checking condition evidence against the submitted Phase 1 survey map.
Skills that use this tool
Roles that use this tool
Alternatives