When and how to measure a habitat or development’s impact on biodiversity.
Applies to England
This draft guidance explains what will apply when BNG becomes mandatory. We will continue to update this guidance.
What the biodiversity metric is
The biodiversity metric is a formula used for measuring biodiversity across all types of habitat, including:
- watercourses such as rivers and streams
For the purposes of biodiversity net gain (BNG), biodiversity is measured in standardised biodiversity units.
The biodiversity metric measures the biodiversity value of habitats by calculating the number of biodiversity units. The formula calculates:
- how many units a habitat contains before development takes place
- how many units are needed to replace the units of habitat lost and to achieve 10% BNG, through the creation or enhancement of habitat
The formula takes different factors into account, including the habitat’s:
For created or enhanced habitats, the formula also takes account of:
- difficulty of creation or enhancement
- the time it takes a habitat to reach its target condition
- distance from the habitat loss
To prove you have accurately calculated the number of biodiversity units for existing habitat, or habitat enhancements to achieve BNG, you must use the calculator called the statutory biodiversity metric tool. This accurately applies the statutory (official) biodiversity metric formula.
Who should use the biodiversity metric tool and why
If your development has to meet mandatory biodiversity net gain (BNG) requirements, it will need to use the statutory biodiversity metric tool.
If your development qualifies as a small site, you can, if you prefer, use a simpler version of the biodiversity metric tool, called the small sites metric (SSM). There is information on the SSM.
The requirement to use the metric will take effect at different times, depending on the size of the development. The relevant tool should be used for:
- major developments when BNG becomes mandatory
- small sites from April 2024
- nationally significant infrastructure projects from late November 2025
Except for small sites using the small sites metric, a developer or land manager must hire a competent person such as an ecologist to use the tool and advise on the tool’s calculations. The local planning authority (LPA) will review submitted calculations.
An ecologist can enter information into the biodiversity metric tool about the existing habitat and any planned development or enhancement. The tool will calculate the biodiversity value of the habitat, and how a development, or a change in land management, will change the biodiversity value. For example, the tool can measure the impact of building houses, planting a woodland or sowing a wildflower meadow.
Early and repeated use of the biodiversity metric tool can help:
- assess a site to find the number of biodiversity units an existing habitat has – its biodiversity value
- compare BNG proposals for a site - such as creating or enhancing habitat, on-site or off-site
- plan habitat management decisions that promote biodiversity
Using the biodiversity metric tool
Information to gather before using the tool
For full details on how to find the information you need to use the tool and how to use it, read the biodiversity metric tool user guide.
You should also consult a professional ecologist, who will use the tool for your project, including gathering, inputting and assessing data. The Chartered Institute of Ecology and Environmental Management publishes guidance on how to find a suitable consultant.
To use the biodiversity metric calculation tool, your ecologist will need to know:
- the types of habitat, both present and planned
- the size of each ‘habitat parcel’ in hectares, or kilometres for linear habitats (watercourse and hedgerows)
- the condition of each ‘habitat parcel’
- whether habitat parcels are in locations identified as local nature priorities in specific local plans or local nature recovery strategies
A ‘habitat parcel’ is a smaller area within a site, which contains one type of habitat. For example, a site may have a parcel of wet woodland habitat and a parcel of priority watercourse habitat.
If you are making an on-site calculation, you will need to include everything that exists within a development’s red line boundary. This includes all features, whether or not they may be lost, retained or improved.
Which version of the metric tool to use
A statutory (official) version of the biodiversity metric tool is available from 29 November 2023. We will provide a summary of what has changed from version 4.0 to the statutory (official) version.
You must use the statutory version of the tool when BNG becomes mandatory. Do not use previous versions (4.0 or before) as these calculations will not be accepted in planning applications.
The metric tool has been used voluntarily in the planning system for several years and there have been several versions of it. All previous versions of the metric tool are published by Natural England.
If you already started calculations in version 4.0 or before, you will need to copy and paste these into the statutory biodiversity metric tool. The outcomes of calculations for most habitats will not change between metric version 4.0 and the statutory version. We will provide a summary of what has changed from version 4.0 to the statutory version.
More information for developers
As a developer, you can use the statutory biodiversity metric tool to assess:
- the number of biodiversity units your proposed development has – its biodiversity value
- how this value could be lost through development
- what you need to do to achieve BNG
You should enter details of your development site into the on-site part of the metric tool. If you haven’t achieved the 10% BNG, you will have to change the site design or consider off-site gains. You can use the tool to work out what off-site gains you would need on other land you own, or if you choose to buy off-site units from a landowner, you can:
- choose an available gain site by exploring the marketplace
- use that site for your calculations with the tool
If you are not able to meet BNG requirements by making on-site or off-site gains, you can buy statutory biodiversity credits as a last resort, after submitting your biodiversity gain plan to your LPA demonstrating why statutory credits are the only option.
There is guidance on statutory biodiversity credits, which explains how to use the statutory biodiversity metric tool to calculate how many statutory credits may be needed.
When to use the statutory biodiversity metric tool
The biodiversity metric tool is not intended to be a one-off step in your design and planning process. It’s advisable to use it repeatedly as you refine your plans. You should aim to get the best possible gains on site, before considering making off-site gains, or (as a last resort) buying statutory credits. The tool will tell you where you could create more gains. Then you can make changes to your designs, and use the tool again to check your decision.
Within the tool, you can save your iterations as versions (for example 1.1, 1.2, 1.3). Submit the final version to the LPA as part of your biodiversity gain plan.
Start using the tool as early as possible in the process. This will help you make decisions and avoid losses to biodiversity.
For example, the tool might calculate that developing on the woodland on your site would cost you 20 units, plus 2 to achieve net gain. But developing on modified grassland would only cost you 8 units, plus 0.8 to achieve net gain.
Small sites: exceptions to having to use the statutory biodiversity metric tool
Small sites are only required to achieve BNG from April 2024.
Small developments are defined as not major developments under the Town and Country Planning (Development Management Procedure) (England) Order 2015. This means:
- residential development where the number of dwellings is between 1 and 9, or if this is unknown, the site area is less than 0.5 hectares
- commercial development where floor space created is less than 1,000 square metres or total site area is less than 1 hectare
- development that is not the winning and working of minerals or the use of land for mineral-working deposits
- development that is not waste development
If your project qualifies as a small site, you can use the statutory biodiversity metric tool, or if you prefer, you can use a simpler version of the biodiversity metric tool, called the small sites metric (SSM).
However, the SSM cannot be used on sites where:
- habitats not available in the SSM are present
- priority habitats are within the development site (excluding some hedgerows and arable field margins)
- European protected species are present on the development site
If you use the SSM, you do not have to do a condition assessment as part of your survey of existing habitat, as the values are fixed for all habitats assessed.
If you use the SSM, you do not need an ecologist. You can use the simpler small sites biodiversity metric tool yourself, or someone who is familiar with the site can do a survey and make a calculation. This could be the project manager, a gardener or a landscape architect. There is also a growing market of service providers offering automated net gain plans using the SSM.
If your small site requires any off-site units, use the SSM to calculate any on-site gains, and submit this as part of your biodiversity gain plan. The land manager for your off-site units will complete the off-site tab of the statutory biodiversity metric calculation tool and submit this when recording the allocation on the biodiversity gain sites register. You will then need to summarise how you have met your BNG requirement in your biodiversity gain plan.
More information for land managers
If you are a land manager, and considering selling habitat to a developer, you must use the metric tool to prove you have accurately measured the biodiversity value of your habitat.
When you have agreed to sell units to a development, fill in the off-site part of the statutory biodiversity metric tool for all habitat enhancements you are delivering for that development. Developers are responsible for submitting information about on-site gains, using the statutory biodiversity metric or the SSM.
You will need to submit a completed metric tool calculation with your application to register your site for off-site gains.
More information for local planning authorities (LPAs)
There is further information for LPAs.