When and how to measure a habitat or development’s impact on biodiversity.
Applies to England
Biodiversity net gain (BNG) is mandatory from 12 February 2024.
What the biodiversity metric is
The statutory (official) biodiversity metric is the way of measuring biodiversity value for the purposes of BNG. It measures all types of habitat, including:
- watercourses such as rivers and streams
For BNG, biodiversity is measured in standardised biodiversity units.
The statutory biodiversity metric measures the biodiversity value of habitats by calculating the number of biodiversity units. It 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:
- strategic significance
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
The statutory (official) biodiversity metric calculation tool must be used in order to demonstrate that you have calculated the number of biodiversity units for existing habitat or habitat enhancements in accordance with the statutory biodiversity metric.
Who should use the metric tool and why
If your development has to meet mandatory biodiversity net gain (BNG) requirements, it will need to use the metric tool.
The requirement to use the biodiversity metric will take effect at different times, depending on the size of the development. The relevant metric tool should be used for:
- major developments
- small developments from 2 April 2024
- nationally significant infrastructure projects from late November 2025
You can choose to use a simpler version of the metric tool, called the small sites metric (SSM) if your development meets the criteria to do so.
Except for small developments using the SSM, a developer or land manager should hire a competent person such as an ecologist to use the metric tool and advise on the metric tool’s calculations. The local planning authority (LPA) will review submitted calculations and may reject them if they do not believe they have been completed by a competent person.
An ecologist can enter information into the metric tool about the existing habitat and any planned development or enhancement. The metric 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 metric can measure the impact of building houses, planting a woodland or sowing a wildflower meadow.
Early and repeated use of the 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 metric tool
Information to gather before using the metric tool
For full details on how to find the information you need to use the metric tool and how to use it, read the biodiversity metric tool user guide.
You should also consult a professional ecologist, who will use the metric 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 metric tool, your ecologist will need to know:
- the types of habitat, both present and planned
- the size of each ‘habitat parcel’ in hectares or metres squared, or for linear habitats (watercourse and hedgerows) in kilometres or metres
- the condition of each ‘habitat parcel’
- whether the proposed actions for the habitat parcel are mapped in the local nature recovery strategies local habitat map (in draft) or identified in other alternative 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
You must use the statutory biodiversity metric tool for mandatory BNG. 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 are submitting a new planning application and 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. There is information on what has changed and how to copy and paste from version 4.0 to the statutory version.
More information for developers
As a developer, you can use the metric tool to assess:
- the number of biodiversity units your proposed development site 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 metric 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 metric 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.
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 metric tool
The 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 metric tool will tell you where you could create more gains. Then you can make changes to your designs, and use the metric tool again to check your decision.
Within the metric 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. Refer to the planning practice guidance for when you need to submit your metric calculation tool.
Start using the metric tool as early as possible in the process. This will help you make decisions to avoid losses to biodiversity.
For example, the metric tool might calculate that developing on the woodland on your site would cost you 20 units, plus you would need to generate 2 units to achieve a 10% net gain. But developing on modified grassland would only cost you 8 units, plus you would need to generate 0.8 units to achieve a 10% net gain.
The small sites metric
Small developments are only required to achieve BNG from 2 April 2024.
A small development does not fall into the category of a major development, as defined in article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2015.
A small development means:
- residential development where the number of dwellings is between 1 and 9 on a site of an area 1 hectare or less, or if the number of dwellings 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 development, you can use the statutory biodiversity metric tool, or if you prefer, you can use a simpler version of the metric tool, called the small sites metric (SSM).
However, the SSM cannot be used on sites where the following are present:
- priority habitats (excluding some hedgerows and arable field margins)
- protected sites
- European protected species
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 qualify to use the SSM, you do not need an ecologist for BNG, though you may still choose to seek ecological advice. You can either use the the simpler small sites biodiversity metric tool yourself if you are familiar with the site and the habitats on it, or someone else who can do a survey and make a calculation. This could be the project manager, a gardener, or a landscape architect.
The SSM is a simplified version of the metric tool and does not have an off-site section. However, if you are using the SSM you can still use off-site units. If you require any off-site units, you just need to submit your completed SSM to the relevant planning authority as part of your biodiversity gain plan. Your biodiversity gain plan will then need to summarise how you have met your BNG requirement, including details of the off-site habitat enhancements and the trading summary. You should send your completed SSM to the off-site provider you are buying units from. When an allocation is recorded on the biodiversity gain site register, the on-site section of the metric tool should be completed alongside the off-site section.
More information for land managers
If you are a land manager, and considering selling biodiversity units 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 metric tool for all habitat enhancements you are delivering for that development.
You should 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.