What is BNG, and how it is calculated, secured and monitored?
Last updated: 2 July 2025
Biodiversity net gain (BNG) was introduced by the Environment Act 2021 as a way of enhancing biodiversity and contributing to nature’s recovery in England through the planning process. BNG is designed to be a consistent way of ensuring that nature is restored alongside the delivery of new development. Development needs to have a measurably positive impact (‘net gain’) on biodiversity, compared to what was there beforehand. Or put more simply, that wildlife habitats are left in a better state than they were before development. Note that BNG only relates to habitats, not specific species that may be present.
In England, BNG became a mandatory requirement in February 2024 under Schedule 7A of the Town and Country Planning Act 1990 (as inserted by Schedule 14 of the Environment Act 2021). Scotland, Wales and Northern Ireland are developing their own proposals for securing biodiversity gains in new developments.
BNG is calculated using a statutory biodiversity metric tool (in other words, a standard system of measurement) created by Defra (the Government’s Department for Environment, Food and Rural Affairs) and Natural England. There is also a small sites metric, which is a simplified version of the statutory biodiversity metric, that is used for small developments.
Most developments that apply for planning permission will need to deliver gains in their biodiversity metric score of at least 10%. This means that a development will be required to create an increase of 10% in their metric score, compared with how the site scored before the development. This can be achieved by either creating new habitat or enhancing existing habitat.
All BNG habitats must be secured for at least 30 years.
Some local authorities have adopted a higher BNG standard (above the mandatory 10%), which will be specified in their local plans.
BNG applies to most residential and commercial developments in England, although some very small types of development are exempt from the regulations (eg, extensions to existing homes).
BNG will apply to Nationally Significant Infrastructure Projects from late November 2025.
The metric measures the biodiversity value of a site before a development takes place (the pre-development value) and then estimates the biodiversity value of the site after it has been developed (the post-development value).
This assessment is carried out using the biodiversity metric calculation tool, which converts habitat areas and wildlife features (such as hedgerows and ditches) into standardised biodiversity units. The number of biodiversity units a site contains will depend on its size, condition, distinctiveness and location. The metric measures how many biodiversity units a habitat contains before development takes place (the ‘baseline’) and how many units would be needed to replace any lost habitat plus the mandatory additional 10% BNG.
To find out more about the statutory biodiversity metric and the small sites metric, including how the calculation tools are used, take a look at this guidance produced by Defra.
BNG can be achieved on-site within the red line boundary of the development (the planning application site) and/or off-site by third party ‘donor’ sites. The rules for biodiversity gain require developers to consider on-site BNG first, followed by registered off-site provision, with statutory biodiversity credits as a last resort.
Off-site BNG should be located as close to the development site as possible, and this is incentivised by how the BNG metric works. Developers are able to source off-site BNG units outside the development site and the Local Planning Authority (LPA) area, but the units effectively become more expensive the further away these are located from the development.
Statutory biodiversity credits, provided by the Government at greater cost to the developer, could be located anywhere in the country. The RSPB, alongside a number of other environmental NGOs (non-governmental organisations), is setting up several habitat banks to provide off-site BNG to developers.
The Government provides a mechanism of ‘last resort’ for those who are unable to secure BNG units either on their own development or off-site. These have been intentionally priced high to discourage their use. Find further information on statutory credits here.
A biodiversity gain plan is a plan that demonstrates how a development will achieve its BNG requirement. When planning permission is granted by the LPA, it will include a condition requiring a biodiversity gain plan to be submitted. The LPA must approve a developer’s biodiversity gain plan before the development can begin. Developers are also required to submit their completed biodiversity metric calculations as part of the biodiversity gain plan, to show that their calculations have been carried out correctly.
As with most applications to discharge planning conditions, the LPA is not required to consult any statutory bodies regarding a biodiversity gain plan, or to publicise it before the application is determined.
Land managers or owners have to use an online ‘biodiversity gain sites register’ to record any land that provides off-site BNG units. The register will be used by developers to record where off-site BNG units are linked to their development. The register provides a publicly accessible source of information about off-site BNG, including the baseline biodiversity value of the delivery site and the expected future biodiversity value of that site, as well as who owns it.
Search the biodiversity gain sites register here.
Currently when BNG is provided within the development site it doesn’t need to be recorded on the register. The Government has committed to find ways to link to data held by LPAs, which would enable people to identify and monitor the implementation of on-site BNG, but at present there is no such system in place.
BNG is secured for a minimum of 30 years through a planning condition or legal agreement – either a Section 106 agreement between the LPA and a landowner, or a conservation covenant between a responsible body and a landowner (which establishes that the land will be used for a conservation purpose). A responsible body can be a local authority, a relevant public body/charity or a private sector organisation (where at least some of its main activities relate to conservation).
BNG is monitored by the LPA if it is secured via a Section 106 agreement, or by the responsible body if it is secured via a conservation covenant. If the terms of the condition or legal agreement are not met, the LPA or responsible body can take enforcement action.
BNG is separate to existing wildlife protections and does not change or override these. It is important to note that significant BNG/off-site BNG provision is not acceptable on sites with existing national or international conservation designations (eg, Sites of Special Scientific Interest, Special Areas of Conservation and Special Protection Areas) as there are existing requirements to ensure they are maintained in a favourable condition. BNG also only takes account of biodiversity within the development site, so does not consider ‘indirect’ impacts to other nearby wildlife as a result of, for example, disturbance or pollution effects.
If anything concerns you about a development’s BNG proposals, do get in touch with your local planning authority who are responsible for ensuring its proper implementation.