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Understanding BNG Legislation

Introduced in February 2024, biodiversity net gain is part of UK planning law. The concept is designed to improve the state of the natural environment, and it is an important requirement of planning permission.

A biodiversity net gain plan is a mandatory consideration for local planning authorities in determining planning applications. All planning applications (there are few exemptions) must provide evidence of how they will deliver biodiversity net gain.

Why was biodiversity net gain BNG introduced?

The 2019 `State of Nature’ report highlighted significant declines in the UK’s wildlife, revealing that 41% of species, including birds, mammals, and insects, have decreased since 1970. Habitat loss, intensive agriculture, and climate change were identified as the main reasons for biodiversity loss. Many species are now at risk: 15% face extinction, and insects, which are crucial for pollination, are showing the steepest declines.

The report also found that despite nature conservation efforts, improvements have been limited. While there has been some success in reintroducing endangered species and protecting certain natural habitats, funding for conservation is still insufficient to reverse overall trends. The report called for urgent action to restore natural spaces, reduce pollution, and promote sustainable farming practices to halt the rapid loss of biodiversity and help the UK achieve its conservation targets.

What is biodiversity net gain?

Biodiversity net gain aims to ensure that the condition of the natural world is left in a measurably better state following the completion of a development than it was predevelopment. This is evidenced via a biodiversity net gain plan involving an ecological consultant assessing a development site using a statutory biodiversity metric to provide a predevelopment BNG score. Existing natural habitat will be assessed, and using a biodiversity gain hierarchy, the consultant will create a BNG plan that will preserve as much habitat as possible and protect ecological features. In some situations, it may be necessary to destroy trees that impede development, and relocate species present on the site. To counter this, the BNG plan must replace the elements of the site that are lost to development and increase the biodiversity on site by 10%. The improved site must also be maintained for 30 years, evidenced via a habitat management and monitoring plan.

Mandatory net gain applies to small scale and major development, commercial schemes and nationally significant infrastructure projects. A developer needs to understand statutory biodiversity credits and the current biodiversity metric.  The current Defra biodiversity metric 4.0 replaced the biodiversity metric 3.1. It allows users to calculate the net gain or biodiversity loss resulting from a project, and the opportunity to demonstrate that a biodiversity gain of at least 10% is achievable on conclusion of the development. A small sites metric was introduced in April 2024 and applies to developments where the number of dwellings is between 1-9 on a site measuring one hectare or under. Where the number of dwellings is unknown, the `small site’ area definition is less than 0.5 hectares. On commercial developments, a small scale site is defined as where the floor space created is under 1,000 square metres, or the site area is under one hectare.

Biodiversity net gain legislation

Biodiversity net gain was introduced in the Environment Act which achieved royal assent and became law in November 2021. Prior to that date, environmental protections existed under EU law, but following Brexit in December 2020, these policies did not apply, and the UK needed a new legal framework to protect biodiversity. The policy underwent a two-year transition period, becoming law in February 2024.

The Act includes BNG rules for planning applications, nationally significant infrastructure projects, and details on the biodiversity gain sites register. It also includes details on the main aims of BNG, the importance of on site and off site biodiversity gains and the 30-year element of monitoring all projects.

Biodiversity net gain exemptions 

• Small scale developments: projects impacting less than 25 square meters of non-zero distinctiveness habitat or five square meters of hedgerow.
• Householder applications: small changes, like home extensions, are excluded from BNG requirements, though major redevelopments are not.
• Self-build and custom-build housing: projects with up to nine homes on less than 0.5 hectares.
• High-Speed Rail Network (HS2): infrastructure linked to the HS2 rail network is exempt due to its specific legislative permissions.
• Biodiversity gain sites: projects aimed solely at biodiversity improvements are exempt, preventing them from needing to meet BNG requirements on the same land.​

The mandatory biodiversity net gain principles:

There are ten good practice principles when it comes to achieving biodiversity net gain, according to the Chartered Institute of Ecology and Environmental Management (CIEEM).

1. Apply the mitigation hierarchy to minimise a development’s impact on biodiversity.
2. Avoid a loss of biodiversity that cannot be offset elsewhere.
3. Be inclusive and fair and involve all stakeholders.
4. Address risks and variable elements fully in achieving net gain.
5. A net gain should be measurable.
6. Secure the best outcome for the natural environment.
7. Exceed biodiversity net gain requirements on a site where possible.
8. Create a long-term net gain legacy.
9. Make optimising sustainability a priority.
10. Be transparent in communicating biodiversity net gain outcomes.

The biodiversity net gain calculator

The metric 4.0 requires the following information:

• Habitat type.
• Size of habitat parcels.
• Condition of habitat parcels.
• Locations.

This information is inputted into the calculator to create a biodiversity unit score.

Delivering net gain

During biodiversity net gain assessments, a site’s overall biodiversity value will be split into two categories – hedgerow and habitat. Each biodiversity net gain unit is given a value enabling biodiversity metric calculations to reveal a comparison between pre and post-development.

A conservation covenant

This is a legal agreement between a developer and a responsible body such as a conservation charity, or local planning authority. A conservation covenant may be used to protect a site of strategic significance, ensuring that even if it is sold, the covenant remains in place to protect the habitats present. Landowners or land managers agreeing to a conservation covenant would receive benefits such as off site compensation, financial support and tax benefits for any type of conservation work on the site, which must be maintained for 30 years.

Offsetting and biodiversity credits

Ideally, delivering biodiversity net gain should happen on-site, through improving habitat, habitat creation, planting native trees and shrubs. Failing that, off site delivery of biodiversity net gain is needed. The third option is to buy statutory biodiversity credits or units to enable the required number of biodiversity net gain units to be generated elsewhere.

Biodiversity net gain examples

Contrasting options will exist for delivering biodiversity net gain on different plots of land. A site with few trees, wildlife species or existing habitat would, for instance, be relatively simple to offset and increase the biodiversity value to achieve net gain on completion of the development.

If protected species are found on a development site, it will be more difficult to achieve the necessary net gain. A protected species survey may be needed to gather details and devise a subsequent biodiversity gain plan to protect it. In cases where it would be impossible to avoid disturbing a protected species, resulting in biodiversity loss, the species could be relocated elsewhere, and the licensed ecologist would need to factor this into the bigger plan. Calculations would need to be completed for the site to achieve the same amount of predevelopment biodiversity before work began, and a 10% increase must be achieved on completion.

The mitigation hierarchy must be used to work out the optimal way to achieve the required amount of net gain, by protecting the animal species present, enhancing habitat values and delivering net gain calculated using the statutory biodiversity metric. The hierarchy states that avoiding harm or disruption is the priority, followed by mitigation measures to minimise any impact on the site’s ecological value: this may involve training a workforce about protecting certain species or excluding certain areas of the development from disruption.

Deliver measurable improvements to the natural environment

When an impact on species or wildlife habitats cannot be avoided, an ecological surveyor must as a last resort option, propose the best way to compensate for losses. This may involve relocating species or creating new habitats on-site, off-site, or a combination of both. Biodiversity net gain rules allow for change to take place outside the site in question providing the 10% net gain increase is achieved.