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This POSTnote outlines the mandatory biodiversity net gain policy introduced in England in 2024 and the risks and challenges for delivering its objectives.

Documents to download

• ‘Biodiversity net gain’ report (822 KB , PDF)

Mandatory biodiversity net gain (BNG) is a new requirement in addition to existing biodiversity and wildlife planning policy. The 2019 impact assessment for the policy states that developers causing the most environmental damage should face the highest costs to steer development towards the least damaging areas and designs.

A baseline survey maps the size, state and presence of differing patches of vegetation types on a development site using the UK Habitat Classification system. The BNG metric, set out in secondary legislation, is used to determine thevalue of these habitats in “biodiversity units”; the pre-intervention score. Using the metric, biodiversity gain plans demonstrate how the developer will deliver a minimum of a 10% gain in these units; the post development score. These planned units can be on the development site, off the development site or delivered through the purchase of statutory credits as a last resort.

A 2024 NAO report found widespread support for mandatory BNG, but that implementation risks were also being raised. The majority of habitat creation and enhancement is projected to occur on development sites. Researchers suggest this may not deliver optimal outcomes for nature recovery objectives, and more holistic approaches are needed to restoring habitats strategically across landscapes are required. The NAO set out risks local authorities will not be able to discharge legal, compliance and enforcement obligations in relation to BNG, as a result of challenges such as insufficient access to ecological expertise.

Key points

• In 2024, mandatory biodiversity net gain (BNG) was introduced as a planning requirement. This is a new process that requires the habitats lost on a development site to be accounted for and losses addressed.
• On the development site, patches of differing vegetation types, such as woodland, are mapped. Data on their size, ecology and location are used to derive ‘biodiversity units’. This involves the use of metrics set out in legislation and supplied in spreadsheets.
• A biodiversity gain plan must be submitted showing how at least a 10% gain in units will be delivered over a 30-year post-development period.
• This gain is achieved by enhancing or creating the same types of habitats that will be lost. This can be on the development site, on separate registered BNG site, or the purchase of statutory credits as a last resort.
• The policy is also intended to ensure a consistent approach to mitigating impacts on biodiversity within the planning system, provide greenspace and economic opportunities for those providing biodiversity units.
• Academic researchers raise concerns about most habitat creation being projected to occur on development sites. They suggest greater biodiversity gains would likely come from requiring the habitat creation to be in areas of strategic conservation importance throghout landscapes.
• Other concerns raised include ecological skills and capacity gaps, and the enforcement and monitoring of the delivery of biodiversity units.

Acknowledgements

POSTnotes are based on literature reviews and interviews with a range of stakeholders and are externally peer reviewed. POST would like to thank the interviewees and peer reviewers for kindly giving up their time during the preparation of this briefing, including:

Members of the POST Board*

Documents to download

• ‘Biodiversity net gain’ report (822 KB , PDF)