What Does Mandatory Biodiversity Net Gain Mean?

21 February 2024 by Georgie Tugwell

In a world where environmental concerns are at the forefront of public discourse, countries are increasingly recognizing the importance of safeguarding biodiversity. A significant step towards ecological sustainability is England’s pioneering initiative, Biodiversity Net Gain (BNG). This innovative approach has garnered widespread attention as it seeks to avoid biodiversity loss that is typically associated with development projects and to instead generate net biodiversity gains. As of 12/2/2024, BNG became mandatory in England. Now an integral part of the developmental landscape it requires calculation of biodiversity units on construction sites and the achievement of a total of 10% BNG post-development.


Understanding Biodiversity Net Gain (BNG)

DEFRA and Natural England designed the BNG policy to ensure that development projects not only avoid reducing biodiversity but actively contribute to its enhancement. The concept is simple yet powerful – for every bit of natural habitat lost due to development, an equivalent amount of biodiversity plus an additional 10% must be created on or off the development site. The goal is for England’s net biodiversity to increase with each development that takes place.

To reach this goal, developers must quantify baseline levels of biodiversity on project sites prior to development activities using the Biodiversity Metric 4.0. The biodiversity lost due to developments must then be restored with a BNG of 10% post-development and the policy offers flexibility in how this mandatory target is met. The strategies that can be used are:

⬜ Creating on-site biodiversity

⬜ Creating off-site biodiversity

⬜ Purchasing statutory biodiversity credits from the government

These uplift strategies are not mutually exclusive and a combination can be adopted to achieve BNG, however, on-site or local biodiversity enhancements are preferable. Meeting BNG on-site is prioritised and the metric’s spatial risk multiplier penalises off-site compensation distant from the original development site where biodiversity was impacted. Habitat creation for BNG performed in advance of development is incentivised by not being subjected to such risk multipliers (e.g. spatial or temporal). Only in instances where on and off-site habitat creation or enhancement is not possible, can developers purchase statutory biodiversity credits to achieve BNG; these are sold by Natural England on behalf of the government.


The Biodiversity Metric

Under the BNG framework, developers are required to assess the baseline biodiversity value of a site before any development begins using the Biodiversity Metric 4.0. Assessments require remote analysis, on the ground ecological assessment of habitat type and condition and metric calculations. The metric quantifies the Biodiversity Units present on sites by considering habitat size, distinctiveness, condition and strategic significance and deals with three types of Biodiversity Unit: Area-based Habitat Units, Hedgerow Units and River Units.

The baseline number and type of Biodiversity Units present becomes the reference point against which the success of the project’s ecological impact is measured. Developers must then show a measurable increase in biodiversity due to their habitat creation or enhancement plans. It is important that any habitats lost due to development are replaced on a like-for-like or like-for-better basis especially when it comes to habitat trading.


The Benefits of BNG

What sets BNG apart is its transition from a voluntary measure to a mandatory requirement in England. With this policy now enshrined in law, developers must calculate, deliver, and maintain quantifiable net gains in biodiversity when seeking planning permission for certain projects. This move demonstrates a commitment to prioritizing the preservation and restoration of ecosystems amid the growing urban landscape. BNG will increase the workload of local planning authorities (LPAs) and as such the government has dedicated £16 million in funding to support LPAs through BNG implementation and recruitment of ecologists and specialists that will be key to BNG’s success.

Successful BNG implementation will reap numerous benefits and by fostering healthier ecosystems, this policy contributes to improved air and water quality, enhanced flood resilience, and increased opportunities for communities to engage with nature. Moreover, BNG can benefit businesses and developers, as a commitment to biodiversity will likely enhance a project’s reputation, increase property value, and align with the growing demand for environmentally responsible practices. Engaging stakeholders, from developers to local communities, will be important in ensuring the effective implementation of this policy and its long-term success.



The mandatory implementation of BNG addresses a critical need to reverse the nationwide biodiversity loss which was highlighted in the bleak UK State of Nature Report last year. The policy stands as a beacon of progress in England’s commitment to sustainable development and whilst not a complete solution, the now mandatory policy marks a significant step towards a more biodiverse future.


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