A Biodiversity Net Gain (BNG) assessment is a crucial concept for sustainable development practices. This is a way of making sure that development leaves habitats in a measurably better condition than they were before. Under Schedule 7A of the Town and Country Planning Act 1990, introduced by the Environment Act 2021, qualifying developments in England require at least a 10% increase in biodiversity value. This is measured using the statutory biodiversity metric.
| Quick navigation points | |
| 1. Why BNG matters | 2. Which developments are covered |
| 3. How the process works | 4. Protecting biodiversity gains over time |
Development can change or remove habitats, but it can also create space for nature to recover. BNG brings these effects into the planning process so that habitat losses and gains can be measured consistently. The aim is not simply to add landscaping at the end of a project, but to consider biodiversity from the earliest stages of design.
Mandatory BNG applies to many developments requiring planning permission in England. It has applied to major developments since February 2024 and to small developments since April 2024.
From 2 November 2026, the requirement also applies to new applications for nationally significant infrastructure projects. These are large-scale projects, such as major energy, transport, airport, port, water and waste developments, which follow the Development Consent Order process rather than an ordinary local planning application.
Not every development is required to deliver mandatory BNG. Exemptions may apply to:
Other exemptions and transitional arrangements may also apply, depending on the type and timing of the application. For planning applications submitted on or after 6th August 2026, self-build and custom-build developments no longer have their own BNG exemption. However, they may still be exempt if they meet the conditions of another exemption, for example, where the development site is no larger than 0.2 hectares and no onsite priority habitat will be adversely affected.
Being exempt from mandatory BNG does not prevent a developer or landowner from assessing the biodiversity value of a site or choosing to include habitat improvements. A voluntary and proportionate assessment may still support the planning process, inform the design or demonstrate the environmental benefits of a project.
Where BNG applies, the existing habitats are surveyed and their biodiversity value is calculated. The proposed design is then assessed to show the effect of retaining, enhancing, creating or losing habitat. The project must normally demonstrate at least a 10% improvement in each relevant part of the metric.
The preferred approach is to avoid habitat loss wherever possible, then reduce unavoidable impacts and create or enhance habitat. Gains can be provided within the development site, through registered offsite biodiversity units or, as a last resort, through statutory biodiversity credits.
BNG is intended to provide lasting habitat improvements, rather than short-term landscaping. Significant onsite habitat enhancements and offsite biodiversity gains must generally be legally secured, managed and monitored for at least 30 years. The arrangements will depend on the habitats being created or enhanced and may include a habitat management and monitoring plan, planning conditions, a section 106 agreement or a conservation covenant. Considering land ownership, future management, monitoring and funding early in the project helps ensure that the proposed habitats are realistic and can be maintained successfully.
BNG provides a measurable way to understand how development may affect nature and how biodiversity can be protected or improved through the design.
For some developments, achieving at least 10% BNG is a legal requirement. For others, a voluntary assessment can still provide useful evidence, support planning discussions and identify proportionate opportunities for habitat enhancement. Whether BNG is mandatory or voluntary, early consideration of the existing habitats can help produce a clearer, more practical and better-informed development plan.
Related guidance: How BNG assessments and calculations work | BNG for small development sites | August 2026 regulatory update