Biodiversity Net Gain: What It Means for Your Planning Application

Biodiversity net gain (BNG) became mandatory in England for major development on 12 February 2024 and for smaller development on 2 April 2024. The requirement is straightforward in principle: developments must deliver at least a 10% net gain in biodiversity value compared to the pre-development baseline. In practice, understanding whether BNG applies to your project, what it will cost, and what surveys you need is anything but straightforward.

The first question most applicants have is whether their project is exempt. Householder applications (extensions, loft conversions, and outbuildings) are exempt. So are developments that affect less than 25 square metres of on-site habitat and less than 5 metres of hedgerow or other linear habitat, provided no on-site priority habitat is affected. Planning applications made on or after 6 August 2026 for sites of 0.2 hectares or below, measured within the red line boundary, are also exempt, again unless on-site priority habitat is affected. Above that size, new-build houses and other developments requiring planning permission usually fall within scope, including on brownfield sites, because there is no brownfield exemption. A change of use, or a scheme built wholly on existing hard standing, can still qualify for the de minimis exemption if it affects little or no habitat.

The BNG process starts with a baseline habitat assessment. A qualified ecologist visits the site and maps all existing habitats (grassland, hedgerows, trees, scrub, ponds, and even hard standing) against the statutory biodiversity metric published by Defra. Each habitat type is assigned a biodiversity value based on its condition, strategic significance, and distinctiveness. This baseline score is the benchmark against which the 10% gain must be demonstrated.

Achieving the 10% gain typically involves a combination of on-site habitat creation, habitat enhancement, and, where on-site delivery is not feasible, off-site biodiversity units or statutory credits. For minor development applied for on or after 6 August 2026, off-site units carry the same preference as on-site habitat, with statutory credits still the last resort. On-site solutions might include wildflower meadow planting, new hedgerow creation, green roofs, or tree planting. The key is that significant on-site enhancements and off-site gains must be maintained for at least 30 years, secured by a legal agreement (or, for on-site enhancements only, a planning condition).

Timing is critical and this is where many projects get caught out. If your site contains habitats that might support protected species (breeding birds, bats, or great crested newts), the ecological surveys can only be carried out at specific times of year. Bat surveys, for instance, are restricted to May through September. Miss the survey window and your application could be delayed by up to a year. A BNG screening assessment early in the process identifies these timing constraints before they become programme risks.

At Site Intelligence, our BNG Screening and Feasibility report gives you a clear picture of whether BNG applies, what your likely baseline score is, and what surveys and mitigation measures you will need. We use the statutory biodiversity metric and cross-reference with MAGIC Map data, priority habitat inventories, and local nature recovery strategies. The aim is simple: give you the information you need to plan your project without seasonal survey delays blowing your programme.