Delivering 10% biodiversity net gain (BNG) on a small site can feel daunting when space is tight, but with the right strategy it is very achievable — and often cheaper to design in from the start than to bolt on later. This guide walks through what the 10% requirement means on a small plot, the gain hierarchy to follow, practical on-site measures that score well, and the important 2026 small-site exemption you should check before doing anything else.
What 10% net gain means on a small site
Under the Environment Act 2021, most development in England must leave biodiversity measurably better than before — a minimum 10% net gain, secured and maintained for at least 30 years. Gain is measured with the statutory biodiversity metric, which scores habitats by type, condition and size before and after development. On a small site the pre-development baseline is usually modest (a garden, hardstanding or amenity grass), which cuts both ways: the target is smaller in absolute terms, but so is the space you have to create replacement habitat.
Check the 2026 small-site exemption first
Before commissioning a metric, check whether you even need to. The government is introducing a new 0.2-hectare area-based exemption, expected to take effect around the end of July 2026, which its own analysis estimates will remove the mandatory 10% BNG requirement from roughly half of residential planning permissions that previously needed it. Exempt sites still fall under the general environmental protections of the NPPF, but do not have to demonstrate net gain. Our companion post on what a sustainability statement is sets this change in its wider context. If your site is above the threshold, the guidance below applies.
Follow the biodiversity gain hierarchy
The metric rewards a clear order of preference:
- Avoid harm to existing valuable habitat — retaining a mature tree or hedgerow is worth far more than recreating it.
- Create or enhance on-site — deliver as much of the gain as possible within the red line.
- Deliver off-site — use land elsewhere or buy off-site units where on-site space runs out.
- Statutory credits — a last resort, deliberately priced to discourage reliance on them.
Practical on-site measures for small plots
Small sites can score surprisingly well with careful design:
- Retain existing features. Keeping a hedgerow or established tree protects a high-value baseline and avoids a deficit you would otherwise have to make up.
- Native planting. Species-rich native hedgerows, shrubs and wildflower areas score better than amenity grass or ornamental beds.
- Green roofs and living walls. On tight urban plots these add habitat area where ground space is scarce, and lift an Urban Greening Factor score too.
- Small habitat features. Bird and bat boxes, hedgehog highways, log piles and rain gardens add ecological value and read well to planners.
Condition matters as much as area: a small patch of well-managed, species-rich habitat can out-score a larger area of poor grassland in the metric.
Off-site units and statutory credits
Where the numbers still fall short, off-site units — increasingly given equal preference to on-site creation for minor development under the 2026 changes — let you buy gain from a habitat bank, secured through a legal agreement. Statutory credits remain the final fallback, priced to sit at the bottom of the hierarchy.
Evidencing BNG in your sustainability statement
Whatever route you take, the application needs to show its working: a completed biodiversity metric, a biodiversity gain plan, and the mechanism securing the habitat for 30 years. This is normally summarised within the sustainability statement and supported by an ecologist's assessment. Getting the baseline survey done early — before clearing any vegetation — is the single most important step, because a stripped site resets the baseline against you. The carbon and energy side of the scheme is handled separately in an energy statement.
How Fortress Associates can help
Fortress Associates prepares automated, site-specific sustainability statements for planning applications across every UK local authority, and the report is free. We set out biodiversity net gain alongside water efficiency, urban greening and BREEAM where required, so your small-site application arrives with its environmental case made clearly. Start on our sustainability statement page, reach us through the contact page, or see our full range of services.
Sources & further reading
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