Biodiversity Net Gain: The Small-Sites Exemption in Force since 6 August 2026

What changed on 6 August 2026

Biodiversity Net Gain — the requirement that many developments leave nature measurably better off, by at least a 10% improvement secured for 30 years — has applied to major development since early 2024 and to smaller sites since April 2024. On 6 August 2026 a new exemption took it back off the smallest sites. The change is made by a specific statutory instrument now on the books: the Biodiversity Gain (Town and Country Planning) (Amendments and Transitional Provisions) (England) Regulations 2026 (SI 2026 No. 790), which amends the 2024 exemption regulations and came into force on that date.

The headline is a clean, area-based cut-off. Where the application site — the area inside the red line on the plan — is 0.2 hectares or less, mandatory BNG no longer applies at all. There is no reduced percentage and no simplified metric to run: sites at or below the threshold simply drop out of the regime. Nought-point-two of a hectare is about 2,000 square metres, and the test is the site area, not the number of homes proposed, so it is measured the same way whether the plan shows one dwelling or several.

There is one important carve-out built into the new exemption from the start: it does not apply where on-site priority habitat would be negatively affected. Priority habitats are the specific habitat types listed in national guidance — the kind of thing an ecologist identifies, not something most urban infill plots contain — but where they are present, the small-site exemption falls away and BNG still bites.

Why it changes nothing for a home extension

For the readers this journal is mostly written for — someone extending, converting a loft, digging a basement or refurbishing an existing house — the honest answer is that 6 August changed nothing, because a home extension was never caught by BNG in the first place. Householder planning applications are already exempt, and there is a separate de minimis exemption for development that affects less than 25 square metres of on-site habitat. A rear extension over an existing patio or lawn sits comfortably outside the regime without needing the new rule at all.

So if you have read a headline about BNG being 'scrapped for small sites' and wondered whether it affects your extension, it does not — you were already outside it. The value of understanding the change is in knowing what it is not: it is not a new hurdle, and it is not something your architect now has to price into a straightforward householder scheme. We set out where BNG does and does not reach in our guide to biodiversity net gain for home extensions.

Where it actually bites: small plots and new dwellings

The change matters one step beyond the typical extension: the small new-build. If your London project is not an alteration to the existing house but a new dwelling on a modest plot — an infill site, a backland plot, a plot split off a large garden, a knock-down-and-rebuild on a small footprint — then BNG could previously apply, with a metric to run and a 10% gain to secure and monitor for three decades. Since 6 August, where that site is 0.2 hectares or less and no priority habitat is engaged, that obligation no longer applies. The government's own estimate, set out in its April 2026 consultation response, was that the exemption would take around half of the residential planning permissions previously subject to mandatory BNG — the smallest applications the regime caught — out of the requirement.

Two further details are worth flagging for anyone in that small-development bracket. The dedicated exemption for self-build and custom-build homes was removed at the same time — but in practice most self-build plots are well under 0.2 hectares, so they usually fall within the new area exemption instead. And for minor developments that are not exempt, the way gains can be delivered has been eased, so an off-site solution now sits on an equal footing with creating habitat on the plot itself.

None of this touches the separate costs of building — the statutory planning fee, the Community Infrastructure Levy where a new dwelling is created, and the professional fees. If you are weighing up a small plot, our guide to the Community Infrastructure Levy covers the charge that a new home can still attract even after BNG falls away.

The caveat, and what to check

Two points decide whether the exemption is really yours. The first is priority habitat: if any is present and would be harmed, the small-site exemption does not apply, and that is a question for an ecologist, not an assumption to make from a desk. The second is timing. The new rules apply to applications for planning permission made on or after 6 August 2026; applications submitted before that date continue under the previous BNG legislation, so a scheme that was already in the system before 6 August does not pick up the exemption retrospectively.

For the great majority of our clients the right way to read this change is as reassuring context rather than a call to action: your extension was exempt, and it stays exempt. For the smaller group looking at a new dwelling on a compact London plot, it is a genuine simplification worth confirming early — ideally before an application is submitted, and while the design still has room to respond. As with any fast-moving area of planning law, the position above reflects the regulations as made in July 2026; if you are reading later, confirm the current rules before you rely on them.