BNG Small Site Exemption: What It Means for Developers Building Under 10 Homes

From April 2024, planning applications in England for small schemes of 1-9 dwellings became subject to the mandatory 10% biodiversity net gain requirement, unless an exemption applied. For major developments, this was already priced in. For small sites, the cost and complexity of BNG compliance has been disproportionate: a single plot does not generate enough value to absorb £10,000-£20,000 in ecological surveys, habitat creation plans, and any 30-year management commitments. The government recognised this, and the rules for the smallest sites changed on 6 August 2026.

The government announced an area-based exemption for the smallest sites in December 2025, and it came into force on 6 August 2026. For planning applications made on or after that date, BNG does not apply where the site area within the red line boundary is 0.2 hectares or below. This is a significant threshold. A 0.2-hectare site is 2,000 square metres (about half an acre, or roughly 21,500 square feet), large enough for 2-4 houses in a suburban context, or a single substantial detached home with garden. The red line on the location plan submitted with the application is the starting point for measuring it. For developers and self-builders working at this scale, the exemption removes the BNG requirement, and with it the need for a baseline habitat survey for the metric, biodiversity metric calculations, habitat creation plans, and any 30-year management and monitoring obligations. There is one important condition: the exemption does not apply if the development affects priority habitat on the site, and the same rule applies to the de minimis and temporary development exemptions. A development affects a habitat if it decreases its biodiversity value, so check for on-site priority habitat before claiming the exemption.

The financial impact is material. A typical BNG compliance package for a small site includes a Preliminary Ecological Appraisal (£800-£1,500), a biodiversity metric calculation (£500-£1,000), a habitat creation and management plan (£1,500-£3,000), and potentially off-site biodiversity units if on-site net gain cannot be achieved (£20,000+ per unit in some areas). For a 3-unit scheme with a gross development value of £900,000, BNG compliance costs of £5,000-£15,000 represent a meaningful hit to margin. The exemption removes these BNG costs for qualifying sites, although protected species surveys may still be needed.

The government has also consulted on a targeted BNG exemption for residential brownfield development. The consultation ran from 15 April to 10 June 2026 and sought views on how to define brownfield residential development and on a single area threshold for any exemption, with options ranging from 0.5 hectares to 2.5 hectares. As at October 2026 there is no brownfield exemption: the GOV.UK list of exempt developments, last updated on 6 August 2026, does not include one. A brownfield scheme on a site larger than 0.2 hectares is therefore subject to BNG in full, unless another exemption applies.

For sites that do not qualify for an exemption, the BNG requirement remains in full force. The 10% net gain must be demonstrated through the statutory biodiversity metric (small developments can often use the simpler small sites metric), and any significant on-site enhancements and registered off-site gains must be maintained for at least 30 years, secured through a Section 106 agreement or conservation covenant (or, for on-site enhancements only, a planning condition). One change helps here: for minor development applied for on or after 6 August 2026, registered off-site gains carry the same preference as on-site habitat creation or enhancement, so a small scheme above 0.2 hectares can go straight to off-site units, although harm to any on-site habitat of medium or higher distinctiveness should still be avoided or, failing that, mitigated first. Statutory biodiversity credits remain the last resort and are deliberately expensive: they cost from £42,000 per credit, and two credits are needed for each biodiversity unit.

The interaction between BNG exemptions and other ecological requirements is important to understand. Even if your site is exempt from the 10% BNG requirement, you are still subject to the general duty to protect species under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017. If your site contains bat roosts, great crested newt habitat, or nesting birds, you still need appropriate surveys and licences. The BNG exemption removes the mandatory net gain calculation; it does not remove the obligation to avoid harming protected species.

Timing matters. The Biodiversity Gain (Town and Country Planning) (Amendments and Transitional Provisions) (England) Regulations 2026 brought the changes into force on 6 August 2026, and the new exemptions apply only to planning applications made on or after that date. Applications made before 6 August 2026 stay under the earlier rules, even if they are decided later, and a section 73 variation of a permission applied for or granted before that date does not benefit from the 0.2 hectare exemption. The same date cuts the other way for self-builders: the self-build and custom build exemption no longer applies to new planning applications made on or after 6 August 2026, so a self-build plot larger than 0.2 hectares now needs to deliver BNG unless another exemption applies.

What does this mean in practice? If you are a small developer or self-builder with a site of 0.2 hectares or below, a planning application made now can rely on the exemption, provided no on-site priority habitat is affected. If you made an application before 6 August 2026 that is still undecided, the earlier rules apply to it, so ask the local planning authority whether a fresh application would be worthwhile, particularly if BNG compliance would cost you £5,000 or more. If you have a brownfield residential site above 0.2 hectares, plan on BNG applying, because no brownfield exemption is in force. And if your site falls outside the exemptions, factor BNG into your costs from day one rather than treating it as an afterthought.

Our Site Feasibility Report screens your site against BNG applicability as part of its constraint check. It tells you whether BNG applies, whether you are likely to qualify for an exemption based on site area, what the baseline habitat conditions are from desktop data, and what the likely cost range is for achieving 10% net gain. If you are unsure whether BNG applies to your site or what it will cost, this is the report that answers the question.