The rules on biodiversity net gain (BNG) are changing again, and the 2026 reforms are the biggest shift since BNG became mandatory. The headline for small developers is a new 0.2 hectare area-based exemption that the government expects to lift the BNG requirement from around half of the residential planning permissions that currently need it. At the same time, the separate exemption for small-scale self-build and custom-build homes is being removed. This guide explains what is changing and what it means for your sustainability statement.
A quick recap: what BNG requires
Under the Environment Act 2021, most new development in England must deliver a measurable 10% biodiversity net gain — leaving the natural environment in a measurably better state than before. Gains are quantified with the statutory biodiversity metric, delivered on-site where possible, off-site where not, and only as a last resort through statutory biodiversity credits. Since April 2024 the requirement has applied to small sites as well as major development.
The new 0.2 hectare exemption
Following a review of the first two years of BNG, the government confirmed a package of changes in April 2026. The central measure is a new area-based exemption: development on a site below 0.2 hectares (roughly 2,000 square metres) that meets the qualifying conditions will be exempt from the mandatory BNG requirement, with the change expected to take effect from the end of July 2026.
The government estimates this single change will exempt around 50% of the residential planning permissions that would otherwise have needed to deliver BNG — a significant simplification for householders and small builders, whose plots frequently fall under that threshold. The stated aim is to focus BNG effort where it delivers the most ecological value, rather than on very small parcels where the administrative burden can outweigh the gain.
The self-build exemption is being removed
Balancing that, the government is removing the existing exemption for small-scale self-build and custom-build development. Until now, individual self-builders could sit outside the BNG regime; going forward they will be treated like other small sites and assessed against the same area-based threshold. In practice, most modest self-build plots will still fall under 0.2 hectares and so may be exempt on area — but the automatic self-build carve-out no longer applies, so each scheme has to be checked on its own footprint. Our earlier overview of the 2026 BNG exemption changes sets out the wider package.
What this means for your sustainability statement
Exempt from BNG does not mean exempt from demonstrating sustainability. A planning sustainability statement still needs to address the full picture — energy, water efficiency, materials, and biodiversity or green infrastructure — and many local plans and validation checklists require ecological enhancement regardless of the statutory BNG position. Key points to get right in 2026:
- Confirm the exemption properly. Measure the true red-line site area and check it against the 0.2 hectare threshold and the qualifying conditions before claiming exemption — do not assume.
- Evidence it. Where you rely on the area-based exemption, state the site area clearly and show why the exemption applies.
- Still design in nature. Even exempt schemes benefit from native planting, bird and bat boxes, hedgehog highways and permeable, biodiverse landscaping — councils increasingly expect it, and in London the Urban Greening Factor applies separately.
- Watch the wider reforms. BNG is also being extended to nationally significant infrastructure projects from 2 November 2026, part of a steady tightening of the regime overall.
Does the exemption help or hurt applicants?
For most small residential schemes the 0.2 hectare exemption is a welcome reduction in cost and complexity: no metric calculation, no 30-year management plan, no habitat monitoring where the site genuinely qualifies. The trade-off is that boundaries matter more than ever — a plot marginally over the threshold, or one that fails a qualifying condition, is back in the full regime. Careful site-area measurement at the outset is the single most valuable step.
How Fortress Associates can help
Fortress Associates prepares automated sustainability statements for planning applications across every UK local authority. The report is free and ready in minutes, and covers energy, water efficiency (the 110 litres-per-person-per-day calculator), biodiversity net gain, urban greening and BREEAM where required. We will confirm whether your site qualifies for the new BNG exemption and make sure the statement evidences it correctly. See our services or get in touch to discuss a small-site scheme.
Sources and further reading
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