A single line in the Autumn Budget documents has changed the planning arithmetic for small urban housing schemes across England. On 28 October 2026, the government confirmed that residential brownfield sites up to 1.5 hectares will be exempt from mandatory Biodiversity Net Gain. The Biodiversity Net Gain brownfield exemption Budget 2026 announcement has been welcomed cautiously by housebuilders and criticised sharply by conservation groups, and it arrives with several practical questions still unanswered.
This article sets out what has actually changed, what previously developed land means in practice, and why a baseline ecological survey may still be sensible even where BNG is no longer a legal requirement.
Key Takeaways
- Budget 2026 confirmed a Biodiversity Net Gain brownfield exemption for residential sites on previously developed land up to 1.5 hectares.
- The commencement date and the secondary legislation needed to bring the exemption into force have not yet been published.
- Priority habitats and protected species on a site are not automatically exempt, even where the site itself qualifies as brownfield.
- The Home Builders Federation expects the change to affect a relatively small number of urban sites; the Wildlife Trusts argue it reduces opportunities for urban nature.
- Developers should still commission a Preliminary Ecological Appraisal before assuming a scheme is BNG-free.
What the Biodiversity Net Gain Brownfield Exemption Budget 2026 Actually Says
The Environment Act 2021 requires most new developments in England to deliver a 10% net gain in biodiversity value, measured using Natural England's biodiversity metric. Since this became mandatory for major development in February 2024 and minor development in April 2024, it has applied to the large majority of residential schemes, regardless of site history.
The Budget 2026 measure carves out an exception. Residential developments on brownfield land, defined broadly as previously developed sites, will not need to meet the mandatory 10% uplift provided the site area is 1.5 hectares or less.
HBF chief executive Neil Jefferson welcomed the move but was careful not to overstate it, saying it "will have a limited impact and benefit a relatively small number of urban sites." That is a fair reading. Many urban infill schemes already sit below 1.5 hectares, so the exemption formalises a position some smaller sites were close to in practice, rather than opening a wide new category of exempt development.
The Wildlife Trusts took a different view, reported by Albion Environmental, arguing the change reduces access to nature in towns and cities and removes chances to create or restore trees, ponds and small parks within developments. Brownfield sites, despite their name, often support open mosaic habitat and species that have colonised derelict land, and these are not automatically low-value from a biodiversity perspective.
Commencement Still Pending
No commencement date has been published for this exemption, and the secondary legislation needed to implement it has not yet been laid before Parliament. Developers should treat the Budget announcement as a statement of policy intent rather than a change they can rely on today. Current BNG requirements under the Environment Act 2021 continue to apply in full until the relevant regulations come into force.
How This Fits With the Earlier 2026 Defra Reforms
The brownfield exemption did not appear in isolation. Defra consulted earlier in 2026 on a targeted brownfield residential exemption, testing thresholds as high as 2.5 hectares before settling, via the Budget, on 1.5 hectares.
That same consultation round produced other confirmed changes in April 2026:
- A 0.2-hectare area-based exemption for small sites generally, with priority habitats on those sites still protected.
- Removal of the self-build and custom-build exemption that had previously let individual plot-builders avoid BNG obligations.
- Off-site biodiversity gains placed on an equal footing with on-site gains for minor development, removing the previous preference for on-site delivery.
Taken together, these changes show a government trying to reduce BNG's burden on the smallest and most marginal sites while keeping the 10% uplift requirement as the default for everything else.
Which Sites Qualify as Previously Developed Land
"Brownfield" is not a precise legal term on its own; the relevant definition is previously developed land, as used in the National Planning Policy Framework. Broadly, this covers land that is or was occupied by a permanent structure, including the curtilage of that structure, excluding land that has reverted to nature through long-term disuse.
| Likely to count as brownfield | Usually excluded |
|---|---|
| Former industrial or commercial sites | Agricultural and forestry land |
| Disused car parks and garage courts | Private residential gardens (in most contexts) |
| Old institutional buildings (schools, hospitals) | Land that has naturally reverted to wild habitat |
| Vacant infill plots with prior built use | Parks and recreation grounds |
This is a summary only. Site-specific status under the NPPF and local plan policy should be confirmed with the local planning authority before any scheme relies on the exemption.
Why a Preliminary Ecological Appraisal Still Matters
Falling under the Biodiversity Net Gain brownfield exemption Budget 2026 threshold does not switch off every ecological obligation. Three things still apply regardless of site size or history.
Protected species law. Bats, great crested newts, nesting birds and other protected species remain protected under separate wildlife legislation, irrespective of BNG status.
Priority habitats. Where a brownfield site supports a recognised priority habitat, such as open mosaic habitat on previously developed land, that habitat typically retains protection even under the small-sites exemption framework.
Local plan and planning condition requirements. Many local authorities have their own biodiversity or green infrastructure policies that sit alongside, and sometimes exceed, the national BNG regime. A planning condition requiring habitat enhancement can still apply to an exempt site.
A Preliminary Ecological Appraisal, typically costing a few hundred to low thousands of pounds depending on site size, identifies these risks early. Skipping it to save cost on a small brownfield site can expose a developer to a late-stage survey requirement, a refused reserved matters application, or a protected species mitigation licence delay, any of which cost far more than the original appraisal.
"This move will have a limited impact and benefit a relatively small number of urban sites.", Neil Jefferson, Home Builders Federation
Implications for the BNG Credit and Unit Market
The statutory biodiversity credit scheme and the private market in off-site biodiversity units have grown around the assumption that most residential schemes need to buy gains. Removing small brownfield sites from that requirement will modestly reduce demand at the smaller end of the market, though HBF's own assessment suggests the practical effect is limited, since many such sites were already close to self-sufficient in their metric calculations.
Equal footing for off-site and on-site gains on minor development, confirmed in April 2026, is likely to have a larger market effect than the brownfield exemption itself, as it removes the previous bias towards costly on-site habitat creation on constrained urban plots.
Practical Checklist for Developers and Agents
- Confirm the site meets the previously developed land definition under NPPF, not just a general "brownfield" label.
- Measure the site area precisely; 1.5 hectares is a hard cut-off once the exemption commences.
- Check whether the local planning authority applies additional biodiversity policy beyond national BNG requirements.
- Commission a Preliminary Ecological Appraisal before assuming no survey is needed.
- Watch for the commencement regulations before relying on the exemption in a planning submission.
- Factor the OBR's housing supply forecast into scheme timing; the OBR expects net housing additions to fall to around 230,000 in 2025/26 and 190,000 fewer cumulative additions by 2030/31 than projected in March, a backdrop against which the government is trying to ease small-site costs.
Frequently Asked Questions
Is the Biodiversity Net Gain brownfield exemption Budget 2026 already in force?
No. The Budget confirmed the policy, but the secondary legislation and commencement date have not yet been published.
Does 1.5 hectares refer to the whole site or the developable area?
This detail is expected to be clarified in the secondary legislation. Developers should not assume either interpretation until the regulations are published.
Can a brownfield site over 1.5 hectares get partial exemption?
Based on the current announcement, the exemption applies to sites up to the threshold; schemes above it would continue to fall under mandatory BNG in the usual way, subject to final drafting.
Do protected species surveys stop being necessary on exempt sites?
No. Protected species legislation operates separately from BNG and continues to apply regardless of the site's BNG status.
How does this interact with the self-build exemption removed in April 2026?
They are separate changes. The self-build and custom-build BNG exemption has been removed, while the new brownfield exemption applies specifically to qualifying previously developed residential sites up to 1.5 hectares.
Conclusion
The Biodiversity Net Gain brownfield exemption Budget 2026 gives small urban schemes a narrower path through biodiversity regulation, but it is not yet law and it does not remove every ecological obligation from a site. Developers, land agents and ecological consultants should keep tracking the secondary legislation, continue commissioning baseline ecological surveys where there is any uncertainty, and check local plan policy before treating a scheme as BNG-free. Anyone assembling a small brownfield site for residential development should speak to a planning consultant or ecologist before finalising a scheme design around this exemption.
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BNG Brownfield Exemption Quick Checker
Previously developed land (brownfield)
Greenfield / agricultural / reverted land
No known priority habitat / species
Yes, present or suspected
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Biodiversity Net Gain, BNG brownfield exemption, Budget 2026, Environment Act 2021, previously developed land, ecological appraisal, Home Builders Federation, biodiversity credits, housing policy England, planning conditions, protected species survey, urban development