Autumn Budget Brownfield Biodiversity Net Gain Exemption: What the 1.5-Hectare Carve-Out Means for Ecology Surveys, Previously Developed Land Disputes and Expert Ecological Evidence (November 2026)

The Autumn Budget of 28 October 2026 confirmed that residential developments on brownfield sites of up to 1.5 hectares in England will be exempt from mandatory biodiversity net gain [1][2]. Planning Resource described it as a 650 per cent increase in the maximum size of brownfield site that can escape the 10 per cent gain requirement, measured against the 0.2-hectare small sites exemption that took effect in August 2026 [3]. The brownfield biodiversity net gain exemption is not yet law, no commencement date has been set, and the Government has said its full consultation response and impact assessment will follow "in due course" [4]. That gap between announcement and regulation is where the next wave of ecology survey disputes will be fought.

This article sets out what was announced, what remains uncertain, and why the exemption will make expert ecological evidence more important, not less.

Key Takeaways

• The Autumn Budget on 28 October 2026 confirmed a brownfield biodiversity net gain exemption for residential schemes on previously developed land of up to 1.5 hectares in England [1][4].
• The exemption is a policy commitment, not a live rule. It needs secondary legislation, and the consultation response that will define "brownfield residential development" has not been published [4][5].
• The 0.2-hectare small sites exemption has applied since August 2026, subject to conditions protecting priority habitat [6]. From 2 November 2026, BNG is mandatory for Nationally Significant Infrastructure Projects [7].
• Protected species law is untouched. Bats, badgers, reptiles, nesting birds and open mosaic habitat on brownfield land still require surveys and, where needed, licences [1].
• Whether a site qualifies as previously developed land, and whether it supports priority habitat, will be contested at planning committee, at appeal and in judicial review. Expert ecological evidence will decide those arguments.

What the Autumn Budget Said About the Brownfield Biodiversity Net Gain Exemption

The Budget documents framed the change as part of the Government's brownfield-first approach to housebuilding. Residential brownfield developments on sites of up to 1.5 hectares are to be exempted from BNG requirements, with the stated aim of reducing costs and simplifying planning for smaller urban schemes [4]. The Local Government Association's Budget briefing recorded the same commitment and noted that the full consultation response, impact assessment and a timeline for the exemption would be published later [4].

The announcement follows a Defra consultation that ran from 15 April to 10 June 2026. That consultation asked what size thresholds should apply and how brownfield residential development should be defined. As of 2 September 2026, Defra's position was that it had received a large number of responses and was still analysing them [5]. Commentary in the ecology sector reports that thresholds between 0.5 and 2.5 hectares were canvassed, with 1.5 hectares the figure now adopted [1].

"The 1.5-hectare exemption's precise scope and implementation timetable have not been finalised." Envirotec Magazine, 2 November 2026 [8]

Three points matter for anyone instructing or acting as a biodiversity surveyor:

1. The exemption is residential only. Commercial, mixed-use, phased and conversion schemes may or may not qualify, depending on the draft regulations [1].
2. The statutory definition of previously developed land for BNG purposes will be set in the consultation response and secondary legislation. It may not simply copy the National Planning Policy Framework glossary definition [1].
3. Until a statutory instrument is made and commenced, the current rules apply in full. Submitting an application on the assumption that the exemption already exists risks refusal [1].

How the New Exemption Fits With the August 2026 BNG Changes

The brownfield biodiversity net gain exemption is the third major relaxation of the regime in 2026. The Biodiversity Surveyors team covered the earlier changes as they happened, including the 0.2-hectare small sites exemption that came into force in August 2026 and the original brownfield exemption consultation.

Measure	Status in November 2026	Who it affects
0.2-hectare small sites exemption	In force since August 2026, subject to priority habitat conditions [6]	Minor residential and small commercial schemes
Self-build and custom-build exemption	Removed in the August 2026 amendments [6]	Self-builders now rely on the 0.2-hectare rule
BNG for Nationally Significant Infrastructure Projects	Mandatory from 2 November 2026 [7]	Promoters of DCO schemes
1.5-hectare brownfield residential exemption	Announced 28 October 2026; regulations and commencement date awaited [4]	Residential schemes on previously developed land

The August amendments also set a pattern that the brownfield exemption is likely to follow: exemptions are conditional. The 0.2-hectare rule does not apply where a site contains priority habitat, which is why the priority habitat Phase 1 survey remains essential before claiming the small site exemption. Brownfield land is more likely, not less, to carry that kind of condition, because open mosaic habitat on previously developed land is itself a priority habitat.

Why the Brownfield Biodiversity Net Gain Exemption Will Not End Ecology Surveys

Critics of the policy made the ecological point clearly. Penny Simpson of Freeths warned that brownfield sites "can be rich in biodiversity" and that the exemption could weaken demand for biodiversity units that private investment in nature recovery relies on. The Wildlife Trusts and the Bumblebee Conservation Trust raised concerns about the loss of flower-rich habitat and the fragmentation of urban habitat networks [8].

Whatever view a reader takes on the policy, the legal consequence is the same. An exemption from BNG removes the duty to deliver a 10 per cent measurable gain under Schedule 7A of the Town and Country Planning Act 1990. It does not remove:

• The Wildlife and Countryside Act 1981 protections for nesting birds, reptiles and other listed species [1].
• The Conservation of Habitats and Species Regulations 2017 and the need for a mitigation licence where bats or other European protected species are affected [1].
• The Protection of Badgers Act 1992.
• Local plan policies on biodiversity, green infrastructure and trees, and the biodiversity duty on public authorities under the Natural Environment and Rural Communities Act 2006.
• Validation requirements. Local authorities continue to ask for compliant ecological survey reports with applications, and incomplete submissions are a common cause of delay [1].

In practice, a 1.2-hectare former depot in an urban area will still need a Preliminary Ecological Appraisal, a bat roost assessment of any buildings, reptile and invertebrate consideration where there is rubble, bare ground or ruderal vegetation, and a check for open mosaic habitat. The difference is that the outputs feed into protected species and policy compliance rather than a statutory biodiversity metric calculation.

Previously Developed Land Disputes and Expert Ecological Evidence

Three questions that will be argued

The exemption creates three threshold questions, each of which can be contested by objectors, parish councils, neighbouring landowners and local planning authorities:

1. Is the site previously developed land within the statutory definition? Gardens, land with only temporary structures, and sites that have blended back into the landscape are notoriously contested under the NPPF definition, and the BNG definition may differ [1].
2. Is the site 1.5 hectares or less? Red-line boundaries, phased applications and access land will be scrutinised to see whether a larger scheme has been split to fall under the threshold.
3. Does the site contain priority habitat or features that take it outside any conditions attached to the exemption? Open mosaic habitat on previously developed land is the obvious candidate [6].

Where ecologists give expert evidence

These questions are decided on evidence, and the evidence is ecological. Biodiversity surveyors will increasingly be asked to act as expert witnesses in:

• Planning appeals under section 78 of the Town and Country Planning Act 1990, where inquiries follow a formal expert-witness format with proofs of evidence and cross-examination.
• Judicial review of permissions granted or refused on the basis of an exemption, where the court tests whether the authority had adequate ecological evidence before it.
• Enforcement cases where a developer relied on an exemption that did not apply.
• Civil claims between landowners, promoters and consultants over survey scope, missed habitat and the cost of statutory credits.

In each setting the principles of CPR Part 35 and Practice Direction 35 apply directly in the civil courts and by analogy before inspectors. The expert's duty is to the tribunal, not the client. Reports must state the facts and assumptions relied on, set out the survey methods and their limitations, and identify the range of opinion. A UK Habitat Classification survey carried out outside the optimal season, or a metric calculation based on condition assessments the surveyor did not personally make, will be exposed under cross-examination.

What a defensible brownfield ecology report looks like in November 2026

• Survey dates, weather and seasonal constraints stated openly, with any limitations explained.
• A clear statement of whether open mosaic habitat or other priority habitat is present, with the criteria applied.
• A baseline biodiversity metric calculation retained even where an exemption is expected, so that the position is documented if the exemption is refused or delayed.
• Protected species assessments and, where needed, licensing advice separated from the BNG analysis.
• A statement of truth and declaration of independence in the form expected by PD 35, so the report can be deployed at appeal without being rewritten.

What Developers, Landowners and Local Authorities Should Do Now

For developers with brownfield residential schemes below 1.5 hectares, the sensible course is to keep preparing applications under the current rules while tracking the regulations. Those with schemes already in the system should ask their ecologist to document the baseline now, because the statutory credit costs and biodiversity unit pricing remain the fallback if the exemption does not arrive in time.

Local planning authorities should review validation checklists so that applicants claiming any exemption are asked for the survey evidence that supports it. Several authorities have already called for fully compliant environmental survey reports to avoid validation delays [1].

Landowners and habitat bank operators should note the market effect. Fewer small residential schemes buying units may soften demand, while the NSIP duty that began on 2 November 2026 brings large new buyers into the market. The net effect on unit prices is uncertain [8].

Frequently Asked Questions

Is the 1.5-hectare brownfield biodiversity net gain exemption in force now?

No. It was confirmed as policy in the Autumn Budget on 28 October 2026, but it requires secondary legislation and no commencement date has been published [4]. Until then, the existing rules, including the 0.2-hectare small sites exemption, apply.

Does the exemption cover commercial or mixed-use brownfield schemes?

The announcement refers to residential development. Whether mixed-use, phased or conversion schemes qualify depends on the draft regulations and the consultation response, which have not been published [1].

Will an ecology survey still be needed on an exempt site?

Yes in almost every case. Protected species legislation, local plan policy and validation requirements are unaffected by a BNG exemption [1]. Brownfield sites frequently support bats, reptiles, nesting birds and open mosaic habitat.

Who decides whether a site is previously developed land?

The local planning authority decides at application stage, subject to appeal to the Planning Inspectorate and challenge in the High Court. Ecological and planning expert evidence is central to that decision.

Conclusion

The brownfield biodiversity net gain exemption confirmed in the Autumn Budget will reduce the statutory burden on small urban housing schemes once regulations are made. It will not reduce the need for competent ecological survey work, and it will increase the number of disputes about what counts as previously developed land, where the 1.5-hectare line falls, and whether priority habitat is present.

Developers should instruct surveys now and keep a documented baseline. Local authorities should tighten validation. Biodiversity surveyors should prepare reports that meet the expert-witness standards of CPR Part 35 from the outset, because a growing share of their work will end up in front of an inspector or a judge.

References

1. Autumn Budget 2026 Permits Bng Exemption. arbtech.co.uk.
2. 102085 Autumn Budget 2026 Key Measures. localgovernmentlawyer.co.uk.
3. Government Announces 650 Increase Maximum Size Brownfield Sites Exempt Biodiversity Gain Requirement. planningresource.co.uk.
4. Autumn Budget 2026. local.gov.uk.
5. Biodiversity Net Gain Considering a Targeted Exemption for Brownfield Residential Development. gov.uk.
6. New Biodiversity Net Gain Exemptions From August 2026 What Developers Need to Know. footanstey.com.
7. Government Confirms Biodiversity Net Gain for Major Infrastructure From November 2026. cieem.net.
8. Budget Brownfield Exemption Raises Concerns for Nature Recovery. envirotecmagazine.com.