A single hedge row can still decide whether a small housing scheme needs biodiversity net gain sign-off. That is the reality many councils and developers have discovered since 6 August 2026, when the Government's reformed small-sites rules came into force. Two months on, this biodiversity net gain small sites exemption update looks at what planning teams have learned, where confusion persists, and how the sector should now prepare for mandatory BNG on Nationally Significant Infrastructure Projects from November 2026.
As of October 2026, the exemption for non-major sites of 0.2 hectares or smaller is no longer new policy on paper, it is live practice, being tested against real applications, real hedgerows, and real local plan policies.
Key Takeaways
- Since 6 August 2026, non-major sites of 0.2 hectares or smaller are exempt from the 10% biodiversity net gain requirement, but only if priority habitats are not affected.
- An ecological survey is still usually necessary to prove priority habitats are absent, the exemption removes the BNG calculation, not the need for evidence.
- For other non-major (medium) sites, developers can now use off-site units without first proving on-site mitigation is unfeasible.
- The self-build and custom-build exemption has been withdrawn, but most such projects still qualify through the small-sites route.
- Applications submitted before 6 August 2026, and related section 73 applications, remain under the previous BNG regime.
- BNG becomes mandatory for Nationally Significant Infrastructure Projects from November 2026, and developers in that pipeline should be preparing now.
What Changed on 6 August 2026
The reforms confirmed by Government adjusted two separate parts of the biodiversity net gain regime. First, non-major development on sites of 0.2 hectares or smaller became exempt from the standard 10% net gain requirement, provided the site does not affect priority habitats. Second, for other non-major sites, often described as medium sites, the previous "on-site-first" mitigation hierarchy was dropped. Developers no longer need to demonstrate that on-site habitat creation is impossible before buying off-site biodiversity units.
Alongside these changes, the long-standing self-build and custom-build exemption was withdrawn. In practice, most self-build plots are small enough to fall under the new 0.2 hectare small-sites exemption anyway, so the net effect for individual self-builders has been limited.
Biodiversity Net Gain Small Sites Exemption Update: What Councils Have Learned So Far
Two months into implementation, local planning authorities report the same recurring issue: the exemption is simple to state but harder to apply consistently. The 0.2 hectare threshold is straightforward to measure, but the priority-habitat condition is not a tick-box exercise. Officers still need enough ecological information on file to be confident that a hedgerow, pond, grassland strip, or veteran tree does not qualify as a priority habitat before they can wave through an exemption claim.
This has created a practical tension between speed and scrutiny. The exemption was designed partly to reduce burdens on small developers, yet councils remain legally responsible for checking that exempt status is genuinely justified. Where local plan policies already set out biodiversity expectations for small sites, officers are also having to reconcile those policies with the national exemption, which can create duplicate or overlapping requirements if local wording has not yet been updated.
Pull quote: The exemption removes a calculation obligation, it does not remove the need for evidence.
Why an Ecological Survey Is Still Needed to Prove Priority Habitats Are Not Affected
This is the point most often misunderstood by small developers applying the exemption for the first time. Qualifying for the small-sites exemption is conditional, not automatic. A site under 0.2 hectares only escapes the 10% biodiversity net gain requirement if it can be shown that priority habitats are not present or not affected by the proposed works.
In most cases, that means a preliminary ecological appraisal or similar survey is still required to establish the baseline. Without it, neither the applicant nor the local authority can confirm the exemption applies. Developers who assume "small site" automatically means "no ecology work" risk delays when validation checks flag missing evidence.
Typical triggers that still require survey evidence include:
- Mature hedgerows or tree lines on or adjoining the site
- Ponds, ditches, or other wetland features
- Unmanaged grassland, scrub, or woodland edges
- Any site within or near a designated wildlife area
Where a survey confirms no priority habitat is present or affected, the exemption can be applied with confidence and the biodiversity metric calculation is avoided. Where a survey is inconclusive or identifies a priority habitat, the standard 10% requirement is likely to still apply.
Medium Sites: Off-Site Units Without the Old Mitigation Hierarchy
For non-major sites above the 0.2 hectare small-sites threshold, the most significant practical change is the removal of the on-site-first requirement. Previously, developers had to evidence that on-site biodiversity gains were not feasible before turning to off-site units or the statutory credit scheme. That evidential burden has gone.
Developers on medium sites can now purchase off-site biodiversity units directly, without first commissioning assessments to prove on-site habitat creation would not work. This is expected to speed up decision-making for sites where on-site habitat creation was always going to be marginal, tight urban infill plots, for example, but it also means local authorities are seeing more off-site unit purchases where, under the old rules, some on-site provision might have been negotiated.
Comparison: Old Regime vs Post-6 August 2026 Rules
| Site type | Pre-6 August 2026 | Post-6 August 2026 |
|---|---|---|
| Non-major, 0.2ha or smaller, no priority habitat affected | 10% BNG required | Exempt from BNG requirement |
| Non-major, 0.2ha or smaller, priority habitat affected | 10% BNG required | 10% BNG requirement still applies |
| Other non-major (medium) sites | On-site mitigation had to be evidenced as unfeasible before off-site units | Off-site units can be used without evidencing on-site infeasibility |
| Self-build/custom-build | Separate dedicated exemption | Exemption withdrawn; most schemes now covered by small-sites exemption instead |
| Applications submitted before 6 August 2026 (and related section 73 applications) | Old regime applied | Old regime continues to apply under transitional rules |
Transitional Rules Still in Play
Not every live application is subject to the new rules. Applications submitted before 6 August 2026 remain under the previous BNG regime, including the old mitigation hierarchy and the former self-build exemption. Section 73 applications linked to permissions granted or submitted before that date are treated the same way.
This matters for developers managing a pipeline of sites at different stages. Two schemes submitted a week apart, either side of 6 August 2026, could be subject to entirely different BNG obligations. Planning teams are advised to check the submission date against the transitional provisions before assuming which rules apply, rather than relying on the decision date or site characteristics alone.
Preparing for Mandatory BNG on NSIPs from November 2026
While small-sites reform has dominated attention, a separate and larger shift is approaching. Biodiversity net gain becomes mandatory for Nationally Significant Infrastructure Projects from November 2026. Unlike the small-sites exemption, this extends BNG obligations into major infrastructure, energy, transport, and water projects that operate under the Planning Act 2008 regime rather than conventional town planning consents.
Developers and promoters with NSIPs in the pipeline should treat the coming weeks as a final preparation window. Practical steps include:
- Confirming which stage each project has reached, since timing against the November 2026 deadline will determine which regime applies
- Commissioning baseline ecological surveys early, given the scale and habitat diversity typical of infrastructure corridors
- Reviewing biodiversity metric calculations against the 10% requirement now standard across most development types
- Engaging with statutory consultees on habitat strategy well before examination, rather than treating BNG as a late-stage addition
What This Means for Developers and Landowners, and Why a Survey Still Matters
For small developers, the headline message is reassurance tempered with caution. The 0.2 hectare exemption can genuinely remove a costly and time-consuming calculation step, but it does not remove the need for sound ecological evidence. Skipping a survey to save money can backfire if a validation check later finds priority habitat on site, forcing a late return to the full BNG process.
For landowners and developers on medium sites, the relaxed mitigation hierarchy offers more flexibility in deciding between on-site habitat work and off-site units, but it is still worth weighing both options on cost and planning-risk grounds rather than defaulting to off-site credits automatically.
In every case, an independent ecological survey remains the practical safeguard. It protects applicants from delay, gives councils the evidence they need to apply exemptions correctly, and, for NSIP promoters, forms the foundation of a defensible biodiversity strategy ahead of November's mandatory deadline.
Frequently Asked Questions
Does every small site automatically qualify for the BNG exemption?
No. Sites of 0.2 hectares or smaller only qualify if priority habitats are not present or not affected. Evidence, usually from an ecological survey, is normally needed to confirm this.
Can medium sites still choose on-site habitat creation instead of off-site units?
Yes. The change simply removes the requirement to prove on-site mitigation is unfeasible before using off-site units; on-site provision remains an option.
What happened to the self-build exemption?
It was withdrawn on 6 August 2026. Most self-build and custom-build plots are still covered because they typically fall within the 0.2 hectare small-sites exemption.
Which applications are covered by the transitional rules?
Applications submitted before 6 August 2026, and section 73 applications linked to permissions granted or submitted before that date, continue under the previous BNG regime.
When does BNG become mandatory for NSIPs?
From November 2026, biodiversity net gain becomes a mandatory requirement for Nationally Significant Infrastructure Projects.
Is a survey still necessary if a site is clearly under 0.2 hectares?
In most cases, yes. Size alone does not confirm the absence of priority habitats, so a survey or appraisal is still the standard way to evidence eligibility for the exemption.
Conclusion
Two months into the biodiversity net gain small-sites exemption update, the clearest lesson is that simplification has not meant the removal of due diligence. The 0.2 hectare exemption and the relaxed mitigation hierarchy for medium sites genuinely reduce administrative burden, but both still depend on solid ecological evidence and careful attention to transitional timing. Developers should continue commissioning surveys before assuming exempt status, check submission dates against the 6 August 2026 cut-off, and start preparing now for mandatory BNG on Nationally Significant Infrastructure Projects ahead of the November 2026 deadline. Early ecological assessment, not assumption, remains the safest route through this reformed system.