Seven weeks. That is how long England's newest biodiversity net gain rules have been live, and already planning inboxes are filling with the same question: does my site actually qualify? The biodiversity net gain small sites exemption September 2026 update has reshaped how small and medium housebuilders, self-builders and landowners approach ecology on minor schemes, but the early evidence suggests confusion is outpacing clarity. Since SI 2026/790 came into force on 6 August 2026, planning consultants report a steady stream of applicants assuming "small" automatically means "exempt," a misreading that risks refusals, retrospective surveys and delayed starts on site.
This piece unpacks what changed, where developers are tripping up, and what to check before assuming any exemption applies.
Key Takeaways
- SI 2026/790 introduced four changes from 6 August 2026: a 0.2 hectare exemption, an offsite-first option for minor sites, removal of the self-build exemption, and a new temporary-development exemption.
- The 0.2 ha threshold is measured against the red-line boundary, not net developable area, and does not apply if onsite priority habitat would be harmed.
- Self-build and custom-build projects submitted from 6 August 2026 no longer get automatic exemption, full BNG assessment may now be required.
- Applications submitted before 6 August 2026 stay under the old rules; the new exemptions are not retrospective.
- Ecological surveys remain essential even on exempt sites because other legal duties, protected species, hedgerows, Habitats Regulations, do not disappear alongside BNG.
What Changed on 6 August 2026
The Biodiversity Gain Requirements (Exemptions) amendment regulations, SI 2026/790, were laid before Parliament on 13 July 2026 and took effect on 6 August 2026. They amend the exemptions framework sitting beneath the Environment Act's biodiversity net gain regime, rather than the core 10% net gain requirement itself, which still applies to every non-exempt development along with its 30-year maintenance obligation.
Four changes matter most for small and medium sites:
| Change | What it means | Who is affected |
|---|---|---|
| New 0.2 ha (2,000 sqm) exemption | Development on land of 0.2 hectares or less, measured on the red-line boundary, is exempt from BNG, unless onsite priority habitat would be negatively impacted | Small infill plots, single dwellings, small commercial units |
| Offsite-first option for minor sites above 0.2 ha | Applicants can go straight to buying offsite biodiversity units rather than following the previous onsite-first hierarchy | Minor development schemes not qualifying for the small-sites exemption |
| Self-build/custom-build exemption removed | Self-build and custom-build projects no longer get automatic exemption from BNG | Self-builders, custom-build developers |
| New temporary development exemption | Development reinstated within five years or less is exempt, unless priority habitat is negatively impacted | Temporary structures, short-term uses of land |
None of these changes are retrospective. They apply only to planning applications submitted on or after 6 August 2026, a point that is already catching out applicants who submitted in July and assumed the new rules would apply to them.
How the 0.2 Hectare Threshold Is Measured, and Where People Go Wrong
The 0.2 hectare exemption sounds simple, but the detail matters more than most applicants expect.
The measurement is the red-line boundary, full stop. It does not matter how much of that land is hardstanding, garden, scrub or bare ground, the whole area within the application's red line counts toward the 2,000 square metre limit. A site with 1,800 square metres of concrete yard and 200 square metres of rough grass is still measured at 2,000 square metres, not just the "green" part.
Three mistakes are already recurring seven weeks in:
1. Loose or generous red-line drawing. Some applicants have drawn boundaries that include access strips, visitor parking or landscaping buffers without realising this pushes the site over 0.2 ha and removes the exemption entirely. A red line drawn to be helpful for access rights can be unhelpful for BNG.
2. Skipping the priority habitat check. The exemption falls away completely if onsite priority habitat would be negatively impacted, regardless of site size. Priority habitats include things like traditional orchards, wood pasture, lowland meadow and certain hedgerow types, and they are not always obvious from a desktop search. Assuming a small urban plot has "nothing worth surveying" is precisely the assumption that leads to a costly correction later. A proper biodiversity net gain assessment at the outset settles this question before it becomes a planning condition dispute.
3. Phased or split applications. Splitting a larger scheme into several sub-0.2 ha applications to dodge BNG obligations is a strategy some are testing, and one planning authorities are increasingly alert to. Local planning authorities can and do look at the wider site context, cumulative impact and whether phasing looks designed purely to avoid the gain requirement. Consultants advising on 8 biodiversity net gain points on planning your project increasingly flag this as a reputational and legal risk, not just a technical one.
What the Self-Build Exemption Change Means
Until 6 August 2026, self-build and custom-build projects were automatically exempt from BNG requirements. SI 2026/790 removed that blanket exemption. Self-builders submitting applications from 6 August 2026 onward must now check whether their site qualifies under the new 0.2 hectare exemption instead, and if it does not, a full biodiversity gain plan and 10% net gain calculation will typically be required.
This is a meaningful shift for self-builders on larger plots, particularly rural or semi-rural sites where the previous exemption made ecology assessment an afterthought. Anyone with a self-build application in preparation should get an early BNG report commissioned rather than discovering the requirement mid-application.
The Offsite-First Route for Minor Sites
For minor development above 0.2 hectares that does not qualify for exemption, the biodiversity gain hierarchy itself has been loosened. Previously, applicants had to demonstrate they had exhausted onsite options before turning to offsite units. Under the amended rules, applicants "can choose to go straight to offsite in the first instance", buying offsite biodiversity units without first proving onsite delivery is unworkable.
This is a practical win for tight urban and suburban plots where onsite habitat creation is genuinely difficult to deliver and maintain for 30 years. It does not remove the 10% net gain requirement, it simply changes the route by which that gain is achieved.
In principle, offsite units are purchased from registered habitat banks or landowners who have created and committed to maintaining biodiversity units, and the transaction is recorded on the national biodiversity gain sites register. Costs vary by habitat type, distinctiveness and location, and this article does not attempt to quote figures, anyone assessing viability should get a bespoke calculation rather than relying on rules of thumb. For a fuller picture of how this compares with onsite delivery, see biodiversity net gain off-site or on-site delivery and BNG off-site land banking versus habitat banking. Those sourcing units directly can also review buying biodiversity units or, for landowners with habitat to offer, selling biodiversity units.
Transitional Traps: Applications Lodged Before 6 August 2026
The transitional rule is straightforward on paper but easy to misapply in practice: the new exemptions apply only to planning applications submitted on or after 6 August 2026. Anything submitted before that date, including pending applications and permissions already granted, continues under the previous BNG rules for the life of that application.
This creates a genuine trap for developers with applications straddling the change. A self-build application submitted on 4 August 2026 still benefits from the old self-build exemption. The same project resubmitted or amended after 6 August could lose that protection and fall under the new regime instead. Anyone unsure which rules apply to a live application should check the submission date against the commencement date before assuming either exemption or full liability.
Why Ecological Surveys Are Still Needed on Exempt Sites
This is the point most likely to catch out developers celebrating an exemption: qualifying for the BNG small sites exemption does not remove other ecological obligations.
Planning policy, the Conservation of Habitats and Species Regulations, and wildlife legislation covering protected species and hedgerows all operate independently of the BNG regime. A site exempt from the 10% net gain calculation can still require:
- A Preliminary Ecological Appraisal or ecological walkover to identify constraints before design work locks in
- Protected species surveys where bats, badgers, great crested newts or nesting birds are plausible, particularly on sites with mature trees, outbuildings or hedgerows
- Hedgerow assessments where removal or alteration is proposed, given separate hedgerow protection rules
- Habitats Regulations screening near designated sites, regardless of BNG exemption status
Skipping these checks because "the site is BNG-exempt" is a common but risky assumption. Local planning authorities can still refuse or condition applications on ecological grounds unrelated to net gain. A short walkover early in the process, of the kind described in how to conduct a biodiversity impact assessment, is far cheaper than a late-stage refusal.
What to Watch Next
Several further changes are in the pipeline and worth tracking:
- Brownfield residential exemption. A consultation on exempting certain brownfield residential development closed on 10 June 2026. The government response is still awaited, and it could materially widen exemptions for urban regeneration schemes.
- Statutory metric changes. Future legislation is expected to update the statutory biodiversity metric used to calculate net gain, which could shift baseline and target calculations for schemes already in the pipeline.
- Conservation and parks exemptions. New exemptions are expected for development whose primary purpose is biodiversity conservation, and for enhancing parks, playing fields and public gardens.
None of these have taken effect yet, and applicants should plan against current rules rather than anticipated ones. The wider political backdrop is also worth noting: on 26 September 2026 the Prime Minister announced the "Your First Home" equity loan scheme for new-build homes, with full detail expected at the 28 October Budget. Combined with the BNG exemption changes, it signals continued government pressure to accelerate housing delivery, which makes getting exemption assessments right, rather than assumed, more important, not less.
FAQ
Does the 0.2 hectare exemption apply automatically to any small plot?
No. It applies only where onsite priority habitat would not be negatively impacted, and only to applications submitted on or after 6 August 2026. The area is measured on the red-line boundary, not the buildable footprint.
I submitted my self-build application in July 2026, do the new rules apply to me?
No. Applications submitted before 6 August 2026 continue under the previous rules, including the old self-build exemption, for the life of that application.
Can I split a larger site into smaller applications to stay under 0.2 hectares?
This is high risk. Planning authorities can consider cumulative impact and whether phasing appears designed to avoid BNG obligations, and refusal or challenge is possible.
If my site is BNG-exempt, do I still need an ecology survey?
Often yes. Protected species, hedgerow protections and Habitats Regulations obligations exist independently of BNG and can still require assessment even on an exempt site.
What happens to the biodiversity gain hierarchy for minor sites above 0.2 hectares?
Applicants can now choose to go straight to offsite unit purchase rather than proving onsite delivery first, though the underlying 10% net gain requirement and 30-year maintenance obligation still apply.
Is a brownfield exemption coming?
A consultation closed on 10 June 2026 but the government response has not yet been published. Developers should not assume a brownfield exemption exists until legislation confirms it.
Conclusion
Seven weeks into the biodiversity net gain small sites exemption September 2026 regime, the pattern is clear: the rules are genuinely more flexible for small and minor sites, but they reward precision, not assumption. Red-line boundaries, priority habitat checks, submission dates and the separate web of wildlife legislation all still need proper scrutiny before anyone treats a site as exempt or a project as low-risk.
Before submitting or amending a planning application, get the site checked against the current exemption criteria, commission a Preliminary Ecological Appraisal where there is any doubt, and confirm which side of the 6 August 2026 transitional line your application sits on. Biodiversity Surveyors carries out exemption checks, priority habitat assessments, full BNG metric calculations and offsite unit sourcing for developers, self-builders and consultants navigating this changed landscape, early advice now avoids costly surprises later. For further background on the underlying framework, see biodiversity net gain explained and exempt projects.