The self-build and custom build BNG exemption

Self-build and custom housebuilding is exempt from the biodiversity gain condition, and the exemption is narrower than most plot buyers assume. The self-build BNG exemption applies only where the development consists exclusively of self-build or custom-build dwellings, there are no more than nine of them, and the site is no larger than half a hectare. Miss any one of the three and the deemed condition under Schedule 7A to the Town and Country Planning Act 1990 applies in full, with a biodiversity gain plan needed before work can begin.

What the exemption actually removes

The exemption is from the statutory biodiversity gain condition created by the Environment Act 2021 and set out in the 2024 exemptions regulations. Nothing else goes with it.

What remains in place:

• protected species law, including the licensing regime for bats, great crested newts and other European protected species
• the duty on the authority to have regard to conserving biodiversity under the Natural Environment and Rural Communities Act 2006
• any local plan policy requiring biodiversity gain, which many authorities have and which operates independently of the statutory condition
• tree protection, including Tree Preservation Orders and conservation area tree notifications
• the Framework's protection for irreplaceable habitats, which does not turn on the metric at all

So an exempt self-build plot with a mature hedgerow, a pond or a building with bat potential still needs ecological survey work, and may still need a condition or a section 106 obligation on habitat. The exemption removes one procedural requirement rather than the ecology.

The three conditions that have to be met

Each of the three has to be satisfied on its own terms, and the drafting is strict.

The third is the one that fails. A scheme of eight plots where six are sold to individuals and two are built speculatively by the developer for open market sale is not exclusively self-build, so the exemption is unavailable for the whole application. Splitting those two units into a separate application changes the position, provided the split is real and not a device.

Site area is measured as the application site, which includes access, parking and garden land. A plot of 0.4 hectares with a 0.15 hectare access strip crosses the threshold. Draw the red line deliberately and check the area before submitting.

What counts as self-build or custom build

The 2015 Act defines self-build and custom housebuilding as the building or completion of houses by individuals, by associations of individuals, or by persons working with or for them, to be occupied as homes by those individuals.

The exclusion in the Act is the important part. It does not include the building of a house on a plot acquired from a person who builds the house wholly or mainly to plans or specifications decided or offered by that person. A developer offering three house types from a brochure and building them to order is outside the definition, however much the buyer chooses kitchen finishes.

The practical distinction is who controls the design. A serviced plot sold with planning parameters, where the purchaser appoints their own designer and builder, sits comfortably inside the definition. A turnkey arrangement where the plot and the completed house come from the same party on standard plans does not.

Custom build arrangements in the middle are common and need care. Where a developer provides a shell and the purchaser completes the interior to their own design, the question is whether the purchaser holds real control over the plans. Document that control, because it is what the authority will test.

Evidence the authority will want

Councils have become more careful about this exemption since 2024, and a bare assertion on the form is often queried.

1. A site plan with the red line area stated in hectares, calculated rather than estimated.
2. A schedule of the dwellings, showing the count and confirming that none is for open market sale.
3. The plot sales mechanism, including draft plot passports or design codes showing that purchasers appoint their own designers.
4. Evidence of the self-build route, which may include entries on the authority's self-build and custom housebuilding register, or correspondence with prospective purchasers.
5. A draft condition or section 106 obligation securing self-build occupation, which many authorities will ask for as the price of accepting the exemption.
6. An ecology report, because the authority still needs to understand the site even where no gain plan is required.

Point five is worth proposing rather than waiting for. An applicant who offers a mechanism tying occupation to self-build delivery looks credible, and the obligation is the thing that makes the exemption safe from the authority's point of view.

A preliminary ecological appraisal remains the sensible first step on any plot with hedges, trees, outbuildings or standing water, and it usually costs less than the delay caused by discovering a constraint after the plots are marketed.

How the exemption is claimed on the forms

Applications now carry a biodiversity net gain section. The applicant states whether they believe the biodiversity gain condition applies and, where they believe it does not, which exemption they rely on. Where the condition does apply, the application must also include information about the pre-development biodiversity value and the date it was assessed.

Three points on the mechanics.

The statement is the applicant's belief rather than a determination, so a wrong statement does not create an exemption. If the authority disagrees, the condition applies and the development cannot lawfully begin without an approved gain plan.

The decision notice should record the position. Ask for the decision notice to state that the development is exempt and on what basis, because that is the document a lender, a purchaser or a building control body will look at later.

And complete the pre-development value fields anyway where the exemption is marginal. Having the baseline recorded costs a survey and protects the scheme if the exemption is lost.

Losing the exemption after permission

The exemption is assessed against the permission as granted, so changing the permission reopens it.

A section 73 application to vary a condition results in a new permission, and the exemption has to be re-assessed against that new permission. Adding a tenth dwelling, or extending the red line beyond half a hectare, removes the exemption for the whole scheme. So does changing the delivery model, which is the quiet risk: a self-build scheme that stalls and is taken over by a developer building for open market sale no longer consists exclusively of self-build dwellings.

Two protective habits help. Keep the dwelling count and site area well inside the thresholds where the layout allows, so a minor amendment does not cross a line. And before applying under section 73 for anything, check the biodiversity position first, because the cost of a late gain plan can exceed the value of the amendment.

Where the exemption is lost, the pre-development baseline is assessed as at the date of the new application unless an earlier date is agreed, and the anti-degradation rule for habitat cleared without authorisation on or after 30 January 2020 still applies. A biodiversity net gain assessment draft gives the structure, and a competent person must complete the statutory metric.

Where the exemption does not help

Several situations look exempt and are not.

• A single dwelling on a plot of more than half a hectare, which fails the area test even though it easily meets the dwelling test.
• A scheme of nine self-build plots plus a converted barn sold on the open market, which fails the exclusivity test.
• Demolition and site clearance carried out before the application, which engages the anti-degradation rule and can create a baseline obligation that survives.
• Sites with irreplaceable habitat, where the Framework protection and the separate compensation requirements apply regardless.
• Development affecting protected species, where survey work, mitigation and a licence may be needed and the demolition method statement for protected species does the work the gain plan would not have done anyway.

The honest summary is that the exemption saves the gain plan, the metric and the 30-year management obligation. It does not save the ecology survey, and on a wooded or hedged plot the survey is where most of the cost and most of the programme risk sits.

See which ecology documents a self-build plot needs by browsing the report categories and starting a draft.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.