Self-build BNG exemption fails at Maidstone appeal over missing legal agreement

A self-build exemption from BNG has failed at appeal in Maidstone after the proposed one-home scheme was not backed by a planning obligation securing its self-build status.

Inspector P Terceiro dismissed Appeal 6007158 at Brishing Court Farm, Boughton Monchelsea, on 28th September 2026. The biodiversity point was not the main reason the scheme failed, but it produced a useful warning for older self-build applications caught by the pre-August 2026 biodiversity net gain regime.

The application, validated in January 2026, was for the change of use of part of a former traditional orchard and the erection of a single-storey self-build dwelling. Maidstone Borough Council argued that the old self-build exemption could not safely be relied upon because the dwelling’s status had not been secured through a legal agreement.

Why the old self-build exemption failed

For applications made before 6th August 2026, the former exemption could apply to qualifying self-build and custom-build schemes of no more than nine dwellings on sites no larger than 0.5 hectares. The key issue here was not the size of the proposal. It was whether the self-build status was secured with enough certainty.

The Council pointed the Inspector to an earlier Maidstone appeal, APP/U2235/W/25/3364165 at Minton House, where an Inspector concluded that a planning obligation was needed to secure the dwelling as self-build for BNG purposes and that a condition would not satisfy the national policy tests. In the Brishing Court Farm appeal, the appellant indicated that an agreement could be provided if necessary, but none was submitted.

Inspector Terceiro recorded that “no legal agreement has been submitted” and found there was insufficient evidence that a condition would do the same job. The result was straightforward: without an appropriate mechanism securing the self-build status, the claimed exemption was not established and mandatory BNG applied.

That is a useful distinction. Calling a dwelling self-build on the application form, or intending to occupy it as a self-build home, is not necessarily the same as legally securing that status where an exemption depends upon it. Our guide to self-build planning permission explains the wider planning route and the separate practical tests.

The missing biodiversity metric then mattered

Once the Inspector found that BNG applied, the application had to meet the minimum information requirements for development subject to the statutory biodiversity gain condition.

A metric summary had been supplied, but not the completed statutory biodiversity metric calculation tool. Inspector Terceiro noted that “the completed metric calculation tool has not been provided”. On the material before him, he could not be certain that the biodiversity gain objective would be met.

Current government guidance still requires an application subject to BNG to include the pre-development biodiversity value and the completed metric calculation showing how that figure has been reached. The gain plan comes later, after permission, but the application-stage metric information cannot simply be left until discharge of the statutory condition.

Planning Geek’s biodiversity net gain guide covers the 10% objective, current exemptions, metric requirements and the changes that took effect in August 2026.

This decision does not revive the self-build exemption

There is an important transitional wrinkle. Brishing Court Farm was an older application, validated on 12th January 2026, so the Inspector had to consider the self-build exemption that still existed when the application was made.

That is not the position for a fresh application today. Government guidance was updated on 6th August 2026: the separate self-build and custom-build exemption no longer applies to new planning applications made on or after that date. Older applications made before 6th August can still rely on the previous exemption where its requirements are actually met.

At the same time, a new exemption was introduced for development sites of no more than 0.2 hectares, provided the development does not negatively impact onsite priority habitat. A new self-build proposal may therefore be exempt for another reason, but being self-build by itself no longer provides the exemption.

BNG was not the only reason the appeal was dismissed

The biodiversity issue should not be mistaken for the whole decision. The Inspector dismissed the appeal on a much broader planning balance.

He found moderate harm to the significance of the Grade II* listed Brishing Court through development in its setting. The proposal would also erode the rural and former-orchard character of the site, sit outside the settlement strategy in a location where day-to-day trips were likely to depend on the car, and lacked the reptile survey needed to rule out unacceptable harm to protected species.

Maidstone could demonstrate only 4.5 years of deliverable housing land. The Inspector also gave substantial weight to the benefits of an additional dwelling, the intended self-build contribution, economic benefits and proposed orchard restoration. Even so, he found those benefits insufficient to overcome the identified harms.

That makes the BNG finding an additional technical problem rather than the sole cause of dismissal. The Inspector expressly said there was no need to take the BNG issue further because the appeal was being dismissed on other matters.

What applicants should take from the Maidstone appeal

For the shrinking pool of pre-6th August 2026 applications still relying on the former self-build BNG exemption, the lesson is to check how the self-build status is secured rather than assume a planning condition will be accepted. The earlier Maidstone decision cited in this appeal shows why a Section 106 obligation or unilateral undertaking may be necessary.

For applications made on or after 6th August 2026, start somewhere else: check the current exemptions, including the 0.2 hectare and de minimis routes, and then provide the full metric information where BNG applies. Do not import the old self-build exemption into a new application simply because an older appeal decision discusses it.

The current GOV.UK exemptions guidance confirms the transitional position. Appeal 6007158 can be read on our locally hosted copy of the decision and on the Planning Inspectorate appeal record.