Biodiversity net gain, or BNG, requires development within its statutory scope to deliver at least a 10% increase in biodiversity value. Start by checking whether the requirement applies to your application. The exemption rules changed on 6 August 2026, so older advice about small sites and self-build homes may now be wrong.
This guide concerns ordinary planning applications in England. It does not cover the separate nationally significant infrastructure regime.
What does 10% mean?
BNG is measured using biodiversity units through the statutory metric, rather than simply allocating 10% of a plot to planting. Existing habitat type, condition and other factors affect the calculation. Off-site gains and statutory credits are distinct ways of addressing a shortfall. See the government's explanation of biodiversity net gain.
A landscaping plan and a BNG calculation are therefore not interchangeable. A design may look greener while failing to provide the required habitat value or long-term management arrangements.
Check the exemption before ordering a full calculation
The government's exempt developments guidance identifies several routes:
| Possible exemption | Key question |
|---|---|
| Householder application | Is this genuinely householder development, such as an extension, rather than a separate new dwelling? |
| Site no larger than 0.2 hectares | Was the new application made on or after 6 August 2026, and does the development avoid impacting onsite priority habitat? |
| De minimis | Does it avoid priority-habitat impact and remain below the applicable habitat-impact thresholds? |
| Permitted development | Is permission actually granted by a development order for this proposal? |
| Earlier application or permission | Which commencement or saving provisions apply to this application's history? |
The new 0.2-hectare test uses the site within the red line boundary, not just the building footprint. That is 2,000 square metres. Measure the actual application site and provide a clear plan; a rough plot estimate is insufficient near the threshold.
The August 2026 changes need a date check
SI 2026/790 introduced the new small-site exemption and an exemption for qualifying wholly temporary development lasting no more than five years. Both require no impact on onsite priority habitat and contain application-date and section 73 restrictions. Successive permissions for the same temporary development cannot simply restart the five-year allowance.
The same instrument removed the standalone self-build and custom-build exemption for new applications made on or after 6 August 2026. Qualifying earlier applications retain the saved exemption, and specified section 73 variations of originally exempt permissions are protected. A new self-build proposal may qualify under another exemption, but being self-build is no longer enough by itself.
The new exemptions also do not automatically remove an existing permission's BNG condition. Record the original application date, permission date and subsequent variations before deciding which rules apply.
Evidence for an exemption
A useful exemption statement identifies the provision relied upon and supplies the facts needed to assess it. For a 0.2-hectare claim, assemble a measured red-line plan, the application date and evidence concerning priority habitat. For a householder claim, make the proposed use and relationship to the existing home clear.
Do not label a proposal “householder” merely because its applicant owns a house. A separate dwelling in the garden needs its own assessment. Likewise, a planning application needed because permitted development rights have been removed is not exempt simply because similar work elsewhere could use those rights.
Where several exemptions could apply, the planning practice guidance recommends focusing on the most appropriate, readily assessed route rather than proving every possible exemption.
If BNG applies, plan for it before submission
Establish the baseline habitat evidence before site clearance and coordinate it with the proposed layout. Identify what can be retained, what will change and how the necessary gains will be delivered. Choose the appropriate metric and a suitably competent person for the assessment.
For non-major development under the new rules, onsite enhancement, onsite creation and registered offsite gains have equal preference at the compensation stage. Statutory credits remain the last option. The preceding avoidance and mitigation requirements for relevant higher-distinctiveness habitat still matter. Earlier applications and certain variations retain the previous hierarchy under regulations 2 and 4 of SI 2026/790.
For an in-scope permission, the Biodiversity Gain Plan normally requires approval before development starts. Significant onsite gains and registered offsite gains involve at least 30 years of maintenance. Identify responsibility, funding and monitoring early, rather than leaving them as unexplained notes on a planting plan. See the government's BNG process guidance.
Worked example: a new garden dwelling
Imagine a new application submitted in September 2026 for one house on a 0.18-hectare site. The applicant should not claim the removed self-build exemption. Instead, assess the new small-site exemption, including whether the proposal impacts onsite priority habitat.
If the red line were actually 0.24 hectares, the small-site route would not apply. Another exemption might still be relevant, but it would need separate evidence. Neither scenario can be resolved from the proposed house's floor area alone.
An exemption is not an ecology clearance
Check protected species, designated sites, trees and applicable planning policies separately. Keep the BNG exemption reasoning alongside those assessments so an exemption is not mistaken for evidence that no ecological effects exist.
Sources and review
SI 2026/790, government exemption guidance updated on 6 August 2026 and the linked BNG guidance were compared on 8 September 2026 with automated research assistance. This source check is not professional ecological or legal sign-off; habitat conditions and permission history require project-specific verification.