Biodiversity Net Gain (BNG) Explained: A Practical Architects Guide for 2026

Mandatory BNG does not apply to every project. The government’s planning guidance sets the eligibility and transition rules. Applications from 6 August 2026 have a new small-site exemption at no more than 0.2 hectares, subject to no impact on onsite priority habitat. Earlier self/custom-build exemptions and section 73 cases need their own date-sensitive checks.

Keep evidence of the site area and habitat assessment used for an exemption. An exemption from the biodiversity gain condition does not answer separate questions about designated sites, protected species, ancient woodland or local policy.

A competent assessment must distinguish habitats, their extent and condition. Existing mapping can guide survey scoping but may miss recent changes or habitats beneath a broad land-use label. Ask for the survey date, methodology and any inaccessible areas. Investigate whether past habitat degradation affects the statutory baseline rather than assuming the current appearance is always sufficient.

Mark retained habitat and construction protection on the same coordinated plan as building footprints, levels, drainage, utilities and temporary works. This reveals conflicts before a preferred option becomes expensive to revise.

Use the statutory metric guidance and tools, including the Small Sites Metric only where its eligibility conditions are met. Do not call the current statutory tool “Metric 4.0”. Area habitats, hedgerows and watercourses use separate unit types and trading rules. A positive total in one category cannot automatically repair a deficit in another.

A proposed woodland does not instantly have the value of an established woodland. Delivery time, difficulty and location can affect the calculation. Ancient woodland and other irreplaceable habitats need their own treatment; they cannot be treated as ordinary high-value habitat that any surplus can replace.

Compare habitat retention and enhancement with the realistic space needed for the project. A green roof, rain garden or planting proposal needs appropriate habitat classification and management assumptions; it is not automatically a fixed biodiversity credit. Obtain quotes for the actual delivery route rather than using an unsourced nationwide price per unit.

The August 2026 minor-development changes allow qualifying non-exempt minor schemes to consider onsite and offsite gains together. For other cases, establish the applicable hierarchy. Distinguish registered offsite gains from government statutory credits, and record who will arrange any allocation, registration and legal securing.

Identify the biodiversity information required with the planning application and the gain plan that normally needs approval before commencement. Do not defer habitat investigations until that condition is due: the early layout must remain capable of delivering the assumed outcome.

For significant onsite enhancements and offsite gains, establish the applicable long-term management and securing arrangements, generally at least 30 years. Agree responsibilities, access, monitoring, costs and remedies with the relevant advisers. Avoid promising that every minor landscape feature must appear in the same legal agreement.

Issue the ecologist a boundary plan and a revision-controlled layout including temporary works and service corridors. Request a marked-up baseline, proposed habitat plan, metric file, assumptions, outstanding surveys and delivery issues.

Carry habitat decisions into specifications and maintenance information. If a retained tree, drainage feature or roof build-up changes during design, ask whether that affects the assessment. The Atlasly unit-uplift calculator can illustrate arithmetic using supplied metric units; it is not the habitat survey, statutory metric or approval process.