Biodiversity Net Gain (BNG) requires qualifying developments in England to leave habitats in a measurably better state than before development. This normally means achieving at least a 10% increase in biodiversity value, calculated using the statutory biodiversity metric. Mandatory BNG has applied to small developments since April 2024. However, applications made on or after 6 August 2026 may be exempt where the development site is no larger than 0.2 hectares and no onsite priority habitat is impacted.
What is a small development site
For BNG purposes, a development is generally classified as small where it meets the following criteria:
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Residential development: between one and nine dwellings on a site smaller than one hectare; or, where the number of dwellings is not known, a site smaller than 0.5 hectares.
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Non-residential development: less than 1,000 square metres of new floorspace or a site smaller than one hectare.
These definitions determine whether a project is classed as a small development. They do not automatically determine whether the project is exempt from BNG.
For example, a residential development of five dwellings on a 0.4-hectare site would be classed as a small development. However, because the site is larger than 0.2 hectares, it would not qualify for the exemption for developments on sites of 0.2 hectares or below.
When does mandatory BNG apply to a small development?
Where no exemption applies, a small development must normally achieve at least a 10% increase in biodiversity value.
Some small developments are exempt from mandatory BNG. The principal exemptions include:
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householder applications, such as extensions or alterations to an existing home;
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developments that meet the de minimis exemption because their habitat impacts fall
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below the specified thresholds and no priority habitat is affected;
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developments on sites of 0.2 hectares or below, where no onsite priority habitat will be adversely affected; and
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qualifying temporary developments lasting no more than five years, where no onsite priority habitat will be adversely affected.
For planning applications submitted on or after 6 August 2026, self-build and custom-build developments no longer have their own BNG exemption. They may still be exempt if they meet the conditions of another exemption, such as the exemption for developments on sites of 0.2 hectares or below. Other exemptions and transitional arrangements may apply. The date of the original application is particularly important where a planning permission is later varied.
The Small Sites Metric
The Small Sites Metric is a simplified version of the statutory biodiversity metric. It was developed to make BNG calculations more proportionate for eligible small developments.
The metric compares the biodiversity value of habitats before and after development. It considers information such as:
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the type and area of each habitat;
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the condition of the habitat;
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habitats that will be retained or lost; and
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habitats that will be created or enhanced.
The Small Sites Metric cannot be used for every small development. Its suitability depends on the habitats present and the ecological complexity of the site. Sites containing priority habitats, protected features or other ecological constraints may require the full statutory metric and additional ecological assessment.
A project should therefore confirm that it meets the Small Sites Metric eligibility criteria before using it.
How can a small development achieve BNG?
A small development can achieve BNG by retaining, enhancing or creating habitats within the site or by using registered offsite biodiversity gains.
A typical approach includes:
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Assessing the existing site: Identify and map the habitats within the planning boundary.
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Avoiding unnecessary habitat loss: Retain existing features where this is practical and compatible with the development.
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Reviewing the proposed design: Identify opportunities to enhance existing habitats or create appropriate new ones.
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Calculating the biodiversity change: Use the relevant statutory metric to compare the baseline and proposed development.
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Addressing any shortfall: Where the required gain cannot be achieved onsite, registered offsite biodiversity units may be used.
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Planning habitat management: Set out how significant habitat improvements will be established, maintained and monitored.
For small developments that remain subject to mandatory BNG, onsite habitat creation or enhancement and registered offsite gains can be considered together. Statutory biodiversity credits remain a last-resort option.
Can an exempt small development still undertake a BNG assessment?
Yes. An exemption removes the legal requirement to achieve mandatory BNG, but it does not prevent a developer or landowner from assessing the site or voluntarily including biodiversity improvements.
Although individual small developments may affect relatively limited areas, their combined effects can gradually reduce or fragment habitats across a wider area. Features such as hedgerows, trees, gardens, grassland and ditches can provide food, shelter and movement routes for wildlife. Considering biodiversity early in the design process can help identify which existing features should be retained and where appropriate habitat improvements can be incorporated.
Therefore, a voluntary assessment may help to:
- inform the design of the development;
- demonstrate how existing habitats will be retained;
- support discussions with the local planning authority;
- respond to local planning policy or planning conditions; and
- identify realistic opportunities to improve habitats.
The appropriate level of assessment will depend on the site, the proposed development and the reason the biodiversity information is required.
The Bottom Line
Small developments are not automatically exempt from BNG. Whether the mandatory requirement applies depends on the size and type of the development, the habitats affected, the application date and any relevant exemption.
Where BNG is required, the assessment should be proportionate but sufficiently detailed to establish the existing habitat value and demonstrate how the required gain will be achieved. Where a project is exempt, biodiversity can still be considered voluntarily to support site design, planning discussions and appropriate habitat improvements.
If the status of a development or the appropriate assessment method is unclear, Tunley Environmental’s biodiversity team can help explain the requirements and identify a proportionate approach for the site.
Related guidance: Current BNG regulations and exemptions | How BNG assessments work