A guide to planning

From 6 August 2026, the exemptions for sites of 0.2 hectares or less, de minimis development and temporary development do not apply where the development negatively affects an onsite priority habitat.

Applicants relying on an exemption must clearly identify the exemption and provide sufficient information with the planning application to demonstrate that it applies. The Council may request further information where it is not possible to verify an exemption from the information submitted.

The self-build and custom-build exemption does not apply to new planning applications submitted on or after 6 August 2026. It may continue to apply to an application submitted before that date, or to certain subsequent variations of an existing permission that was exempt when originally granted.

Development exempt from mandatory BNG remains subject to relevant planning policies and other environmental protections. Development should continue to avoid harm to biodiversity and incorporate appropriate opportunities for habitat enhancement and nature-friendly features wherever practicable.

Measuring biodiversity value

Habitats affected by development are converted into measurable biodiversity units using the statutory biodiversity metric.

The metric uses habitat type, condition, distinctiveness, size and strategic significance, together with factors relating to the difficulty and time required to create or enhance habitat. It provides a standardised proxy for measuring changes in biodiversity value.

The statutory biodiversity metric includes three types of biodiversity unit:

These unit types are calculated separately and cannot be added together, exchanged or converted from one type into another. The requirement to achieve at least a 10% net gain applies separately to each relevant type of biodiversity unit.

Statutory biodiversity metric documents

The statutory biodiversity metric comprises:

The statutory biodiversity metric calculation tool

This is an Excel spreadsheet used to calculate pre-development and post-development biodiversity values and the resulting percentage change.

A separate Small Sites Metric calculation tool is available for eligible small developments.

Habitat condition assessment sheets

These provide the statutory criteria for assessing the condition of habitats recorded in the metric.

Separate condition assessment sheets are not generally required when the Small Sites Metric is used because its habitat assessment requirements are incorporated into the Small Sites Metric process.

User guides

The Statutory Biodiversity Metric User Guide and Small Sites Metric User Guide explain how the calculation tools must be completed and set out the applicable rules and principles.

The relevant calculation tool, condition assessments and supporting habitat plans must be submitted together. The calculation must follow the relevant statutory guidance to be accepted.

The current statutory metric tools and guides are available on the GOV.UK statutory biodiversity metric tools and guides page. The Government confirms that any published version of the statutory metric tools may be used, although applicants are encouraged to use the most recent version.

Using the Small Sites Metric

The Small Sites Metric may be used for eligible small developments that are not major development, but it is not appropriate for every small site.

Subject to the restrictions set out in the Small Sites Metric User Guide, it may generally be used for:

The Small Sites Metric must not be used where the development does not meet the eligibility requirements in the current Small Sites Metric User Guide. This includes circumstances where:

The Small Sites Metric should only be used by a competent person who has the knowledge and skills necessary to identify the habitats present and apply the metric correctly.

If information submitted with an application indicates that protected species, priority habitats or other habitats not covered by the Small Sites Metric may be present, the Council may require additional ecological surveys and the completed statutory biodiversity metric before determining the application.

The latest Small Sites Metric and accompanying User Guide should always be checked before deciding whether it is appropriate for a particular development.

Strategic significance

Strategic significance is one of the factors used by the statutory biodiversity metric to calculate the biodiversity value of an individual habitat parcel.

The metric assessor must assign the appropriate strategic significance category to each relevant habitat parcel at the baseline and post-development stages. An explanation of the category selected should be provided in the comments section of the metric.

The statutory metric applies the following strategic significance multipliers:

The Kent and Medway Local Nature Recovery Strategy was published in November 2025. Its local habitat map and mapped potential measures now supersede the earlier Interim Strategic Significance Guidance for Biodiversity Net Gain in Kent and Medway.

Applicants and metric assessors should therefore use the Kent and Medway Local Nature Recovery Strategy mapping and the current Strategic Significance Guidance for Biodiversity Net Gain in Kent and Medway when completing the metric.

Commentary should be included in the metric explaining how strategic significance has been determined for every relevant habitat parcel.

Information and calculations submitted to meet the mandatory BNG requirement must comply with the applicable biodiversity gain hierarchy.

For development other than minor development, this means taking action in the following order:

For relevant minor development submitted on or after 6 August 2026, the hierarchy has been modified so that the development may use registered offsite biodiversity gains without first demonstrating that all on-site habitat creation or enhancement options have been exhausted.

This modification does not remove the requirement to avoid and mitigate harm to on-site habitats of medium, high or very high distinctiveness. It also does not remove the need to comply with planning policy, protected-species requirements or the wider ecological mitigation hierarchy.

Developers should consider biodiversity and the biodiversity gain hierarchy from the earliest stages of site selection and design. Planning authorities must take the applicable hierarchy into account when considering whether the biodiversity gain objective has been met and when deciding whether to approve a Biodiversity Gain Plan.

Biodiversity metric rules

The statutory biodiversity metric is subject to four rules.

Rule 1: Trading rules

The trading rules within the statutory biodiversity metric must be followed.

These rules control how the loss of one habitat type may be compensated by the creation or enhancement of another habitat. Habitat losses must be compensated for on a like-for-like or like-for-better basis, as required by the metric.

Rule 2: Separate unit types

Biodiversity unit outputs for area habitats, hedgerows and watercourses must not be added together, traded or converted between unit types.

The requirement to achieve at least a 10% net gain applies separately to each relevant unit type.

Rule 3: Statutory calculation tools

The statutory biodiversity metric calculation tool must be used to calculate biodiversity value.

The Small Sites Metric may be used where the development and habitats meet its eligibility requirements. The calculation tools summarise the resulting biodiversity value and indicate whether the biodiversity gain objective has been met.

Rule 4: Exceptional ecological circumstances

In exceptional ecological circumstances, a deviation from the statutory metric methodology may be permitted by the relevant planning authority.

Any proposed deviation must be supported by appropriate ecological evidence and must be discussed and agreed with the Council before the Biodiversity Gain Plan is submitted. A deviation will not be accepted solely because applying the statutory methodology would be inconvenient or result in a lower biodiversity-unit outcome.

Biodiversity metric principles

The statutory biodiversity metric is underpinned by the following principles:

Competent persons and realistic habitat proposals

The Council draws applicants’ attention to Principle 1 and the requirement for the metric assessment to be completed by a competent person.

For the main statutory biodiversity metric, this would normally be a suitably qualified ecologist with appropriate experience of the habitats present on the site.

The Council will also have regard to Principle 7, which requires proposed habitat interventions to be realistic and deliverable.

The Council is unlikely to accept proposals that rely on the creation or maintenance of high-quality or priority habitat where:

Where native hedgerows or trees are proposed within private residential gardens, they may need to be recorded as part of the vegetated garden habitat rather than as separate newly created hedgerow or urban-tree units. This will depend on the design, ownership, accessibility, management arrangements and applicable statutory metric guidance.

Habitat condition assessments and plans

Statutory habitat condition assessments are an important part of the biodiversity metric and will be carefully reviewed by the Council.

Where an applicant concludes that a habitat does not meet a particular condition criterion, detailed evidence may be required to support that conclusion. Original ecological survey data should therefore be supplied wherever possible.

This is particularly important for grassland habitats. A Phase 1 Habitat Survey or Preliminary Ecological Appraisal may not provide sufficient information to accurately assign the appropriate UK Habitat Classification habitat type and condition.

Detailed botanical surveys may be required for semi-improved or other potentially species-rich grasslands. These surveys must be completed at an appropriate time of year and using an appropriate survey methodology.

The application should include clearly presented pre-development and post-development habitat plans. These plans must:

The Council must be able to understand, verify and follow the metric calculation by comparing it directly with the submitted plans, surveys and condition assessments.

Further information

Applicants should refer to the following sources before preparing a planning application or Biodiversity Gain Plan: