Biodiversity Net Gain

1. There are strict legal requirements in terms of what an application must contain in relation to BNG before it can be accepted as valid. This is set out in Schedule 7A (Biodiversity Gain in England) of the Town and Country Planning Act 1990 and requires the completed metric calculation tool showing the calculations of the pre-development biodiversity value of the onsite habitat on the date of application and other information.
2. In addition, and in accordance with the PPG Local authority guidance - the council is setting out what further information it requires in terms of BNG to make an application valid. The Councils updated Validation List is set to be published Autumn 2026. In the meantime, the current requirements can be found on our Local validation requirements webpage. The councils draft validation list requires applicants to submit a Biodiversity Gain Statement.
3. The Biodiversity Gain Statement, which will include the completed Metric and habitat condition assessment sheets should set out how the development will achieve a minimum 10% gain in Biodiversity through on and/or offsite provision and how such provision will be secured (condition/planning obligation/financially), maintained and monitored for a minimum period of 30 years. A commentary of how the proposals comply with the Biodiversity Gain Hierarchy should also be included. Applicants are advised to use the council’s Biodiversity Gain Statement template.
4. It will also be helpful to have information about any planning obligation which may need to be entered into connected to the application and the relevant draft heads of term.
5. Paragraph 13 of Schedule 7A of the Town and Country Planning Act 1990 requires that a Biodiversity Gain Plan (BGP) be submitted and approved to discharge the general biodiversity gain condition prior to the commencement of development. A government template for the Biodiversity Gain Plan can be found here. Applicants may submit a draft of this at application stage instead of the Biodiversity Gain Statement if they wish.
6. Applicants are generally encouraged to submit as much information on BNG as possible with their application to assist the council with BNG considerations and all material matters that may interact with BNG. Limited information may result in requests for further information and/or delays in determination of an application.
7. Further information is provided below on particular local matters related to use of the Metric and application of the legislation and guidance. In some cases a lack of information may constitute a reason for refusal.

8. Certain applications are exempt from the BNG regulations. These have been amended as of 6 August 2026. Exemptions include:

• Householder development as defined within article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2015.
• Development granted planning permission by a development order under section 59. This includes permitted development rights.
• Development no larger than 0.2 hectares (applies form 6 August 2026). Development is exempt where it is carried out on a site with an area no larger than 0.2 hectares and it does not impact an onsite priority habitat. This exemption applies only where the application for planning permission was made on or after 6 August 2026.
• Development subject to the de minimis exemption - development that does not impact a priority habitat and impacts on-site habitat less than 25 square metres (e.g. 5m by 5m) of habitat, or 5 metres of linear habitats such as hedgerows.
• Temporary development that is permitted for a period of 5 years or less (from 6 August 2026). Development is exempt where the whole of the development consists solely of temporary development and it does not impact an onsite priority habitat. This exemption applies only where the application for planning permission was made on or after 6 August 2026.
• Development of a biodiversity gain site. Development which is undertaken solely or mainly for the purpose of fulfilling, in whole or in part, the biodiversity gain condition which applies in relation to another development. (This must be connected to a development).
• Retrospective applications made under section 73A of the Town and Country Planning Act.
• Self-build and custom build development are no longer exempt (from 6 August 2026).

• the Biodiversity Gain Requirements (Exemptions) Regulations 2024
• the Biodiversity Gain (Town and Country Planning) (Amendments and Transitional Provisions) (England) Regulations 2026
• Biodiversity net gain - GOV.UK
• PPG – Biodiversity net gain: exempt developments

11. Biodiversity Net Gain (BNG) is an approach to development that leaves biodiversity in a better state than before. Development can produce a biodiversity net gain if it seeks to make its impact on the environment positive, delivering improvements to biodiversity through habitat creation or enhancement after avoiding or mitigating harm.
12. To arrive at a figure for biodiversity gains and losses that result from development, habitats are converted into measurable units which is done using the Statutory Metric. This uses UKHabs definitions and provides a proxy value for biodiversity that can be measured and monitored and is applicable to all terrestrial and intertidal habitats.
13. The Statutory Metric has three components which are:

• Metric calculation tool and Small sites Metric calculation tools – an excel spread sheet which calculates loses and gains.
• Condition assessment sheets (not required for Small Sites Metric) – an excel document with condition assessment sheets for each habitat present on site or to be created.
• User Guides Metric and Small Sites Metric – a pdf document which provides rules and principles that must be followed and detailed guidance on using the above.

14. Together these form the statutory element of the metric so that the calculation tool and where required, the condition assessment sheets, must be provided and the guidance followed for the application/metric to be valid.
15. On smaller sites that are not major the Small Sites Metric (SSM) may be used but there are limitations to what the SSM can cover and so it may not be always be the most suitable choice. Instances where this may be used are set out in the Small Sites Metric (Statutory Biodiversity Metric): User Guide but in summary it can only be used:

• For residential development where the number of dwellings is between 1 and 9, or if this is unknown, the site area is less than 0.5 hectares
• For commercial development where floor space created is less than 1,000 square metres or total site area is less than 1 hectare

And the following apply:

• the habitats on site are available in the SSM
• There are no priority habitats are present on-site
• There are no statutory protected sites or habitats present
• There are no European protected species present

16. If the Small Sites Metric is used and it subsequently transpires that protected species or priority habitats are likely to be present then the application may be made invalid and the applicant required to submit the main metric before the application can proceed.

Applying Strategic Significance to the Metric

17. The Statutory Metric User Guide requires that BNG assessors should assign a strategic significance category for each individual habitat parcel (each row of the metric spreadsheet) both at baseline and post-intervention.
18. The categories are High, Medium and Low which multiply the value of the habitat by 1.15, 1.10 and 1.0 respectively. The values are based on whether the habitat has been identified within a Local Nature Recovery Strategy.
19. The LNRS can be found on The Kent and Medway Local Nature Recovery Strategy (LNRS) website.

Key Rules and Principles

20. Information and calculations submitted to comply with BNG requirements will only be accepted where it complies with the rules and principles within the guidance. Key amongst these is the biodiversity gain hierarchy (See PPG Paragraph: 008 Reference ID: 74-008-20240214) which means taking action in the following order of priority:
    • first, in relation to onsite habitats which have a medium, high and very high distinctiveness (a score of four or more according to the statutory biodiversity metric), the avoidance of adverse effects from the development and, if they cannot be avoided, the mitigation of those effects; and
    • then, in relation to all onsite habitats which are adversely affected by the development, the adverse effect should be compensated by prioritising in order, where possible, the enhancement of existing onsite habitats, creation of new onsite habitats, allocation of registered offsite gains and finally the purchase of biodiversity credits.
21. It is important that developers follow the Biodiversity Gain Hierarchy at every stage including site selection as planning authorities must take into account the Biodiversity Gain Hierarchy when considering whether the biodiversity objective has been met and when determining whether to approve the Biodiversity Gain Plan.
22. The Biodiversity Metric Rules are set out in table 2 of the User Guide

Table 2 Biodiversity metric rules

23. As Rule 4 states deviation from the guidance is only applicable in exceptional circumstances which the guidance goes onto explain are exceptional ecological circumstances that must evidenced and agreed with the LPA prior to the submission of the Biodiversity Gain Plan.
24. The Biodiversity Metric Principles are set out in table 4:

Table 4 Biodiversity metric principles

25. The council would draw applicants attention to principle 1 and the need to use a competent person which would usually be a qualified ecologist with appropriate expertise in the habitats found on site.
26. Attention is also drawn to Principle 7 that requires that interventions are realistic. The council is for instance unlikely to accept that priority habitats of a high quality can be created and maintained where they are under recreational pressure on a development site, are small in scale or require specialist long term management which is unlikely to be consistently provided.
27. In addition where native hedgerows and trees are planted in residential gardens these are likely to be treated as part of a vegetated garden area habitats rather than as newly created native hedgerow linear habitats or urban trees.

Habitat Condition Assessments and Plans

28. The statutory Habitat Condition Assessments are a key part of the metric that needs careful scrutiny. Where applicants consider that a habitat fails to meet certain criteria this may need detailed evidence if it is to be agreed. It is important therefore that original survey data is supplied wherever possible. This is particularly important for semi-improved grasslands where a phase 1 or Preliminary Ecological Appraisal is seldom sufficient to accurately assign a UKHabs grassland type in the metric. For most semi-improved grasslands detailed botanical surveys will be required at an appropriate time of year.
29. In order for the council to understand and follow the metric it is important that clear pre and post development plans are provided that have separate ids for each individual habitat area or linear feature that is listed in the metric.

30. Offsite and significant on site BNG will be secured by legal agreement for a minimum of 30 years managed under an agreed Habitat Management and Monitoring Plan (HMMP) which should ideally follow the Natural England template.
31. In addition, where long term landscape and biodiversity objectives need to be met by a development, the future management may secured by legal agreement or condition for the lifetime of the development. This may be under a HMMP or a landscape and Ecological Management Plan (LEMP). The LEMP often covers matters beyond what is necessary for a HMMP such as community engagement. One document may cover all that is required but the requirements under BNG for 30 years are likely to be more onerous than the longer-term requirements commonly required under a LEMP and the legal agreement will need to reflect this.
32. Early discussion on prospective legal agreements is advised to ensure that all matters are correctly dealt with.

33. Tunbridge Wells Borough Council are working with Kent Nature Partnership and KCC Ecology to continue to develop BNG Policy, practice and guidance.
34. The council is continuing to develop longer term solutions to off-site BNG that will help strategic BNG objectives.
35. The council has indicated that it will produce a Biodiversity Net Gain Supplementary Planning Document (SPD) once it has adopted the new Local Plan. The need for a BNG SPD will be kept under review.

For further information please contact:

David Scully
Landscape and Biodiversity Officer
david.scully@tunbridgewells.gov.uk

For enquiries about legislation and government guidance on biodiversity, please contact Natural England:
BiodiversityNetGainEnquiries@naturalengland.org.uk

Links to useful government guidance and the statutory metric

Guidance – Information you need for Biodiversity Net Gain (BNG)

• Understanding biodiversity net gain
• Biodiversity net gain: exempt developments
• Meet biodiversity net gain requirements: steps for developers
• Biodiversity Gain Plan
• Statutory biodiversity metric tools and guides
• Local authority guidance
• Extract of Draft Validation Guidance