Biodiversity Net Gain (BNG)

What is Biodiversity Net Gain? 

In November 2021, the Environment Act laid out statutory targets for the recovery of the natural environment in four priority areas: air quality, biodiversity, water and waste, with an overarching target to halt species decline by the end of 2030.

Specifically, in response to ‘biodiversity’, the Act paved the way for a new legal requirement that development must achieve a minimum 10% net increase in biodiversity, to be maintained for a minimum period of 30 years, so as to leave the natural environment in a measurably better state than it was before development occurred.

When does BNG apply and which types of development will it apply to?  

Biodiversity Net Gain will become a mandatory requirement from the 12th February 2024  for planning applications submitted for major development made under the Town and Country Planning Act 1990.  

“Major development” means development involving any one or more of the following –

    • the winning and working of minerals or the use of land for mineral-working deposits;
    • waste development;
    • the provision of dwellinghouses where—
      1. the number of dwellinghouses to be provided is 10 or more; or
      2. the development is to be carried out on a site having an area of 0.5 hectares or more and it is not known whether the development falls within sub-paragraph (c)(i);
    • the provision of a building or buildings where the floor space to be created by the development is 1,000 square metres or more; or
    • development carried out on a site having an area of 1 hectare or more

BNG for small sites, which do not fall into the category of major development will have an extended transition period and will apply from the 2nd April 2024.

"Small development" means development involving any one or more of the following -

    • 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
    • commercial development where floor space created is less than 1,000 square metres or total site area is less than 1 hectare
    • development that is not the winning and working of minerals or the use of land for mineral-working deposits
    • development that is not waste development

Please note: Not every major and small development will be required to deliver BNG. A set of exemptions are set out in The Biodiversity Gain Requirements (Exemptions) Regulations 2024.

BNG Validation Update

When submitting an application that is subject to Biodiversity Net Gain, on or after the mandatory ‘go live’ dates as stated above, please refer first to the Council’s latest Validation Checklist, which specifies the additional information to be provided alongside the application in order to meet the basic BNG requirements. Please note that this document will be subject to frequent review, at this time.

Delivering Biodiversity Net Gain 

Measuring BNG

Applicants for ‘major development’ will need to submit a completed DEFRA Statutory Biodiversity Metric as part of their application, which must be completed by a ‘competent person’. A Small Sites Metric (SSM) will be available for small sites from the 2nd April 2024.

Put simply, the Biodiversity Metric is a calculator tool for biodiversity. It works by assigning a unit value to habitats present within the development red line boundary, according on their importance for biodiversity. It then calculates how many Biodiversity Units will be generated post-development, accounting for both losses of habitat to the development footprint and habitats created and/or enhanced through the landscaping strategy.


There may be some instances where activities will have been undertaken prior to the submission of a planning application that will reduce the baseline biodiversity value of the site. Where this has occurred since 30th January 2020 and is not in accordance with a planning permission, then the pre-development biodiversity value of the site should be taken to be its biodiversity value immediately before these activities were undertaken.

Achieving BNG

When preparing and designing development proposals to achieve BNG, applicants will need to consider and implement a range of measures, in line with the mitigation hierarchy, where the impact on biodiversity must first be:

  • Avoided - Seek options that avoid harm to ecological features, such as locating development on an alternative site.
  • Mitigated - Minimise negative impacts to ecological features using mitigation measures, such as through good project design or by guaranteeing measures that avoid or minimise negative impacts; and then
  • Compensated - Where any significant negative impacts on biodiversity remain, these should be offset using appropriate compensation measures, such as enhancing existing or creating new, off-site habitats; or purchasing statutory credits.

In addition to the mitigation hierarchy, legislation and policy sets out a sequential hierarchy of BNG delivery, which prioritises on-site BNG delivery first, then off-site BNG delivery, and finally the purchasing of statutory credits as a last resort.

The approach to the mitigation hierarchy and how the application proposes to achieve the minimum 10% BNG, should form part of the Biodiversity Gain Plan. The Biodiversity Gain Plan will need to  be submitted following grant of planning permission, in order to discharge the Statutory Biodiversity Gain Condition.

Securing BNG

Any significant on-site biodiversity gains; and all off-site biodiversity gains must be secured by a S106 legal agreement for at least 30 years; and accompanied with a Habitat Management and Monitoring Plan.

Further information

There is a commitment across the Kent Authority’s to develop a shared and consistent approach to BNG. Kent’s LPAs (including Ashford Borough Council) are jointly funding the development of guidance and resources to inform BNG delivery. This joined up approach is being supported and coordinated by Kent County Council (KCC).

For further information, guidance documents, and resources such as the new Kent BNG Sites Register, please visit: Biodiversity Net Gain for Kent and Medway | Making Space For Nature Kent