What Is Biodiversity Net Gain (BNG)?

Biodiversity Net Gain (BNG) can be defined as an approach to development that ensures habitats for wildlife are in a measurably better state post-development than prior to development.

The BNG can be delivered either entirely on-site, or part on-site and part off-site, or via the purchasing of biodiversity units or statutory biodiversity credits. The BNG framework requires preference to be given firstly to on-site delivery. In all cases where mandatory BNG applies, the development must result in an overall ‘net gain’ relative to the pre-development biodiversity value.

The post-development percentage uplift of BNG required under The Environment Act 2021 is 10%, relative to the pre-development biodiversity value of the site. A range of qualitative conditions must also be met to claim BNG at a site.

The Environment Act 2021

For all development defined as ‘Major’, as of the 12th of February 2024, delivering a 10% Biodiversity Net Gain is a mandatory legal requirement. Major development includes:

• the provision of dwellinghouses where—
  • the number of dwellinghouses to be provided is 10 or more; or
  • 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 is 10 or more dwellinghouses;
• the provision of building(s) where the floor space to be created by the development is 1,000 square metres or more;
• development carried out on a site having an area of 1 hectare or more;
• the winning and working of minerals or the use of land for mineral-working deposits;
• waste development.

For small developments BNG became mandatory from 2 April 2024. These include:

• residential development of 1 to 9 dwellings on a site of an area 1 hectare or less, or if the number of dwellings 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 the 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.

Some sites are exempt from the BNG requirements under The Environment Act. These include:

• existing planning applications, where planning permission was made prior to 12 February 2024
• development that does not impact priority habitat and impacts less habitats than:
  • 25 square metres (5m by 5m) of on-site habitat; or
  • 5 metres of on-site linear habitats such as hedgerows;
• householder applications;
• self-build and custom build applications, meeting all of the following criteria:
  • no more than 9 dwellings; or
  • on a site no larger than 0.5 hectares; or
  • consists exclusively of dwellings that are self-build or custom house building;
• developments that are granted planning permission by a development order (including permitted development rights).

The National Planning Policy Framework (NPPF)

The NPPF (last updated 12 December 2024) sets out that ‘Planning policies and decisions should contribute to and enhance the natural and local environment by: […] minimising impacts on and providing net gains for biodiversity, including by establishing coherent ecological networks that are more resilient to current and future pressures’.

Local Planning Authorities can request that developments deliver a measurable Biodiversity Net Gain via a Biodiversity Metric even where The Environment Act 2021 does not apply, using the government’s planning policies set out in the NPPF alongside local planning policy.

Local Planning Policy

At a local level, planning policy can require that developments deliver in excess of the 10% Biodiversity Net Gain requirement under The Environment Act 2021. Examples of areas where an above 10% BNG policy is in draft, emerging, or adopted planning policy are:

• The Royal Borough of Kingston upon Thames Council – 30% BNG
• Greater Cambridge – 20% BNG
• East Devon District Council – 20% BNG
• Canterbury City Council – 20% BNG
• Maidstone Borough Council – 20% BNG
• Medway Council – 20% BNG
• Swale Borough Council – 20% BNG
• London Borough of Richmond upon Thames – 20% BNG
• Mole Valley District – 20% BNG
• Surrey Heath Borough Council – 20% BNG
• Swindon Borough Council – 20% BNG

Best Practice Guidance Framework

Leading up to and post The Environment Act 2021 there has been a rapidly evolving resource of best practice guidelines, which form the benchmark for assessment of net gain plans within the planning application process. Of most relevance are:

• The British Standard BS 8683:2021 Process for designing and implementing Biodiversity Net Gain — Specification (BSI, 2021)[1]
• Biodiversity Net Gain Report and Audit Templates (CIEEM, 2021)[3]
• Biodiversity Net Gain. Good Practice Principles of Development. A Practical Guide (CIEEM, CIRIA, IEMA 2019)[4]
• Guidelines For Ecological Impact Assessment in the UK and Ireland (CIEEM, 2018)[5]
• Biodiversity Net Gain. Good Practice Principles for Development (CIEEM, CIRIA, IEMA 2016) [2]

Underpinning Biodiversity Net Gain assessments is the use of a biodiversity metric calculator to calculate both the baseline biodiversity units at a site, and the projected number of biodiversity units post-development, calculating the percentage uplift or shortfall. The Environment Act 2021 makes provisions for the use of a statutory issued metric. To input site habitat data into the metric, the UK Habitat Classification System (UKHAB) [6] is used.

Achieving BNG is much more than just a number. The British Standard for BNG BS 8683:2021, states that:

“In order to achieve BNG, a project has to follow the mitigation hierarchy and be able to demonstrate that it has followed the BNG Good Practice Principles for development.”

Under the Good Practice Principles for Development, making a measurable Net Gain contribution (i.e. measurable via a biodiversity metric calculator) is just one of ten principles to satisfy when claiming BNG for a project with reference to best practice guidelines. BNG requires satisfying a much broader range of qualitative principles, underpinned by the quantitative proxy outputs of biodiversity metric calculators.

Will BNG Be Achievable On My Site?

In the majority of cases BNG is achievable, including off-site delivery of biodiversity units following the required spatial hierarchy. However, assessing the feasibility of a development project to deliver BNG as early as possible in the site selection and design stage is key to BNG delivery and avoiding significant cost and time delays.

It is possible to make a quick high-level assessment of the BNG feasibility of a project based on reasonable assumptions via a desktop study.  This is particularly useful when considering different site options and can also be used to estimate likely BNG costs and requirements, before commissioning a full net gain plan suitable for submission as part of a planning application.

Early consideration for a project includes searching for potential ‘show stoppers’ for BNG such as irreplaceable habitats and/or designated sites for nature conservation. For example, the new British standard for BNG [1] states that “Projects with impacts on irreplaceable habitats cannot achieve BNG.”

Other important factors to be considered early include:

• Apply mitigation hierarchy of avoiding, then mitigating, and as a last resort compensating for ecological impacts, which is also a condition of claiming BNG.
• The current Local Planning Policy Biodiversity Net Gain requirement, and similar sites as case studies within the Local Planning Authority area.
• Opportunities and risks presented by the Local Biodiversity Action Plan targets.
• Can trading rules be satisfied?
• Carrying out a high-level biodiversity metric calculation at site selection stage based on reasonable assumptions, to assess:
  • Could BNG feasibly be delivered within the site
  • Could land adjacent to the site be acquired to deliver BNG
  • Is offsetting likely to be required, is there potential to use environment banks or can a potential shortfall in biodiversity units be purchased via the Local Planning Authority?

As a general rule, BNG is about enhancing not transforming the natural environment and so trading rules are built into the Biodiversity Metric which can have significant implications for BNG delivery. Under trading rules habitats created to compensate for losses should be of the same broad habitat type (e.g. woodland for woodland, grassland for grassland), unless there is good ecological reason to do otherwise. For example, it may be more appropriate to restore a habitat back to a more historical baseline habitat of ecological value, prior to its modification into what its baseline is today.

References

[1] British Standards Institute (BSI) (2021). BS 8683:2021 Biodiversity. Process for designing and implementing Biodiversity Net Gain — Specification. British Standards Institute, London.

[2] Biodiversity Net Gain. Good Practice Principles for Development (CIEEM, CIRIA, IEMA 2016)

[3] CIEEM (2021). Biodiversity Net Gain Report and Audit Templates Chartered Institute of Ecology and Environmental Management, Winchester, UK.

[4] Baker, J. Hoskin, R. Butterworth, T (2019) Biodiversity Net Gain. Good Practice Principles for Development. A Practical Guide. CIRIA. ISBN: 978-0-86017-791-3

[5] CIEEM (2018) Guidelines for Ecological Impact Assessment In The UK and Ireland. Chartered Institute of Ecology and Environmental Management, Winchester.

[6] Butcher, B., Carey, P., Edmonds, R., Norton, L. and Treweek, J. (2020). The UK Habitat Classification User Manual Version 1.1 at http://www.ukhab.org.

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