What Is The Definition of Biodiversity Net Gain (BNG)?


The British Standard for Biodiversity Net Gain (BNG) [1] defines BNG as a:

Quantifiable outcome from project activities that deliver demonstrable benefits for biodiversity compared to the baseline situation

The Biodiversity Net Gain Good Practice Principles [2] define BNG as:

“Development that leaves biodiversity in a better state than before, and an approach where developers work with local governments, wildlife groups, landowners and other stakeholders in order to support their priorities for nature conservation.”

Broadly speaking, BNG is the use of a biodiversity metric calculator to calculate both the existing baseline biodiversity units at a site, and the projected number of biodiversity units post-development, comparing the two to calculate either a projected ‘net gain’ in biodiversity, or ‘net loss’. A range of qualitative conditions must also be met to claim BNG at a site, and BNG does not change the existing obligations for planning applications regarding ecology, so it’s critical to keep in mind that BNG is not simply a metric calculator output.

Biodiversity Net Gain is not a new concept globally, with nations such as Germany, USA and Australia already following BNG principles in decision making.

Why Is BNG Becoming Mandatory?


The Environment Act 2020 received Royal Assent in 2021, making the Bill an Act of Parliament. The resulting Environment Act 2021 sets out amendments to the Town and Country Planning Act with a new Biodiversity Net Gain objective. Schedule 14 of the Act sets the BNG objective as ‘The biodiversity gain objective is met in relation to development for which planning permission is granted if the biodiversity value attributable to the development exceeds the pre-development biodiversity value of the onsite habitat by at least the relevant percentage.’ The ‘Relevant Percentage’ is currently 10% in the Act, however the Secretary of State may by regulations amend this figure.

The Act also sets out that the mandatory BNG in planning decisions is to be carried out in accordance with a Secretary of State issued biodiversity metric calculator and that the planning authority must approve a biodiversity gain plan for the planning application site, informed by the metric.

The management and protection of Biodiversity Net Gain habitats sites must be secured for at least 30 years via obligations or conservation covenants under the Act. Biodiversity units can be delivered on-site, off-site or via statutory biodiversity credits and will be recorded in a national register for Biodiversity Net Gain sites.

The Environment Act 2021 does not change the existing protections afforded to protected species, and developments are still required to follow the mitigation hierarchy of avoiding, mitigating, and as a last resort compensating for biodiversity impacts. Biodiversity Net Gain is a concept of additionality.

Is BNG Mandatory Now?


Not yet. However, this varies between Local Planning Authorities and on a site by site basis.

The Timeline of Mandatory BNG

The Environment Act 2021 contains provisions to mandate a 10% Biodiversity Net Gain for all Town & Country Planning Act developments, however secondary legislation is required to bring the Act into law. The expected timeline for mandatory net gain for planning applications is as follows:

  • In Spring 2022 a DEFRA consultation is expected, to inform secondary legislation and detail
  • In Spring 2023 it is expected that a BNG sites register, and a statutory biodiversity unit credit sales platform will be released
  • In Autumn 2023 BNG is expected to become mandatory for all Town & Country Planning Act (TCPA) developments.

It is anticipated that it will take approximately 2 years for BNG to become mandatory via a range of secondary legislation. The journey to mandatory BNG will be informed by public consultation and more detail on BNG implementation will be provided, covering:

  • Biodiversity net gain plans
  • Exemptions
  • Irreplaceable habitats
  • Phased developments
  • Net gain sites register (not yet in place)
  • Additionality limitations, including ‘stacking’ with other environment schemes
  • Purchasing of statutory biodiversity credits (not yet in place)
The Current National Context In England

The National Planning Policy Framework 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’, and that ‘To protect and enhance biodiversity, plans should identify and pursue opportunities for securing measurable net gains for biodiversity’.

The NPPF also sets out that when determining planning applications, Local Planning Authorities should apply a principle that ‘development whose primary objective is to conserve or enhance biodiversity should be supported; while opportunities to improve biodiversity in and around developments should be integrated as part of their design, especially where this can secure measurable net gains for biodiversity’.

The current NPPF does not mandate BNG and does not set a clear benchmark, where requested, of what percentage uplift in biodiversity units should be provided post-development vs. the baseline situation. These factors have in recent years resulted in confusion of its implementation where it is requested by Local Planning Authorities in determining planning applications.

The Current Local Context In England

For now, BNG policies are still highly variable between Local Planning Authorities. Some have no requirements for Biodiversity Net Gain in their Local Planning Policies, whilst some have a mandatory requirement for all developments but no percentage uplift assigned.

In some instances, Local Planning Authorities (LPAs) are requesting a Biodiversity Net Gain assessment where there are no BNG provisions in local planning policy, relying on the NPPF only. We expect to see an increase of LPA requests for Biodiversity Net Gain assessment over the next two years in the run up to its mandatory implementation for all Town & Country Planning Act developments.

Best Practice Guidance Framework


Leading up to The Environment Act 2021 there has been a rapidly evolving resource of best practice guidelines, which will almost certainly form the benchmark for assessment of net gain plans within the planning application process when mandatory BNG comes into force. 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. The metric currently used is the Natural England Biodiversity Metric 3.0 [6], also termed the ‘Defra Metric’. To input site habitat data into the metric, the UK Habitat Classification System (UKHAB) [7] 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 mitigation 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.” In the UK there is currently no definitive list of irreplaceable habitats. However, the National Planning Policy Framework and British Standard [1] provide context for professional judgement to be applied.

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 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 3.0 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.


[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] Natural England, 2021. The Biodiversity Metric 3.0 (JP039). Natural England Joint Publication.

[7] 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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