Definition

Biodiversity Net Gain (BNG) is a legal requirement, introduced under the Environment Act 2021, for most new developments granted planning permission in England to deliver a measurable net increase in biodiversity value compared with the pre-development baseline. It became mandatory for most major developments from February 2024, and for most small sites from April 2024, replacing the previous, largely voluntary approach to biodiversity in planning.

Why this matters for Sustainability

BNG is one of the most significant recent shifts in UK development law: it moves biodiversity from a "do no harm" consideration to a mandatory, quantified, legally secured obligation attached to almost every planning permission. For RICS surveyors advising on acquisition, development appraisal, or planning strategy, BNG is now a direct cost and risk factor that has to be priced into a scheme from the earliest stage, not an environmental add-on assessed after design is fixed.

1. The statutory biodiversity metric

BNG is measured using Defra's statutory biodiversity metric, which converts habitat type, condition, distinctiveness and strategic significance into standardised "biodiversity units." A baseline survey establishes the pre-development unit value; the post-development and any off-site or enhancement units are then compared against it to confirm whether the mandatory 10% uplift has been achieved.

2. The mitigation hierarchy

BNG follows a clear order of preference: avoid harming existing valuable habitat through site layout and design; minimise any unavoidable impact; restore or enhance habitat on-site; and only then compensate for any shortfall using off-site provision or, as a genuine last resort, statutory biodiversity credits bought from the government at a deliberately high price to discourage their use.

3. On-site, off-site, and statutory credits

On-site delivery is the preferred route and avoids ongoing third-party arrangements, but many urban or constrained sites cannot physically accommodate enough habitat area to reach 10%. Off-site provision — buying biodiversity units from a registered habitat bank — is the common fallback, secured by a conservation covenant or a planning obligation and recorded on the national biodiversity gain sites register. Statutory credits sit below both as the least favoured, most expensive option.

4. The 30-year secured period

Whichever route is used, the gain must be secured and actively managed for a minimum of 30 years, typically through a conservation covenant or a Section 106 planning obligation with an accompanying habitat management and monitoring plan. This creates a long-term cost and monitoring obligation that must be reflected in a scheme's viability appraisal, not treated as a one-off capital cost at construction stage.

Assessor tip

Be ready to explain why statutory biodiversity credits are deliberately the most expensive route, not just one option among equals — assessors use this to check whether a candidate understands BNG as a genuine mitigation hierarchy, with a built-in financial incentive to deliver gains close to the development, rather than a simple offsetting scheme a developer can buy their way out of.

Relevant RICS guidance and legislation

  • Environment Act 2021, Schedule 14 (biodiversity gain)
  • Town and Country Planning Act 1990 (as amended) — biodiversity gain condition
  • Defra Biodiversity Metric 4.0 and statutory biodiversity metric guidance
  • RICS Sustainability and ESG professional statement

Ethics and Rules of Conduct angle

Rule 4 of the RICS Rules of Conduct requires members to act with integrity. A surveyor advising a client to rely heavily on cheap, poorly substantiated off-site units, or to understate a scheme's true long-term BNG management cost to improve an appraisal's headline viability figure, risks producing advice that misleads the client about real project risk and the scheme's genuine environmental credentials — a growing area of scrutiny as BNG delivery is monitored over its full 30-year term.

APC questions and answers

Q (Level 1)What is Biodiversity Net Gain (BNG)?

BNG is a legal requirement under the Environment Act 2021 for most new developments in England to deliver a measurable net increase in biodiversity value compared with the pre-development baseline, rather than simply minimising harm.

Q (Level 1)What percentage uplift does mandatory BNG require?

A minimum 10% net gain in biodiversity value, measured using the statutory biodiversity metric and expressed in standardised “biodiversity units.” The gain must be secured for a minimum of 30 years.

Q (Level 2)What is the mitigation hierarchy in the context of BNG?

Avoid harm to existing habitats first; minimise any unavoidable impact; restore habitat on-site where possible; and only then compensate using off-site habitat creation or, as a last resort, statutory biodiversity credits purchased from the government. On-site gains are preferred over off-site, and off-site is preferred over statutory credits.

Q (Level 2)A client's development site has limited space for on-site habitat creation. How would you advise them on meeting their BNG obligation?

I would first establish the baseline biodiversity value using the statutory metric and confirm how much on-site gain, if any, is realistically achievable within the scheme. Where on-site provision cannot deliver the full 10% uplift, I would advise sourcing the shortfall through an off-site biodiversity gain site, typically via a habitat bank, secured by a conservation covenant or planning obligation and registered on the national biodiversity gain sites register, before falling back on statutory credits, which are priced to be a last resort and the most expensive option.

Q (Level 3)How does BNG affect the valuation of a residential development site, and what should a surveyor check before advising on viability?

BNG introduces a direct cost line into the development appraisal: the cost of on-site habitat works, any off-site unit purchase or statutory credits, and the long-term (minimum 30-year) monitoring and management commitment, which is often secured through a conservation covenant with its own ongoing cost. I would check the developer's biodiversity metric calculation has been independently reviewed, confirm whether a habitat management and monitoring plan has been costed into the appraisal rather than treated as a one-off capital item, and flag BNG as a material consideration affecting residual land value, particularly on sites with little spare land for on-site delivery.