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Biodiversity Net Gain: prepare for mandatory 10% gain requirements

Biodiversity Net Gain (BNG) became mandatory for most planning permissions in England from February 2024, requiring developers to deliver a 10% increase in biodiversity value. Small sites followed in April 2024. December 2025 reforms introduced significant new exemptions for sites up to 0.2 hectares. NSIPs will be subject to BNG from May 2026. Here is what developers and landowners need to know.

Change event: Biodiversity Net Gain mandatory for planning permissions in England from February 2024 Effective 12 February 2024

What Biodiversity Net Gain requires

Biodiversity Net Gain (BNG) is a mandatory planning requirement in England introduced through Schedule 7A of the Town and Country Planning Act 1990 (inserted by the Environment Act 2021). It requires all planning permissions — except those specifically exempted — to deliver at least a 10% increase in biodiversity value relative to the pre-development baseline.

The biodiversity gain is measured using the statutory biodiversity metric, a standardised assessment tool published by Natural England. The 10% gain must be maintained for at least 30 years through planning conditions, section 106 agreements, or conservation covenants.

BNG has been phased in over two years:

  • 12 February 2024: Major developments (10+ dwellings or sites over 1 hectare; non-residential development over 1,000 square metres)
  • 2 April 2024: Small developments (9 or fewer dwellings on sites under 1 hectare; non-residential development under 1,000 square metres)
  • May 2026: Nationally Significant Infrastructure Projects (NSIPs) — originally planned for November 2025 but delayed
Mandatory gain required
10% increase in biodiversity value (measured using statutory metric)
Maintenance period
At least 30 years
Cost per biodiversity unit
£9,000–£15,000 (market varies significantly)
Geographic scope
England only (planning is devolved)
NSIPs subject to BNG
From May 2026
Exemption (December 2025 reform)
Sites up to 0.2 hectares (~60% of planning applications)

Who must comply

BNG applies to all planning applications submitted on or after the relevant commencement date (12 February 2024 for major developments, 2 April 2024 for small developments), unless the application falls within a specific exemption.

Transitional provisions apply: applications made before the commencement dates are not subject to BNG requirements, even if determined after those dates.

Local planning authorities are responsible for determining biodiversity gain plans, monitoring delivery of onsite and offsite gains, and taking enforcement action where BNG obligations are not met.

Exemptions from BNG

The following developments are exempt from BNG requirements under the Biodiversity Gain Requirements (Exemptions) Regulations 2024 (SI 2024/47):

  • Householder applications: Extensions, loft conversions, and other minor domestic works
  • De minimis developments: Developments impacting under 25 square metres of non-priority habitat or 5 metres of linear habitat (excluding priority habitats entirely — any impact on priority habitat triggers BNG)
  • Self-build and custom build: Maximum 9 dwellings on sites up to 0.5 hectares
  • Permitted development: Works that do not require planning permission
  • Retrospective permissions: Applications under section 73A of the Town and Country Planning Act 1990

In December 2025, the government announced significant reforms to BNG, introducing a new area-based exemption for developments on sites up to 0.2 hectares. This exemption applies to approximately 60% of all planning applications and represents a major reduction in the scope of BNG coverage.

The government is also consulting on a targeted brownfield exemption for residential development up to 2.5 hectares. These reforms have generated criticism from environmental groups, who argue they weaken nature protections.

⚠️ 30-year maintenance obligation

Developers and landowners must ensure biodiversity gains are maintained for at least 30 years. This obligation is secured through planning conditions, section 106 agreements, or conservation covenants. Failure to maintain the required habitat can result in enforcement action by the local planning authority.

If you are delivering BNG through onsite habitat creation, you must have a credible long-term management plan in place before the planning permission is granted. If you plan to sell the site after development, ensure the BNG obligations are properly transferred to the new owner.

How to deliver Biodiversity Net Gain

Developers must submit a biodiversity gain plan to the local planning authority demonstrating how the 10% gain will be achieved. There are three routes to deliver BNG:

  • Onsite habitat enhancement: Creating, restoring, or enhancing habitats on the development site itself. This is the preferred approach and often the most cost-effective.
  • Registered offsite biodiversity gains: Purchasing biodiversity units from landowners who have registered habitat creation or enhancement sites on Natural England's Biodiversity Gain Site Register.
  • Statutory biodiversity credits: As a last resort, purchasing statutory credits from Natural England. Credits are significantly more expensive than onsite or offsite gains and should only be used when other options are not feasible.

Natural England operates the Biodiversity Gain Site Register for registered offsite gains and administers the statutory biodiversity credits scheme. Developers can search the register to identify available offsite units.

Costs of Biodiversity Net Gain

The cost of BNG varies significantly depending on the delivery route and local market conditions. Estimates suggest:

  • Onsite habitat creation: Varies widely based on site characteristics, existing habitat baseline, and the type of habitat being created. Can range from low thousands to tens of thousands of pounds depending on scale and complexity.
  • Offsite biodiversity units: £9,000–£15,000 per biodiversity unit in the emerging market (though prices vary significantly by region and habitat type).
  • Statutory credits: Priced to be the most expensive option to incentivise onsite and offsite delivery first.

For a typical small housing development, BNG costs might add £20,000–£50,000 to the project, though this is highly site-specific. Developers should budget for BNG costs early in the project planning process and consider how to integrate habitat creation into the design to reduce overall costs.

December 2025 reforms and what they mean

In December 2025, the government announced significant reforms to BNG implementation, introducing:

  • New 0.2 hectare exemption: Developments on sites up to 0.2 hectares are now exempt from BNG. This affects approximately 60% of all planning applications and substantially reduces the number of small housing and commercial developments subject to BNG.
  • Proposed brownfield exemption: The government is consulting on an exemption for residential development on brownfield sites up to 2.5 hectares. If implemented, this would further reduce BNG coverage.

These reforms have raised concerns among conservation organisations about weakening nature protections. Environmental groups argue that the exemptions undermine the original intent of the Environment Act 2021 and reduce the total habitat gain that would have been delivered through the BNG framework.

For developers, the reforms create uncertainty about the stability of the regulatory framework. Projects in the pipeline may need to reassess whether they will be subject to BNG, depending on site size and timing of the planning application.

Nationally Significant Infrastructure Projects (NSIPs)

The extension of BNG to Nationally Significant Infrastructure Projects (NSIPs) has been confirmed for May 2026, following a delay from the initially proposed November 2025 introduction date.

NSIPs are large-scale infrastructure projects — such as power stations, major transport schemes, waste facilities, and water infrastructure — that require development consent from the Secretary of State rather than planning permission from a local planning authority.

NSIP promoters should begin preparing for BNG requirements now, including baseline biodiversity assessments and consideration of how to deliver the 10% gain through onsite habitat creation or offsite units.

❗ What developers should do now

If you are planning a development in England:

  • Check whether your site is subject to BNG — use the December 2025 reforms (0.2 hectare exemption) and other exemptions to determine whether BNG applies to your project
  • Conduct a baseline biodiversity assessment — use the statutory biodiversity metric to calculate the pre-development baseline
  • Explore onsite delivery first — integrate habitat creation into your design to reduce costs and deliver the 10% gain onsite where feasible
  • Search the Biodiversity Gain Site Register — if onsite delivery is not feasible, identify available offsite units in your area
  • Budget for BNG costs — factor in £9,000–£15,000 per unit (or onsite creation costs) when preparing development appraisals
  • Prepare a biodiversity gain plan — your planning application must include a biodiversity gain plan demonstrating how the 10% gain will be achieved and maintained for 30 years

ℹ️ England only

BNG applies only in England. Planning is a devolved matter, with Scotland, Wales, and Northern Ireland operating separate planning systems. If you are developing in Scotland, Wales, or Northern Ireland, check the applicable planning requirements in those nations.

ℹ️ Local planning authority capacity concerns

The National Audit Office raised concerns in 2024 about whether local planning authorities have sufficient access to ecological expertise to effectively discharge their BNG compliance and enforcement obligations. 41% of councils reported capacity concerns.

Developers may experience delays in planning determinations as local planning authorities build capacity to assess biodiversity gain plans. Submit high-quality, complete biodiversity gain plans to reduce the likelihood of requests for further information or delays.

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