What is Biodiversity Net Gain (BNG)?

Biodiversity Net Gain (BNG) is an approach to development and land management that ensures new construction projects leave the natural environment in a measurably better state than it was before work began.  

Since April 2024, BNG has been a mandatory legal requirement for all major and minor planning applications in England (excluding standard householder applications). Under this law, developments must deliver a minimum 10% net gain in biodiversity compared to the site’s original baseline. Some local areas require a higher level of biodiversity improvement.  

• The 10% Rule Explained: If a development site is assessed to have a baseline value of 10 BNG units before construction, the developer must ensure the site provides at least 11 BNG units post-development.

• The 30-Year Rule: As set out in the Environment Act 2021, any habitats created or enhanced to meet BNG requirements must be legally secured and maintained for at least 30 years.  

• Like-for-Like Replacement: The legislation requires that lost habitats are replaced on a "like-for-like" or better basis to prevent the loss of ecologically distinct landscapes.

How is BNG Calculated?

Biodiversity Net Gain is calculated using the official Statutory Biodiversity Metric, developed by Defra and Natural England. This digital tool calculates the numerical value of a site's baseline and post-development habitats by looking at four core factors:  

• Size: The physical scale of the habitat.

• Condition: How healthy and well-managed the ecosystem is.

• Distinctiveness: How rare or ecologically valuable the habitat type is.

• Strategic Significance: Whether the habitat aligns with local nature recovery priorities.

The metric categorises habitats into specific groups, each governed by strict trading rules. Depending on the category, developers must meet rules that require them to replace habitats like-for-like, or upgrade to a "better" or more distinct habitat type to ensure a genuinely positive outcome for nature.

Onsite vs. Offsite Mitigation

When planning a project, developers must follow the mitigation hierarchy, which prioritises avoiding and reducing habitat destruction from the very beginning of the design phase. If habitat loss is unavoidable, the deficit must be addressed using one of three pathways:

1. Onsite Units: The first stage is to restore or create habitats directly within the development's redline boundary. This can include landscaping, public open spaces, wildlife-friendly gardens, and urban trees.  

2. Offsite Units: On dense or highly built-out schemes, delivering a 10% gain onsite is often physically impossible. In these scenarios, developers must purchase offsite BNG units from dedicated "habitat banks" elsewhere in the country.

3. Statutory Credits: If no onsite or market-led offsite units are available, developers can purchase Statutory Credits directly from the government as a last resort. However, the government intentionally prices these credits significantly higher than the offsite market value to discourage their use.  

Understanding the Spatial Multiplier

When purchasing offsite units, developers must factor in the spatial multiplier built into the statutory metric. This rule financially penalises projects that buy units outside the development's local area:

• Local Delivery: Buying units within the same Local Planning Authority (LPA) or National Character Area (NCA) incurs no penalty.

• Neighbouring Delivery: If the offset site is in a neighbouring NCA or LPA, the units are worth 0.75 (meaning you must buy more units to achieve the same score).

• National Delivery: If the offset site is anywhere else in the country, the units are worth 0.5, doubling the requirement for offset.

The Legal Framework

To prove compliance, BNG habitats must be legally bound. This is achieved through one of two mechanisms:  

• Section 106 Agreements: Legally binding deeds agreed directly with the Local Planning Authority.  

• Conservation Covenants: Legal agreements made with an officially designated "Responsible Body."  

These legal tools ensure that the habitat is monitored, funded, and managed in line with an approved Habitat Management and Monitoring Plan (HMMP) for the full 30-year term. Additionally, a legal charge is placed on the land's title deeds, protecting the habitat bank even if the land is sold or transferred to a new owner.

The Biodiversity Gain Sites Register

Once a legal agreement is secure, the offsite habitat bank must be registered on the Biodiversity Gain Sites Register managed by Natural England and Defra. This public register ensures transparency, tracks unit allocations, and prevents the "double counting" of credits. Under BNG guidelines, offsite units cannot legally be allocated to a planning application until the host habitat bank is fully registered.  

Latest Updates to BNG Legislation (GOV.UK)

As the framework evolves, the government continues to refine the system based on implementation feedback. The latest changes include:

1. New Small Site Exemption (0.2 Hectares or Less)

Published: December 2025 / Outcome Framework April 2026

The Update: To drastically reduce administrative and financial burdens on minor developments, secondary legislation has introduced a blanket exemption for sites measuring 0.2 hectares or less. Defra estimates this change exempts roughly 50% of minor residential planning applications that previously triggered BNG, enabling Local Planning Authorities to focus their resources on larger, high-impact developments.  

2. Transition Plan for Nationally Significant Infrastructure Projects (NSIPs)

Published: April 2026. In force from November 2026.

The Update: BNG framework expansion rules have been finalised for Nationally Significant Infrastructure Projects (such as major rail, energy, and water schemes). Following consultations, the mandatory 10% BNG framework for NSIPs will formally launch in November 2026. This gives infrastructure developers adequate time to integrate the final statutory frameworks into their early designs. To remain proportionate, the metric will apply strictly to the habitats directly impacted by the development rather than the entire baseline site boundary.  

3. Tightened Rules for Short-Term & Temporary Permissions

Published: April 2026  

The Update: New regulatory updates explicitly address temporary planning permissions. Temporary applications granted for a maximum duration of 5 years or less are now subject to clear, targeted exemptions and relaxed criteria, ensuring that short-term site uses are not hindered by standard 30-year infrastructure mandates while still protecting immediate local ecological baselines.  

4. New Voluntary Quantitative Reporting for LPAs

Published: April 2026  

The Update: Under the Environment Act, LPAs are required to formally report on local biodiversity actions. To standardise the data, Defra released its official BNG Reporting Template (Version 1.0). While compiling detailed quantitative unit data remains voluntary for under-resourced councils, this standardised digital system will stream directly into Defra and MHCLG's national monitoring systems to track nature recovery targets and adjust statutory credit pricing in the future.  

Need to Secure BNG Units for Your Project?

Navigating statutory metric trading rules, spatial multipliers, and local register requirements can be complex. If you have any questions about how these latest legislative updates impact your upcoming planning application, or if you are ready to purchase high-integrity, legally secured offsite BNG units, our team is here to help.

Contact BNG Partnership today to speak with our planning and land experts, or get in touch directly to secure the exact habitat units your project needs for approval.

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