Making Biodiversity Net Gain Work Better for Small Developers

In May 2025, Defra launched a consultation aimed at improving how the Biodiversity Net Gain (BNG) requirement is applied to minor, medium, and brownfield developments. Introduced under the Environment Act 2021, BNG mandates most developments in England to deliver a 10% net gain in biodiversity. While the policy has been broadly successful for large-scale projects, smaller developers have raised concerns over complexity, cost, and practicalities especially when working on constrained or low-impact sites.

The government’s goal is to keep the BNG policy effective while making it more proportionate and accessible for SMEs. This latest consultation explores how to reduce burdens without compromising environmental outcomes.

What’s Being Proposed?

1. Smarter Exemptions. Defra is considering new and revised exemptions, including:

• Replacing the current self-build exemption with a simpler one for single dwellings under 0.1 hectares.

• Raising the “de minimis” threshold—currently 25m²—to as much as 250m² to exempt more low-impact development.

• A potential full exemption for all minor developments (up to 9 dwellings), to reduce costs and help unlock stalled small sites.

• New exemptions for:

  • Parks and playing fields with only low-value habitats,

  • Projects that aim to restore or enhance nature,

  • Temporary permissions (up to 5 years).

    
    

2. A Better Small Sites Metric (SSM)The Small Sites Metric. A simplified biodiversity calculator for minor development could be:

• Rebranded as the “low impact metric” and expanded to medium-sized sites (<1ha),

• Made easier to use by removing rigid “trading rules” and reducing the number of habitat types,

• Supported by new guidance and clearer definitions for non-ecologists,

• Adjusted so that developers aren't penalised when working with poor condition habitats.

  
  

3. More Flexible Off-site BNG. To ease delivery challenges, proposals include:

• Relaxing the biodiversity gain hierarchy for minor development, allowing more freedom to go off-site,

• Removing the spatial risk multiplier (which currently makes buying distant units more expensive).

  
  

4. Brownfield Focus: Open Mosaic Habitat (OMH). Recognising the difficulty of recreating OMH, the consultation suggests new definitions and the possibility of substituting similar habitat mosaics when OMH cannot be practically delivered.

Why It Matters

Minor and medium developments make up the majority of planning applications. These changes could help unlock small sites, cut unnecessary costs, and support SME builders—all while keeping the focus on reversing biodiversity loss.

The consultation is open until 24 July 2025, and Defra is seeking feedback from developers, planners, ecologists, and the wider community. If you’re involved in delivering small-scale development, now is the time to have your say. Email us at david@maddoxplanning.com if you'd like to know more and you think that these changes would help you.

The changes could result in a full exemption for minor developments