DEFRA consults on bringing NSIPs into the BNG regime and changing it for minor, medium and brownfield development

brownfield development

Just when you thought it was safe to move away from the gov.uk notifications you get a flurry of announcements and consultations.  We here at Irwin Mitchell we have often said it feels like the government knows when we are trying to take some time away, but perhaps in hindsight the week after the whole of the development sector got together in Leeds for UKREiiF it is not surprising we have these now.

Yesterday, DEFRA published two separate consultations concerning how statutory biodiversity net gain (BNG) operates in England:

• Improving the implementation of Biodiversity Net Gain for minor, medium and brownfield development; and
• Consultation on Biodiversity Net Gain for Nationally Significant Infrastructure Projects

Both consultations close on 24 July 2025.

Whilst the NSIP consultation is very much a stand-a-lone document, the consultation about how BNG should be implemented for minor, medium and brownfield sites is not. This morning, MHCLG published a working paper on development thresholds, which is very much a companion piece to this consultation. More on that working paper can be found here, and we really do recommend reading the two together. 

Improving the implementation of Biodiversity Net Gain for minor, medium and brownfield development

The overall intention of this consultation appears to be twofold:

1. To address some of the problems that developers and ecologists have identified when applying statutory BNG to small sites and minor development; and 
2. To test how the BNG regime should respond to MHCLG’s proposal to introduce a new “medium development” classification for developments of between 10 and 49 dwellings or up to 1 hectare in size.

It seeks views on a number of measures that are intended to “streamline” the BNG regime for these sites and make it easier for SME developers to engage with.

The proposals fall into four main categories:

• Changes to the statutory exemptions to BNG 
• Changes to how the BNG Metrics operate
• Making easier for minor developments to access the market for off-site BNG units; and
• Changes to the treatment of Open Mosaic Habitat on brownfield sites. 

1. Smarter Exemptions

The government is considering expanding exemptions to reduce the burden on low-impact developments. Proposals include:

• Replacing the self-build exemption with a broader exemption for single dwellings.
• Raising the ‘de minimis’ threshold to exempt developments with minimal habitat impact.
• Introducing exemptions for temporary developments, biodiversity-focused projects, and improvements to parks and playing fields.

2. Streamlining the Small Sites Metric

The Small Sites Metric (SSM), a simplified tool for calculating biodiversity impacts, could be extended to medium developments (up to 49 homes). Other proposed changes include:

• Removing or amending trading rules to allow more flexible habitat compensation.
• Fixing habitat condition assumptions to better reflect typical site conditions.
• Simplifying habitat categories and improving guidance for non-specialists.
• Digitising the metric tools for improved usability.

3. Greater Flexibility for Off-Site Gains

To ease compliance for minor developments, the consultation proposes relaxing the biodiversity gain hierarchy. This would allow off-site habitat enhancements to be treated equally with onsite gains. Additionally, disapplying the Spatial Risk Multiplier (SRM) could reduce costs and improve access to off-site units.

4. Addressing Brownfield Challenges

Brownfield sites, especially those containing Open Mosaic Habitat (OMH), present unique difficulties. The consultation explores updating guidance for OMH identification and allowing alternative habitat mosaics to serve as compensation when OMH cannot be feasibly recreated.

To quote the consultation “We will work with experts and use the responses to this consultation to inform which alternative habitats may be suitable to compensate for the loss of OMH. A key consideration will be the need to provide a range of structurally diverse microhabitats to ensure the habitat supports invertebrates”.

A Balanced Approach?

It is clear the Government has listened to issues faced by SME builders and the different federations which support them, but does it give the win-win for the economy and the environment? This consultation reflects a broader commitment to balancing environmental protection with economic growth. By refining BNG implementation, the government aims to support housing delivery, encourage private investment in nature recovery, and ensure that biodiversity enhancements are both meaningful and achievable across all scales of development.

Consultation on Biodiversity Net Gain for Nationally Significant Infrastructure Projects

This consultation has been a long time coming. Readers of the Irwin Mitchell’s Environmental Updates or Nicola Gooch’s informed and timely blog posts will be aware that April 2025 was the original date NSIP projects were to be caught by the BNG regime.  On the opposite end of the scale to the smaller site issues set out above, but they have their own problems which need to be overcome

DEFRA’s proposals for how BNG will be applied to NSIP’s can be summarised in the following table:

The consultation proposes a consistent, “core” approach to BNG across all NSIP sectors, with flexibility for sector-specific adaptations if necessary. BNG will apply to all onshore NSIPs in England, including temporary and associated developments within the defined project boundary (order limits). Marine projects beyond the intertidal zone and infrastructure consented through other routes (e.g., hybrid Bills) are excluded for now.

Key Proposals

1. Biodiversity Gain Objective: NSIPs must demonstrate a minimum 10% increase in biodiversity value compared to the pre-development baseline. This must be evidenced through a biodiversity gain plan and calculated using the statutory biodiversity metric.
2. Irreplaceable Habitats: These are excluded from the BNG calculation due to their high ecological value. Any loss must be minimized and compensated through bespoke strategies, but statutory biodiversity credits cannot be used.
3. Calculating BNG: The statutory biodiversity metric will be used to assess habitat value pre- and post-development. Gains can be achieved through on-site enhancements, registered off-site units, or, as a last resort, statutory biodiversity credits.
4. Pre-Development Value: All habitats within the NSIP boundary must be assessed. To prevent habitat degradation before application, earlier dates may be used to determine baseline values if degradation is suspected.
5. Delivering BNG: Unlike Town and Country Planning Act developments, NSIPs can use off-site gains or credits without first exhausting on-site options. Significant on-site enhancements must be secured for 30 years via legal agreements.
6. Temporary Land Use: The consultation seeks views on whether bespoke policies are needed for land used temporarily during construction, which may not be viable for long-term habitat commitments.
7. Integration with Other Policies: BNG must align with existing environmental obligations and the mitigation hierarchy. Enhancements for other purposes (e.g., green infrastructure) may count toward BNG if appropriately evidenced.
8. Evidence and Decision-Making: Applicants must submit a biodiversity gain plan and metric calculation with their development consent order (DCO) application. Post-consent updates may be required, especially for phased developments.

Next Steps

The consultation closes on 24 July 2025. Following analysis of responses, the government will finalize biodiversity gain statements for each NSIP type and publish supporting guidance and templates. The BNG requirement will come into force for NSIPs in May 2026.