Defra has published new guidance setting out how mandatory Biodiversity Net Gain (BNG) will operate for Nationally Significant Infrastructure Projects (NSIPs) in England.

From 2 November 2026, applications for Development Consent Orders (DCOs) will be required to demonstrate a minimum 10% biodiversity net gain, supported by the relevant Biodiversity Gain Statement. The new guidance provides considerably more detail on how this will work in practice, including an important new concept for NSIPs: the BNG boundary.

The BNG boundary is not necessarily the Order Limits

For many NSIPs, particularly large solar, wind and associated grid infrastructure projects, DCO Order Limits can cover very substantial areas. Importantly, the new guidance confirms that the whole of the Order Limits does not automatically need to form the biodiversity baseline.

Instead, applicants must define a separate BNG boundary. This must include habitats within the Order Limits that will either:

• be negatively affected by the development; or
• be used to contribute towards the project’s BNG.

Habitats that will genuinely remain unaffected can therefore be excluded from the BNG baseline, provided an appropriately experienced ecologist can demonstrate that they will not be subject to permanent or temporary habitat loss or deterioration in condition.

This distinction could be particularly important for solar and wind NSIPs, where Order Limits frequently include extensive areas associated with cable corridors, access, construction flexibility, limits of deviation or other land over which powers are required, but where significant areas of habitat may ultimately remain unaffected.

For example, habitats above horizontal directional drilling may potentially be excluded where it can be demonstrated that they will not be affected or degraded. Similarly, habitats beneath or adjacent to overhead electricity infrastructure may be excluded where construction and operation will not adversely affect them. Conversely, working areas, access routes, compounds, cable trenches and other areas subject to temporary disturbance will need to be considered.

Dealing with design uncertainty

The guidance also recognises the reality of the DCO process: the detailed design and precise construction footprint may not be fixed when an application is submitted.

Where the extent of habitat impacts remains uncertain, the BNG assessment should adopt a realistic worst-case scenario, based on the maximum realistic design and construction parameters rather than simply assuming that every habitat within the Order Limits will be lost.

Where the location of impacts remains flexible, the approach becomes more nuanced. If there is a clearly preferred design or route, the BNG calculation can be based upon that option and its realistic worst-case footprint. Where there is no preferred option, a precautionary approach may be required, potentially assuming impacts to the highest-value habitats that could realistically be affected unless these can be protected through secured design or construction controls.

This means that BNG will increasingly need to develop alongside the Rochdale Envelope, design principles and ecological mitigation strategy, rather than being treated as a standalone calculation undertaken towards the end of the DCO process.

Temporary impacts will also matter

Another important consideration for energy NSIPs is the treatment of temporary habitat loss.

Construction compounds, temporary access routes, cable installation and other temporary works may bring habitats into the BNG boundary. However, the guidance allows certain temporarily affected habitats to be treated as retained where they are restored to their original habitat type and condition within specified timescales: five years for low or very low distinctiveness habitats and two years for medium distinctiveness habitats.

For large solar and wind projects with extensive cable routes and temporary construction footprints, accurately distinguishing between permanent loss, temporary loss, reinstatement and genuine habitat enhancement will therefore become increasingly important.

What counts as ‘on-site’ BNG?

The BNG boundary also determines what constitutes on-site and off-site gain.

For NSIPs, habitat creation or enhancement within the BNG boundary is on-site. If habitat elsewhere within the Order Limits is to be used to provide BNG, it must consequently be brought within the BNG boundary.

Conversely, off-site BNG must be outside the BNG boundary and outside the Order Limits.

This has potentially important implications for project design and land strategy. Areas within a large solar or wind DCO that are intended for ecological enhancement cannot simply sit outside the project’s baseline while subsequently being counted as off-site units. If they contribute towards the project’s BNG, they form part of the BNG boundary and their existing biodiversity value must first be accounted for.

Furthermore, all habitat creation and enhancement counted as on-site gain for an NSIP is considered significant and will need to be appropriately secured and managed for at least 30 years.

Earlier integration will be essential

The guidance reinforces the need to consider BNG much earlier in NSIP development.

Applications will require a single statutory biodiversity metric calculation and BNG boundary plan, even where a project crosses multiple local planning authority areas, comprises non-contiguous development areas or is delivered in phases.

For solar and wind developers, the emerging BNG boundary is therefore likely to become an important part of iterative project design. Decisions around Order Limits, cable corridors, construction compounds, access routes, habitat retention and areas proposed for ecological enhancement can all influence both the baseline and the scale of the BNG obligation.

With mandatory NSIP BNG now approaching, projects progressing towards DCO submission should be reviewing their habitat data, emerging design and land strategy now to understand how the new requirements could affect the application — and, importantly, where relatively early design decisions could substantially improve the eventual BNG position.