What to include in biodiversity net gain baselines for NSIPs
Find out which habitats to include when calculating the pre-development biodiversity value for nationally significant infrastructure projects (NSIPs).
Applies to England
Biodiversity net gain (BNG) will be mandatory for nationally significant infrastructure projects (NSIPs) from 2 November 2026.
You must calculate an NSIP’s baseline using the statutory biodiversity metric tool. Find the biodiversity metric tool on Statutory biodiversity metric tools and guides.
The baseline shows the pre-development biodiversity value of on-site habitats. It’s measured in biodiversity units.
NSIPs only need to include certain habitats within their development boundary (‘order limits’) in their baseline calculation.
Habitats to include in your baseline
Your baseline calculation must include the pre-development biodiversity value of any habitats within the order limits that will be:
- negatively impacted by the development
- used to contribute towards BNG
If your final project location or construction area is not fixed, find out which habitats to include in the section ‘If your project design details are uncertain’.
Habitats negatively impacted by the development
Your baseline calculation must include all habitats within the order limits that will be negatively impacted by the development.
This includes all habitats subject to permanent or temporary:
- loss – for example, permanent loss from the development footprint, or temporary loss during construction
- deterioration of habitat condition as measured with the biodiversity metric condition assessments – for example, from shading or disturbance caused by the development
You must include negatively impacted habitats in your baseline calculation even if you will return them to their original type and condition after the impact.
You must consider all expected impacts from the development, including impacts from:
- the development footprint
- enabling and site‑preparation works
- construction activity, including direct habitat clearance, temporary compounds, storage areas and access routes
- associated development included within the development consent order (DCO) application
- expected impacts during the maintenance and operation of the development
Your ecologist should advise on expected negative impacts to habitats, including whether any habitats will experience deterioration in condition as a result of the development.
On Statutory biodiversity metric tools and guides, you can find out how to:
- conduct a condition assessment – use the biodiversity metric condition assessments spreadsheet
- record a deterioration in habitat condition – read the statutory biodiversity metric user guide
Habitats used to contribute towards BNG
Your baseline calculation must include the pre-development biodiversity value of any habitats within the order limits that will be used to contribute towards BNG.
In other words, you should include any habitats where on-site biodiversity gains will be located. These are any habitats that you plan to enhance or create new habitats on, and include in the post-development biodiversity value.
This applies regardless of whether the habitat creation or enhancement is:
- only for the purpose of BNG
- carried out for other purposes and you intend it to count towards BNG outcomes in part or full – for example, Environmental Impact Assessment compensation or green infrastructure
To find out when habitat created or enhanced for other purposes can also be used for BNG, read the guidance on What you can count towards a development’s biodiversity net gain.
Habitats you can exclude from your baseline
You can exclude unimpacted habitats from your baseline.
NSIPs might include significant areas of habitat within their order limits that will remain unimpacted by development. For example, this could happen where limits of deviation are used to account for uncertainty about the exact project location or the area needed for construction.
Where infrastructure will be located above or below ground, the habitat beneath or above is usually included within the order limits. However, this habitat might remain unimpacted.
Your baseline calculation can exclude habitats within the order limits that will remain unimpacted by the development. You should only exclude these habitats if your ecologist can confirm that there are no expected impacts to habitats. This includes permanent loss, temporary loss, or deterioration in condition.
You can also choose to include unimpacted habitats in your baseline calculation. You might do this to:
- simplify mapping and evidence
- demonstrate BNG uplift across a larger area
If there are small parcels of unimpacted habitats, you should consider including them in your baseline calculation. This will make your baseline more proportionate, easier to interpret, and simpler to evidence.
If you choose to include any unimpacted habitats in the baseline calculation, they will be subject to the 10% BNG requirement. These habitats will be included in the post-development biodiversity value and count towards BNG.
Below-ground infrastructure
For below-ground infrastructure or construction techniques, your baseline can exclude habitats above the works if you’re certain they will not be impacted or degraded.
For example, if you use below-ground horizontal directional drilling, you may exclude areas of unimpacted habitat above the drilling area. You must include any habitats that will be impacted, for example during access, set up and working around the drilling access points.
Your ecologist should confirm that your working methods and impact avoidance measures will result in no impact to any areas you exclude from your baseline. They should consider the type of habitats present and the proposed disturbance depth.
Above-ground infrastructure
Your baseline can exclude areas of habitat beneath or immediately next to an above-ground structure if you’re certain they will not be impacted or degraded by your project.
For example, you can exclude habitats in the order limits beneath overhead electricity lines if you’re sure they’ll remain unimpacted by the development. You must still include any habitats that will be impacted by site preparation or construction activity, such as habitats around pylons, access routes, or temporary storage areas.
Your ecologist should confirm that above-ground infrastructure will not negatively impact any habitats excluded from your baseline.
If your project design details are uncertain
If the final project location or construction area is not fixed, NSIPs can include areas of design uncertainty or limits of deviation.
In these cases, you must identify which habitats to include in your baseline using the realistic worst-case extent of habitat loss or deterioration. You should base this on the maximum design and construction parameters, not on a hypothetical or overly precautionary scenario.
For example, your project might have linear limits of deviation to account for uncertainty about the exact location of the infrastructure. In this situation, you can calculate your baseline using the maximum expected construction working width, rather than the full width of the limits of deviation.
There might be areas of higher value habitat within the limits of deviation. If there’s a reasonable likelihood that your works area or route may deviate and impact the higher value habitat, your worst-case impacts should assume it will be impacted. You should only assume there will be no impact if you have measures or processes in place to avoid it.
This approach ensures your BNG strategy demonstrates how 10% BNG will be delivered in a realistic worst-case scenario and avoids underestimating habitat losses.
Providing evidence of what’s in your baseline
You must provide evidence on the habitats included in your baseline as part of your project’s biodiversity gain plans.
For more information about biodiversity gain plans, read the relevant biodiversity gain statement for your NSIP.
Outline biodiversity gain plan
The area of the project development site included in the baseline calculation is called the BNG boundary.
You must provide a BNG boundary plan in your outline biodiversity gain plan.
Your BNG boundary plan must clearly show, and differentiate between, the following categories:
- negatively impacted habitat
- areas of habitat that will be used to contribute towards BNG
- unimpacted habitats included in baseline
- unimpacted habitats excluded from the baseline
Your BNG boundary plan must:
- show the entire order limits, with the BNG boundary overlaid
- be drawn to an identified scale
- show the direction of north
- have a clear legend showing the different categories of habitat listed above
If your baseline excludes areas of unimpacted habitat within the order limits, you should provide supporting information in your outline biodiversity gain plan. This should explain why or how the habitats will remain unimpacted, and set out assumptions or parameters you used.
For example, you could describe any measures you’ll use to prevent negative impact, or provide evidence that construction will avoid the habitat area. This can include cross-referencing other application documents to avoid duplicating information, such as construction or works plans.
Updated or phase biodiversity gain plans
You will need to provide more evidence if you update your baseline in your updated or phase biodiversity gain plans.
You may need to update your baseline if there are changes to the existing habitats or the proposed development.
You must update your baseline where:
- there will be a greater impact on habitat compared to the outline biodiversity gain plan
- there are changes to the area or location of planned on-site habitat enhancement or creation for BNG
- updated surveys identify changes to the existing on-site habitat type or condition
You can also update your baseline if there are reduced impacts to habitats compared to the outline biodiversity gain plan.
If you update your baseline for the updated or phase biodiversity gain plan, your new plan must:
- provide an updated BNG boundary plan if there have been any changes to the extent or location of habitat loss, habitat deterioration, or habitat creation or enhancement for BNG – if none of these areas have changed, you must confirm that the original BNG boundary still applies
- include supporting information to describe and justify the changes
- provide supporting evidence demonstrating how project changes have altered the expected habitat impacts
If you have chosen to update your baseline due to reduced impacts to habitats, you must support these updates with evidence. Evidence could include updated construction or works plans, engineering drawings, or details of impact prevention measures that show how habitat loss or deterioration will be avoided.
You should use version control to clearly label any updates to BNG boundary plans, metric calculation tools, and the supporting justification.