BNG for major infrastructure: setting the boundary and baseline on a linear route

If you apply for a development consent order on or after 2 November 2026, your outline biodiversity gain plan needs a BNG boundary plan and a baseline. The rules sit in the biodiversity gain statements and in the GOV.UK guidance on biodiversity net gain for nationally significant infrastructure projects, updated on 3 November 2026. This guide quotes them on the two choices a linear scheme has to evidence.

Where does the 10 per cent apply on a linear route?

The 10 per cent applies inside the BNG boundary, which can be much smaller than the order limits. The guidance says: "The area on which you calculate the baseline is called the BNG boundary. This may not be the same as the development’s order limits."

For an energy scheme, read it with the biodiversity gain statement for energy (2 June 2026). The guidance says the gain statements "have the same effect as if they were included in the relevant national policy statement".

Sources for the table: GOV.UK guidance (updated 3 November 2026); energy gain statement, paragraph 6.4 (2 June 2026); Defra impact assessment, section 14 (19 March 2026). The widths describe energy transmission schemes, and the second is the construction area. Neither is a rule.

The boundary plan itself has four stated requirements. It 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"

What goes inside the BNG boundary

Three kinds of habitat sit inside the boundary: habitat you impact, habitat you use for gains, and unimpacted habitat you choose to include. Paragraph 6.1 of the energy gain statement covers habitats "that will be negatively impacted by the development (through temporary or permanent habitat loss or deterioration)".

The guidance lists the impacts to count. They include "construction activity, including direct habitat clearance, temporary compounds, storage areas and access routes".

Unimpacted habitat is your choice, on one condition. "You should only exclude these habitats if your ecologist can confirm that there are no expected impacts to them from the development." That covers "permanent loss, temporary loss, or deterioration in condition".

Temporary impacts stay inside the boundary but need not be recorded as lost. The guidance lets you count them as "retained" if you reinstate the original habitat type and condition within a set period.

• "within 5 years of the date the initial impact happened" for "impacts on low or very low distinctiveness habitat"
• "within 2 years of the date the initial impact happened" for "impacts on medium distinctiveness habitat"

"The 2- or 5-year period starts from the date the particular habitat area is impacted." Each section of a route therefore has its own clock.

Section 6 of the user guide confirms the point: "Temporary losses treated as retained in the metric are still negatively impacted habitats and must be included in the baseline and BNG boundary for NSIPs."

Which date fixes the baseline?

The baseline date is the day you submit the outline biodiversity gain plan. The guidance says: "The baseline must use information that reflects the value of habitats on the date you submit your outline biodiversity gain plan."

Survey age is a gain statement matter. Paragraph 6.10 of the energy statement requires survey data "collected within an appropriate timeframe prior to submission of the outline biodiversity gain plan".

Paragraph 6.11 allows older surveys only if the ecologist can confirm "there has been no material change to the on-site habitats since the survey date".

An earlier date comes into play where habitat was degraded. The guidance names "unauthorised loss or impact to the biodiversity value of on-site habitat" before you submitted the application.

"The earlier date should be immediately before the degradation activities were carried out, or the closest date before this for which you have accurate data." You should discuss it "with the Planning Inspectorate as early as possible".

You do not need to "account for any degradation before 30 January 2020". Nor must you "prove the absence of degradation since that date when submitting your DCO application".

An earlier date needs proof: "you must have surveys or other evidence that show you meet these circumstances". Section 8 of the user guide says you can use data records, imagery and historic field surveys to determine habitat types before degradation.

Design not fixed: the realistic worst case

Where the route can still move, the baseline uses the widest impact you could realistically cause, not the whole limits of deviation. The guidance says: "If you do not know the final extent of habitat impacts, your baseline must reflect the realistic worst-case extent of habitat loss or deterioration."

"For example, for a linear scheme, you should use the maximum construction working width rather than the full width of the limits of deviation."

The guidance works through three scenarios for a linear project.

• A likely-final route. "If you have a strongly preferred or most-likely route or design option, you should calculate your baseline using your realistic worst-case extent and that design option."
• Two routes, neither preferred. In the guidance example one route costs 300 units and the other 100. The applicant uses the first, "which has the highest value based on the number of metric units".
• High-value habitat you will avoid. The applicant leaves a wetland out of the baseline and must use "design measures or processes to prevent impact to the wetland habitat throughout the construction and operation phases".

In each case the outline gain plan describes "the assumptions you have made".

What counts as baseline evidence?

Survey by an ecologist is the evidence for habitat type and condition. The guidance says your ecologist "will need to survey the habitat before development and record the data needed for the metric calculation".

Desk evidence has a named place where accurate data is missing. "Where you do not have accurate data, you must calculate your baseline using any evidence or data available". The examples given are "previous site surveys, online data sources and aerial imagery".

"You should apply the highest-value type and condition of habitats that are likely to have been or be present on the site, based on available evidence." Every parcel without accurate data is scored at the highest value the evidence supports.

The summary of consultation responses records challenges "due to seasonal survey windows, access issues, or the scale of land to survey".

Irreplaceable habitat is recorded wherever it lies. The irreplaceable habitat guidance (updated 18 August 2026) lists blanket bog, lowland fens and ancient woodland among others. It says: "You must record all irreplaceable habitat on your site in the metric calculation tool, regardless of whether they will be affected by the proposed development."

The calculation itself runs in the statutory metric tool. The tools and guides page (updated 3 November 2026) lists version 1.1.0.

A desk read of the route comes first. It tells the surveyor where to go and dates what the record shows. Professional survey then determines habitat type. A separate guide covers where baselines on ordinary planning applications go wrong.

Illustrative example

This route is invented and describes no real scheme. A 30 km buried cable crosses farmland, two rivers and a block of wet woodland. The widths sit within those the Defra impact assessment describes.

Now the temporary impacts. Suppose 150 hectares is low distinctiveness habitat reinstated within 5 years, and 40 hectares is medium distinctiveness habitat reinstated within 2 years. Then 190 of the 200 hectares counts as retained, and the other 10 hectares is recorded as lost.

Every figure depends on habitat type, and survey decides habitat type. The kilometres that hold medium and high distinctiveness habitat are where the units are, so the surveyor starts there.

Drawing a boundary on a route like this? Send us your route and we reply within two working days.

Check the wording for your route

What to do next

One item is still open. The government response (15 April 2026) says Defra "also intends to provide other guidance that addresses challenges commonly raised in relation to linear NSIPs". Check the GOV.UK page before you fix the boundary.

Sources

1. GOV.UK: Biodiversity net gain for nationally significant infrastructure projects, GOV.UK guidance, first published 27 August 2026, updated 3 November 2026 (checked 10 November 2026)
2. Defra: Nationally significant infrastructure projects, biodiversity gain statement for energy, published 2 June 2026, updated 16 June 2026 (checked 10 November 2026)
3. Defra: Biodiversity gain statements for nationally significant infrastructure projects, collection, published 2 June 2026, updated 31 July 2026 (checked 10 November 2026)
4. Defra: Statutory biodiversity metric tools and guides, updated 3 November 2026 (checked 10 November 2026)
5. Defra: The Statutory Biodiversity Metric User Guide, last updated November 2026 (checked 10 November 2026)
6. Defra: Biodiversity net gain for nationally significant infrastructure projects, summary of responses and government response, 15 April 2026 (checked 10 November 2026)
7. Defra: Impact assessment, biodiversity net gain for nationally significant infrastructure projects, 19 March 2026 (checked 10 November 2026)
8. Defra: How to treat irreplaceable habitat for biodiversity net gain, GOV.UK guidance, updated 18 August 2026 (checked 10 November 2026)

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Written by the Vireo teamVireo Nature reads land before anyone surveys it. Every figure in our work carries its range, its source and its test.