A development site that straddles two river catchments can now source off-site watercourse units in either of them without a spatial penalty. That is the practical effect of version 1.1.0 of the statutory biodiversity metric 1.1.0 calculation tool, which Defra released on 3 November 2026 alongside a revised user guide dated November 2026 [1]. The update landed one day after mandatory 10% biodiversity net gain took effect for relevant nationally significant infrastructure projects on 2 November 2026 [2].
This article sets out what changed, what did not, and how ecologists and BNG surveyors should adjust their evidence so that metric calculations survive scrutiny at committee, appeal and in court.
Key Takeaways
- Defra published statutory biodiversity metric 1.1.0 on 3 November 2026, with a revised user guide; the document history records the clarification under 2 November [1].
- The change clarifies how the spatial risk multiplier applies to off-site watercourse compensation when an impact site crosses more than one waterbody catchment [1].
- Multipliers are 1.0 within the relevant waterbody catchment (any catchment the site spans qualifies), 0.75 within the operational catchment, and 0.5 outside it [1].
- Earlier calculations remain valid: Defra says any version of the statutory metric tools can be used and updates do not affect unit outputs [1].
- From 2 November 2026 relevant NSIPs must deliver 10% biodiversity net gain, bringing much larger linear schemes into the watercourse module [2].
What the statutory biodiversity metric 1.1.0 update changes
The watercourse module of the statutory metric has always applied a spatial risk multiplier to off-site compensation, reflecting the principle that gains closer to the impact are more valuable to the affected catchment. What was unclear was how to apply that multiplier when a single development crosses a catchment boundary, as linear schemes, large housing allocations and infrastructure projects often do.
Version 1.1.0 resolves it [1]:
Two points of discipline follow. First, these are multipliers within the watercourse module, not percentages of the total BNG requirement. Second, the area-habitat and hedgerow modules use different geographical rules, so the watercourse approach must not be copied across to them [1].
What has not changed
Defra's resource page states: "You can use any version of the statutory metric tools for BNG calculations." Defra adds that updates will not affect biodiversity unit outputs, so calculations already submitted on earlier versions are not invalidated [1]. Projects spanning several catchments should still check whether the clarified interpretation affects the inputs they used.
NSIP biodiversity net gain is now live
Mandatory biodiversity net gain under Schedule 7A of the Town and Country Planning Act 1990, inserted by the Environment Act 2021, has applied to most major planning applications since 12 February 2024 and to small sites since 2 April 2024. The government confirmed earlier in 2026 that the requirement would extend to nationally significant infrastructure projects from November 2026, and that extension took effect on 2 November 2026 [2].
For BNG surveyors this is significant. Road, rail, energy and water schemes cross many more watercourses than a typical housing site, and they are far more likely to span catchment boundaries. The timing of the metric 1.1.0 clarification is no coincidence.
How the biodiversity gain hierarchy interacts with watercourse units
The statutory framework prefers on-site enhancement first, then off-site units, then statutory biodiversity credits as a last resort. For watercourses, on-site delivery is often constrained by engineering and flood risk, so off-site units matter more than in the area-habitat module.
Sourcing off-site watercourse units now involves a clear checklist:
- Identify every waterbody catchment the impact site touches using the Environment Agency's Catchment Data Explorer.
- Record the operational catchment for each.
- Map candidate compensation sites against both, so the applicable multiplier (1.0, 0.75 or 0.5) is evidenced rather than asserted.
- Confirm the units are registered on the biodiversity gain site register and secured by a conservation covenant or section 106 agreement for at least 30 years.
- Keep the version number of the metric tool used in the file, together with the date of the calculation.
Statutory biodiversity metric 1.1.0 and the baseline survey
No multiplier rescues a weak baseline. The metric relies on habitat distinctiveness, condition and strategic significance, and for rivers and streams it relies on a competent assessment of the watercourse using the metric's river condition methodology. Surveyors should make sure that:
- Surveys are carried out at the right time of year, with the dates recorded and any seasonal limitations stated.
- Condition assessment criteria are applied line by line, with photographs tied to each criterion.
- Any assumptions about habitat extent, especially where access was restricted, are flagged in the report.
- The habitat baseline is dated and frozen as at the point the application is validated, so that later changes on site cannot be used to reduce the baseline.
Ecological expert evidence: where BNG assessments are challenged
BNG calculations are increasingly contested. Objectors challenge them at planning committee, appellants and councils argue about them at appeal before the Planning Inspectorate, and judicial review claims have begun to test how authorities scrutinise the biodiversity gain plan. In each setting the ecologist becomes, in substance, an expert witness.
The principles are the same as in the civil courts under CPR Part 35: the expert's duty is to the decision maker, not the client; the report should state the instructions, the data relied on and the limits of the opinion; and alternative conclusions should be addressed rather than ignored. From 30 November 2026 the RICS standard Surveyors acting as expert witnesses, 5th edition, applies to chartered surveyors giving evidence, including disclosure of AI use and of assumptions and alternatives [3]. Ecologists who are not RICS members will still find it a useful benchmark, and the CIEEM code of professional conduct imposes similar duties of objectivity.
An independent audit of metric inputs, carried out by an ecologist not involved in the original assessment, is now routine on larger schemes. It typically checks:
- Habitat classification against UKHab and the field evidence.
- Condition scores against the photographs and survey notes.
- Strategic significance claims against the local nature recovery strategy.
- Watercourse catchment mapping and the multiplier applied.
- The metric version used and whether the inputs are reproducible.
Practical steps for developers and landowners sourcing watercourse units
- Commission the catchment mapping before agreeing a price for off-site units. The difference between a 1.0 and a 0.5 multiplier doubles the units needed.
- Where a site spans two catchments, the 1.1.0 clarification means units in either catchment carry the full multiplier. Use that flexibility to widen the search for habitat bank supply.
- Ask the habitat bank for its own metric calculation, the registration reference and the legal agreement securing the site.
- Keep a dated record of which metric version was used for each submission.
- For NSIPs, start the watercourse baseline early. Linear schemes rarely have the luxury of a single survey season.
Frequently asked questions
Do earlier BNG calculations need to be re-run on metric 1.1.0?
No. Defra states that any version of the statutory metric tools can be used and that updates do not affect biodiversity unit outputs. Projects spanning several catchments should check whether the clarified watercourse interpretation changes their inputs [1].
Does the watercourse multiplier apply to hedgerows or area habitats?
No. The area-habitat and hedgerow modules use different geographical rules. The watercourse approach should not be copied across [1].
When did NSIP biodiversity net gain start?
Mandatory 10% biodiversity net gain for relevant nationally significant infrastructure projects took effect from 2 November 2026 [2].
Who can challenge a BNG assessment?
Objectors, consultees, planning inspectors and, through judicial review, the courts. Robust survey records and an independent audit of metric inputs are the best protection.
Conclusion
Version 1.1.0 of the statutory biodiversity metric is a clarification rather than a reform, but it matters. Developments that cross catchment boundaries can now evidence a full 1.0 multiplier for off-site watercourse compensation in any catchment they span, while the 0.75 and 0.5 steps remain for compensation further afield. With NSIP net gain live from 2 November 2026, the number of schemes affected will grow quickly.
The practical next steps are to map catchments before sourcing units, to record the metric version used, to keep baseline survey evidence to an expert-witness standard, and to have metric inputs independently audited on any scheme likely to be contested. Done properly, BNG survey evidence should be as defensible in front of an inspector or a judge as it is at committee.
References
- Biodiversity Metric Update. planninggeek.co.uk.
- Government Confirms Biodiversity Net Gain for Major Infrastructure From November 2026. cieem.net.
- Surveyors Acting as Expert Witnesses. rics.org.