Defra published a biodiversity metric update on 3 November 2026, issuing version 1.1.0 of England’s statutory biodiversity metric calculation tool and a revised user guide. The main guidance change addresses how the spatial risk multiplier works when a development’s impact site crosses more than one waterbody catchment. For practitioners dealing with rivers, streams and off-site habitat compensation, it is a useful clarification rather than a new biodiversity net gain exemption.
The biodiversity metric update applies to the calculation framework used for mandatory biodiversity net gain in England. It concerns the geographical relationship between a development’s impacts and the off-site watercourse habitats proposed to compensate for them. Depending on that relationship, the metric discounts the units credited to the project. Getting the catchments wrong could therefore distort the apparent adequacy of a proposed gain package.
This is a change to Defra’s technical resources, not a new commencement of the Environment Act 2021 or an automatic requirement to recalculate every existing assessment. Defra continues to permit the use of earlier versions of its statutory metric tools. The important task for an applicant and ecological assessor is to choose the correct spatial risk category, record the supporting geography and understand what the calculation can and cannot establish.
What the biodiversity metric update changes
The updated Defra publication identifies version 1.1.0 of the statutory calculation tool and a revised user guide. Its update note explains that the new guidance deals specifically with projects spanning multiple waterbody catchments. The revised November 2026 user guide records the clarification in its document history under 2 November, while the online resource was updated on 3 November.
The new explanation appears in the guide’s spatial risk section. Where the impact site crosses several waterbody catchments, the assessment can treat off-site watercourse compensation inside any of those catchments as falling within the impact site’s catchment geography. For neighbouring and outside locations, the guidance similarly looks at the relevant operational catchments. It is not simply a matter of measuring straight-line distance between two places.
How watercourse spatial risk is scored
The statutory metric uses three spatial risk categories for off-site watercourse units. A score of 1.0 applies where compensation is within the relevant waterbody catchment. A score of 0.75 applies where it lies outside that waterbody catchment but within its operational catchment. A score of 0.5 applies when it is outside the operational catchment. These are multipliers applied within the watercourse module, not percentages of the total biodiversity net gain requirement.
For a development whose impact site spans two waterbody catchments, the biodiversity metric update means the assessor should examine both catchments rather than selecting just one as the impact site’s sole reference point. Habitat compensation in either spanned waterbody catchment may fall within the 1.0 category. Locations outside those waterbody catchments but within an operational catchment spanned by the impact site may attract 0.75; locations beyond the relevant operational catchments attract 0.5.
The revised guide describes the ‘within’, ‘neighbouring’ and ‘outside’ categories for projects crossing boundaries. Those categories are distinct from the separate rules for area habitats and hedgerows, which refer to local planning authority boundaries, National Character Areas and, for intertidal habitats, Marine Plan Areas. A developer should not copy the watercourse catchment approach across other modules without checking the appropriate metric instructions.
Practical implications for developments
The consequences are most obvious for linear infrastructure and large developments. A road, railway, pipeline, flood alleviation scheme or extensive housing proposal may cross waterbody boundaries while obtaining off-site watercourse units from a different location. The biodiversity metric update makes it important to show which catchments the impact site actually spans and how each off-site intervention relates to that geography.
Applicants should ask the ecologist to identify the impact site’s boundaries, map the relevant waterbody and operational catchments, explain the spatial risk score chosen and retain the supporting evidence within the metric submission. Defra’s guide also asks those using several off-site locations to give each location a unique reference. These steps help the decision maker test the results rather than simply accepting a headline figure.
Geographical eligibility is only one element. The metric still distinguishes watercourse units from area habitat and hedgerow units, and the project must meet applicable habitat trading rules and other statutory requirements. A favourable spatial risk multiplier cannot on its own resolve poor habitat condition, an inadequate management period, weak legal security or a mismatch between the ecological measures offered and the impact being compensated.
Do older biodiversity calculations remain valid?
Defra’s resource page expressly states: You can use any version of the statutory metric tools for BNG calculations.
It adds that updates will not affect biodiversity unit outputs. That means publication of version 1.1.0 does not, by itself, make earlier versions obsolete. Nevertheless, any live assessment should follow the relevant instructions accurately, and a project with several waterbody catchments should check whether the clarified interpretation matters to the inputs previously selected.
There is a distinction between clarifying an existing rule and making a legal change. This biodiversity metric update is described as a tool and guidance revision. Defra does not present it as a new statutory percentage, a wider exemption or a replacement for the environmental information required with an application. Those looking for the wider planning framework can use Planning Geek’s biodiversity net gain guide, while our fish pass and watercourse planning guide covers related river interventions.
What to check before submitting a metric
For ordinary planning applications and qualifying nationally significant infrastructure projects, the latest publication provides an opportunity to review catchment assumptions before a biodiversity gain plan or other required information is submitted. Defra’s guidance now also expressly addresses nationally significant infrastructure projects, for which mandatory biodiversity net gain took effect for relevant applications from 2 November 2026. Planning Geek has separately covered that NSIP commencement.
The practical lesson from the biodiversity metric update is narrow but important: establish the correct catchment geography before putting a spatial risk figure into a spreadsheet. Ecological assessors should consult the November user guide and the current calculation tool rather than assume the multiplier always follows the nearest local authority boundary. Planning officers can then assess whether the submitted evidence supports the category used for each off-site watercourse measure.
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