Spatial risk multipliers and where off-site BNG units sit

Off-site biodiversity units are worth less the further they sit from your development, and the discount is applied inside the statutory metric itself rather than in negotiation. The spatial risk multiplier sorts off-site compensation into three categories by its relationship to the development site, and progressively reduces the units it delivers. Buy habitat in the wrong place and you pay for units you do not get credit for.

The practical effect is that a low-priced unit 90 miles away can cost more per unit delivered than a dearer one in the next parish. That calculation needs doing before you sign anything.

What the multiplier does to unit value

The metric calculates post-intervention biodiversity units from habitat area, distinctiveness, condition and strategic significance, then applies risk multipliers. Two of them matter for off-site delivery: the temporal risk multiplier, which discounts habitat that takes years to reach its target condition, and the spatial risk multiplier, which discounts habitat located away from the impact site.

The spatial multiplier is applied to off-site units only. On-site habitat creation is not discounted spatially, which is one reason the trading sequence starts with what you can deliver inside the red line. For off-site units, the multiplier is a straight reduction in the units credited towards the biodiversity gain objective.

Check the factors in the current version of the statutory biodiversity metric rather than relying on a figure quoted in an older report, because the metric and its user guide are revised and the published values are the ones the authority will apply.

The three spatial categories

The categories are defined by administrative and ecological geography rather than by distance in kilometres, which surprises people who expect a radius.

National Character Areas are Natural England's 159 areas of distinct landscape character, and they do not follow council boundaries. That cuts both ways. A site near a district edge may have two or three authority areas and a National Character Area all within reach, giving a wide search area at the undiscounted rate. A site in the middle of a large unitary authority has a tighter practical radius.

Map both geographies before you start looking for units. The National Character Area boundaries are published by Natural England and the authority boundaries are on the planning data platform, and the overlay is the map your land agent should be working from.

Why local units cost less in metric terms

Think in cost per credited unit rather than cost per unit purchased. If a habitat bank outside your National Character Area and outside neighbouring authorities sells units at a lower asking price, the discount means you need more of them to meet the same gain objective. The effective price per credited unit rises accordingly, and on a scheme needing a meaningful number of units the difference overtakes the headline saving quickly.

There is a second cost that is easy to miss. Where the shortfall cannot be met by off-site units at all, statutory biodiversity credits are the fallback, and they are deliberately priced to be the least attractive route. Anything that reduces the credited units you obtain from a purchase pushes you towards that fallback.

So the order of preference is clear: on-site delivery first, then off-site units inside the same authority area or National Character Area, then neighbouring areas, then anything further, then statutory credits. Our note on soil translocation and habitat creation on stripped ground covers how to get more out of the on-site element before you go looking off site.

Local nature recovery strategies and alignment

Separate from the spatial multiplier, the metric rewards habitat created in a location identified as strategically significant. Local nature recovery strategies, prepared by responsible authorities under the Environment Act 2021, are the main mechanism for identifying those areas, alongside other local plans and strategies that map priority habitat networks.

The two mechanisms stack. Off-site habitat inside your own authority area that also sits within an area identified in the local nature recovery strategy attracts no spatial discount and a higher strategic significance category, which is the best combination available. A habitat bank chosen only on price rarely achieves both.

Ask the responsible authority or the local planning authority's ecologist which strategy applies and whether the mapping is published, because coverage and publication dates vary across England. Where no strategy is yet published for the area, the authority will say what it is using instead.

Negotiating for nearby habitat banks

The market reality is that supply is uneven. Some authority areas have several registered off-site gain sites, others have none, and the registered sites are not evenly distributed within an area either.

Four moves that help on a real scheme:

• commission the metric baseline early, so you know the unit shortfall by habitat type and distinctiveness before you approach the market, since the trading rules mean you cannot substitute freely between habitat types
• search by National Character Area as well as by authority, which often widens the undiscounted pool considerably
• ask the authority whether it holds a list of local off-site gain sites or operates its own habitat bank, as a number of authorities do
• where a landowner adjoining your site has suitable land, a bespoke off-site agreement inside the same parish is usually the strongest option on both multipliers

Allow time. Off-site units must be secured by a conservation covenant or a section 106 agreement, registered on the biodiversity gain sites register, and the habitat must be maintained for at least 30 years. None of that is quick, and it has to be in place before the biodiversity gain plan can be approved.

Evidence of the spatial relationship

The authority will check your categorisation rather than take it on trust. The metric calculation should be accompanied by a plan showing the impact site and each off-site parcel, the relevant authority boundaries, and the National Character Area boundaries, with the category applied to each parcel stated and justified.

Where a parcel straddles a boundary, say how you have treated it and why. Where you have claimed the undiscounted category on the basis of a shared National Character Area rather than a shared authority area, show the National Character Area on the plan with its name and reference. These are the points a county ecologist queries, and an unexplained category is the quickest route to a request for a revised metric.

Keep the metric file itself, not just the output summary. Authorities increasingly ask for the calculation workbook so they can check the inputs, and the biodiversity net gain assessment must be completed by a competent person under the Environment Act 2021 regime, with the biodiversity gain plan approved by the local planning authority after permission is granted.

Effect on the gain plan

The biodiversity gain plan is submitted after permission and before development begins, and it has to set out the pre-development and post-development biodiversity values, the on-site and off-site habitat relied on, and how the gain will be secured and maintained. The spatial categorisation feeds directly into the post-development figure, so an error there is an error in the plan.

Two sequencing points are worth planning around. The plan cannot be approved until the off-site units are registered and secured, so a late unit purchase delays the start on site rather than the application. And if an off-site arrangement falls through after approval, you are back to finding units under time pressure, which is when the spatial discount hurts most because the nearby options have usually gone.

Build a contingency into the land budget rather than into the programme. Our notes on reviewing another consultant's ecology report and on hedgerow units and the Hedgerows Regulations overlap cover the other places where a metric run comes apart under scrutiny.

For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.