Insight · 21 July 2026 · 6 minutes read
Run the biodiversity metric before the layout, not after it
The metric is arithmetic, and arithmetic done at the end of a design is a constraint rather than a tool. Three worked examples of what moving it earlier is worth.
by Nadia Farsi
Since February 2024, most development in England has had to demonstrate a 10% biodiversity net gain, measured with the statutory metric. Two and a half years in, the pattern is clear and slightly depressing: the metric is being run as a compliance check on a finished layout rather than as a design input, and the cost of that sequencing is being absorbed as though it were unavoidable.
It is arithmetic. Habitat area multiplied by distinctiveness, condition, and strategic significance, with a time-to-target-condition discount and a risk multiplier for off-site delivery. Nothing in it is a surprise once you have run it twice. What makes it feel unpredictable is that the numbers for mown amenity grassland and for managed neutral grassland differ by roughly a factor of four, and the drawing looks almost identical either way.
Three examples from our own work. On a 212-unit scheme in Buckinghamshire, changing 1.9 hectares of amenity grass to a meadow regime and relocating an attenuation basin took the scheme from 6.1% to 12.4%, at an increase in commuted maintenance of about £11,000. Without it, the scheme needed three off-site units, quoted at £31,000 each.
On a smaller site in Wiltshire — 34 units, 1.1 hectares — the answer went the other way. There was genuinely no room, and we advised buying off-site units in month two rather than compressing the layout and losing four plots. The client did that, and the four plots were worth considerably more than the units cost.
On the third, a commercial scheme near Swindon, the metric revealed that a hedgerow the layout had treated as expendable was in moderate condition and species-rich, and therefore worth about as much on its own as the entire proposed landscape strategy. It stayed, and the service road moved six metres.
The common thread is not that early metric runs always save money. It is that they turn a compliance figure into a decision you can price. Two of those three schemes ended up spending more on habitat than they had planned to. All three spent less than they would have spent finding out in month nine.
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