Drought, Condition Assessments and What It Means for Your Biodiversity Net Gain Metric
2026 has been a dry year. By early August, over 70% of England's land area was in official drought status, following the driest July on record and successive weeks with less than 10mm of rainfall across large parts of the country. Reservoir storage, river flows and groundwater levels have all fallen well below the long-term average, and the Environment Agency has activated its incident response arrangements for the second year running. Even the Government issued a national alert following wildfire outbreaks.
For ecologists, the consequences of this go beyond the headlines about hosepipe bans and wildfire risk. They are showing up directly in our survey data, and increasingly in the responses we're getting back from Local Planning Authorities (LPAs).
The May–August problem
Industry guidance is clear that the optimal window for botanical and habitat surveys typically runs from May to August, when vegetation is sufficiently developed to allow confident identification and an accurate assessment of species composition and habitat condition. Surveys conducted within this window are generally treated as robust, and results are accepted without further qualification.
This year, that assumption is breaking down. We are increasingly finding that surveys carried out well within the recognised window are drawing scrutiny from LPAs, because the vegetation being recorded simply isn't representative of what the habitat would look like in a normal year.
Calcareous grassland is a good example. These habitats sit on shallow, freely-draining soils over chalk or limestone at the best of times. That's part of what makes them botanically distinctive. In a prolonged drought, that free drainage becomes a liability rather than an asset. Moisture stress sets in earlier and more severely than on other soil types, flowering plants go over early or fail to flower at all, and the diverse, colourful sward that would normally support a condition assessment reflecting high floristic diversity can appear visibly impoverished, even though the underlying seed bank and habitat potential haven't changed.
The same pattern is turning up across other drought-sensitive habitats: species-poor swards where we'd expect diversity, early die-back masking structural features, and reduced detectability of indicator or positive indicator species that condition assessments rely on.
What this means for condition assessment and the metric
This creates a genuine dilemma. Condition assessment underpins the Statutory Biodiversity Net Gain Metric, and the metric's outputs drive how many biodiversity units a site needs to deliver or how much off-site compensation a client needs to secure.
Where survey conditions are constrained in a way that could suppress an accurate reading of condition, industry standard guidance directs us toward the precautionary principle: if we cannot be confident that a lower condition score reflects the true baseline (as opposed to a temporary, drought-induced state), we should not down-grade the habitat on the strength of that survey alone. In practice, this often means defaulting to a higher condition assumption than the on-the-ground observations might otherwise suggest.
That precaution is the right call ecologically: it protects against underselling a habitat's real value. But it has a direct financial consequence for developers and landowners: a higher assumed condition for the baseline habitat increases the unit value being lost through development, which can push the biodiversity net gain calculation toward requiring greater compensation or off-site offsetting than the site might actually need once conditions normalise.
In short: this year's drought risks making 2026 baseline surveys more conservative than the habitat truly is, and more expensive to compensate for.
Our recommendation: talk to us before you commit to a baseline
Laura Grant, Chartered Ecologist and Associate Director at Ecology by Design says “Rather than accepting an overly precautionary metric output as final, I’d advise clients, particularly those with grassland, heathland or other drought-sensitive habitats in their baseline, to treat this year's survey as a provisional position and if possible, plan an update survey from May 2027 onward, once conditions have had a chance to return to typical seasonal norms.”
This approach:
- Provides a more defensible, representative baseline for negotiations with LPAs, reducing the risk of disputes over survey validity.
- Gives the metric outputs a chance to reflect the habitat's genuine condition, rather than a drought-suppressed snapshot, potentially reducing the compensation requirement.
- Strengthens the evidence base behind any BNG submission, ecological impact assessment, or planning application, by demonstrating that atypical conditions were identified, addressed and accounted for rather than ignored.
However, we recognise that this precautionary approach won’t always be possible, so where a scheme's timeline doesn't allow for a delay in to next year, we can advise on interim approaches. We can document drought conditions transparently within survey reports, cross-reference historical records or aerial imagery, and engage early with the relevant LPA to agree how the constraint will be handled.
If your site includes drought-sensitive habitats and your 2026 surveys have come back showing lower diversity than expected, it's worth pausing before that data is locked into a final submission. Get in touch with the Ecology by Design team to discuss whether an update survey, a phased approach, or additional supporting evidence could help you avoid over-compensating for a dry summer.
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