Here’s a scenario worth thinking about before autumn submissions go in. A soakage test carried out in August 2026 comes back with a healthy infiltration rate. The soakaway is sized accordingly, the drainage strategy is approved, and the thing is built the following spring — into ground that behaves nothing like it did in the test pit.
Why this year is different
The Environment Agency’s report for 14 to 20 August 2026 records that the seasonal recession of groundwater levels is continuing, with levels in the Oolite Limestone aquifers exceptionally low in places and some chalk sites in the Wessex Downs, Yorkshire Wolds and the North and South Downs classed as below normal or notably low. A quarter of monitored river sites are exceptionally low, and 71% of England by land area is in drought (GOV.UK).
That follows what the Environment Agency has called the driest July in 190 years. The agency also notes a related effect worth holding onto: with ground conditions this dry, water does not easily soak into baked and compacted soils, increasing run-off risk. Very dry ground can behave in ways that don’t map neatly onto either summer or winter design assumptions.
What BRE Digest 365 actually asks of you
BRE Digest 365, Soakaway design (2016 edition) is still the standard reference for infiltration testing — our BRE Digest 365 soakaway calculator runs the full method, and the percolation test calculator turns the pit timings into the infiltration rate. Two features of the method matter here.
First, it is deliberately conservative about the test itself: the design rate is taken from the lowest of up to three tests in a pit, not an average of them. That guards against one flattering fill-and-drain cycle carrying the design.
Second — and this is the one that gets skipped — the method assumes you know where the water table is going to be. Groundwater levels move seasonally, so evidence of the highest likely water table is normally needed alongside the test result. Standard practitioner guidance for sites with a seasonally high water table, such as lowland areas or land near a watercourse, is to test in winter or spring, when the table is highest — precisely the opposite of the conditions available across much of England right now.
There is a wider professional argument running about how reliable BRE 365 results are in general — susdrain hosts an Environmental Protection Group briefing asking whether the test has had its day. You don’t need to take a side in that debate to accept the narrower point: a test done in the driest ground in living memory carries more uncertainty than usual.
Why the stakes just went up
Drainage evidence used to be something you could largely settle after permission. That’s changed.
Under the August 2026 NPPF, policy F8 requires SuDS on all development with drainage implications to be designed in accordance with Defra’s National Standards, and policy DM2 puts a site-specific flood risk assessment and a SuDS statement on a national validation list. The Environment Agency and CIRIA have published national SuDS Strategy Templates for major and non-major development to standardise what gets submitted.
So the infiltration rate is now load-bearing at validation, not just at discharge of conditions. If it turns out to be optimistic, you’re not correcting a detail — you’re revisiting an approved strategy, potentially with a different discharge route and a different land take.
Five things worth doing
Record the test date and antecedent conditions. Note rainfall in the preceding weeks and the groundwater context. A reviewer six months from now needs to know the test was run in a drought, and so do you.
Get separate groundwater evidence. Don’t infer the winter water table from an August pit. Borehole records, published aquifer data or a monitoring period give you something defensible.
Consider retesting in the wet season on marginal sites, or designing with an explicit margin and stating why.
Know your fallback. If the rate is genuinely marginal — broadly, once you’re heading below about 10⁻⁶ m/s, infiltration stops being practical — establish the next step in the discharge hierarchy early rather than discovering it at construction.
Keep three tests, and use the lowest. It’s the method, and in a year like this one it’s also the only sensible instinct.
Size it properly, quickly
SoakMonkey does BRE Digest 365 soakaway sizing in your browser — enter your test results, get a defensible design and a printable output for the drainage strategy. It won’t tell you whether your test was done in the right season, but it will make the rest of the job take minutes.
Sources: GOV.UK — Dry weather and drought in England: 14 to 20 August 2026 · BRE Digest 365 — Soakaway design (2016) · susdrain — EPG briefing on soakaway testing · Environment Agency blog, 12 August 2026