Asked for a soakaway test and design? Here is what building control needs

A soakaway is one of the few jobs on a small build where you can do the fieldwork yourself: dig a pit, fill it three times, time it. Everything after that is arithmetic, and you can do the arithmetic free. What is worth knowing first is where the rules come from, which test your building control body expects, and what the design has to show.

— reviewed against the regs as they stand.

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Pit dimensions and the three test timings in; the soil infiltration rate and the storage volume the BRE Digest 365 method requires out, with the storm-duration table behind it.

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What building control is actually asking for

Requirement H3 of the Building Regulations covers rainwater from roofs and paved areas, and it sets an order of priority for where that water goes:

  1. an adequate soakaway or other infiltration system; or, where that is not reasonably practicable,
  2. a watercourse; or, where that is not reasonably practicable,
  3. a sewer.

So when an extension or a new house adds roof and paving, the first question building control asks is whether the water can go into the ground on site. Approved Document H, the guidance on meeting H3, adds the thing the soakaway must not do — the water has to be distributed so that it does not damage the foundations of the building or any adjacent structure — and then covers where a soakaway can go and how big it has to be.

That is what “a soakaway design” means in a building control letter: evidence that the soakaway is in an acceptable place and big enough for your ground. The first is a plan. The second is a test and a calculation.

Where it can go

Approved Document H says an infiltration device should not be built:

It sets no distance from the boundary. Some councils do: one building control team asks for 5 m from buildings and boundaries, another for 2.5 m from a boundary. Check your building control body’s own guidance before you mark a position on the plan, because on a small plot the location is settled before the size is.

The test

Everything in the design rests on one measured number: how fast the ground at the soakaway’s depth takes water away, the infiltration rate. Two tests exist, and it is worth knowing which one you are being asked for.

The BRE Digest 365 soakage test is the one planning conditions and lead local flood authorities usually name, and building control bodies commonly ask for it too. Dig a trial pit where the soakaway will go, to the depth it will reach, 0.3 to 1 m wide and 1 to 3 m long. Fill it to at least 75% of its effective depth and time the fall from 75% to 25%. Do it three times, on the same day or consecutive days, so the ground around the pit is saturated. Design on the slowest result — some authorities say so in terms, and tell you not to average. The percolation test calculator turns the pit size and times into the rate, and has a stopwatch for the pit.

The Approved Document H percolation test uses a 300 mm square hole and a timed 150 mm fall, giving a percolation value, Vp, in seconds per millimetre. It is the test for a septic tank drainage field, but Approved Document H also allows it for small soakaways and gives a formula to convert Vp to an infiltration rate. If that is the test your building control body accepts, it is less digging; if the request says BRE 365, it is the trial pit.

Mind the season. Ground that has been dry for months drinks the first fills and flatters the result, and a soakaway sized on a summer test can struggle in a wet winter. Some authorities ask for testing in winter or spring for exactly that reason. We have written about testing in drought conditions and how to show what conditions a test was done in.

Photograph the pit with a tape in it and write down the dimensions and every time. Some building control bodies have their own test record sheet, and ask for it back once the test is done.

The design

The soakaway has to hold whatever rain arrives faster than the ground can take it. BRE Digest 365 sizes it for a 1-in-10-year storm — the return period Approved Document H asks for — checked across a range of storm durations, from a few minutes to a day, because a short cloudburst and a long steady rain load it differently. For each duration, the storage needed is the inflow from the drained area less the outflow into the ground, and the worst case governs. The soakaway must then half-empty within 24 hours, so it is ready for the next storm. Most authorities now add a climate-change allowance to the rainfall, commonly 40%.

What it is built from matters as much as its size. A rubble-filled pit stores only the voids between the stones, 30 to 40% of its volume; modular crates store 90 to 95%. BRE Digest 365 explained sets out every step with a worked example, and what size soakaway do I need? puts typical numbers on it.

Approved Document H has a shortcut for the smallest cases: a soakaway serving 25 m² or less may be designed on an assumed 10 mm of rain in 5 minutes. For larger areas it points to BRE Digest 365 or BS EN 752-4. It still cites the 1991 edition of the digest; the current one is the 2016 edition, and that is the one to work to.

What the design has to show

There is no prescribed form. Whoever checks it will be looking for:

That is what a SoakMonkey report sets out, with a report ID building control can verify online.

When it is due

With the plans on a full plans application, or before the drainage goes in on a building notice — building control inspects drainage before it is covered, and a soakaway is hard to resize once it is buried. The test itself belongs even earlier: before the crates are ordered, and ideally before the design is fixed, because slow ground can move the soakaway, change its construction or rule it out.

If the ground won’t take it

A failed test is not the end of the job; it moves you down the Part H list. The next option is a watercourse, where one can be reached; after that, a sewer — which means applying to the water company to connect, and a surface water sewer where there is one.

Below an infiltration rate of roughly 1×10⁻⁶ m/s — heavy clay, typically — a soakaway usually cannot half-empty within 24 hours at any size, and the answer is attenuation: storage that releases water at a controlled rate to a watercourse or sewer. BRE Digest 365 does not size that. Keep the failed test and put it in the submission, because it is the evidence that a soakaway was not reasonably practicable.

Planning, SuDS and Wales

For a single extension, building control is usually the only one asking. For new homes, planning often asks as well:

The infiltration test is the same whoever asks for it, so one good test can serve both.

What it costs

The test is the part you can do yourself with a digger, a hose or a bowser, a tape and a watch. The design is arithmetic.

Do it yourself, free

SoakMonkey runs the BRE Digest 365 method in your browser: enter the pit size and the three fall times, the area draining to the soakaway and the site’s location, and it works out the infiltration rate, runs the storm analysis with the climate-change allowance, and sizes a rubble pit, crates, concrete rings or a trench, with the half-drain check. It flags ground that is too slow before anything is dug. On site, it installs on your phone and times the test for you.

What you’ll need to hand: the trial pit’s length, width and effective depth; the three fall times; and the roof and paving areas draining to the soakaway, from your drawings. For an extension, a soakaway for an extension walks through the usual case.

If you’d rather hand the design over entirely, ask us.

Common questions

Do I need a soakaway for my extension?

If the new roof or paving needs somewhere to drain, very likely. Requirement H3 of the Building Regulations puts rainwater disposal in an order of priority: a soakaway or other infiltration system first; a watercourse only where that is not reasonably practicable; a sewer only where neither is. So building control will usually want to see that a soakaway has been considered and, if the ground will take it, designed.

What percolation test does building control want?

Usually the BRE Digest 365 soakage test: a trial pit dug where the soakaway will go and to its depth, filled with water three times, with the fall from 75% to 25% of its depth timed on each fill. Approved Document H also describes a smaller test — a 300 mm square hole that gives a percolation value, Vp — and allows it for small soakaways, but planning conditions and lead local flood authorities usually name BRE 365, and building control bodies commonly do too. Ask yours which it wants before you dig.

How far does a soakaway have to be from the house?

Approved Document H says not within 5 m of a building or road, not in unstable ground, not where the water table reaches the bottom of it at any time of year, not so close to other soakaways or drainage fields that the ground is overloaded, and not where contaminated run-off could pollute groundwater. It sets no distance from the boundary, but some councils do — 2.5 m and 5 m are both in use — so check your building control body's own guidance.

What goes in a soakaway report for building control?

The trial-pit dimensions and the fall times; the infiltration rate and how it was worked out; the area draining to the soakaway; the design rainfall, return period and any climate-change allowance; the storm durations checked and the one that governs; the soakaway's construction and dimensions; the time it takes to half-empty; and a plan showing where it goes. No form is prescribed — those are the facts that make the design checkable.

What if my ground fails the percolation test?

Then the Part H order of priority moves on: a watercourse if one can be reached, otherwise a sewer, which means applying to the water company to connect. Below an infiltration rate of roughly 1×10⁻⁶ m/s — heavy clay, typically — a soakaway rarely half-empties within 24 hours at any size, so attenuation with a controlled discharge is the usual answer. Keep the failed test: it is the evidence that a soakaway is not reasonably practicable.

How much does a soakaway test and design cost?

Advertised prices vary widely. One Greater Manchester firm lists a BRE 365 calculation alone at £180, and a day's testing at £600 before any plant or water bowser; one consultancy says its fees for the smallest single house exceed £1,200 + VAT. SoakMonkey sizes the soakaway from your own test free, and the Building Control-ready report is £25 + VAT (£30 inc VAT).

Does planning need anything different?

For a single extension, building control is usually the only one asking. On new homes planning often asks too: the NPPF published in August 2026 expects sustainable drainage designed to Defra's National Standards wherever development could affect drainage, with a statement of how the standards are met in the application. Those standards ask infiltration features to half-empty within 24 hours for storms up to the 1-in-30-year event, while building regulations continue to apply within the curtilage of each property. In Wales, development with a construction area of 100 m² or more needs sustainable drainage approval before work starts.

Test done, or about to be?

Enter the pit size and your three fall times and SoakMonkey works out the infiltration rate, runs the BRE 365 storm analysis and sizes the soakaway — free, in your browser, no sign-up. The Building Control-ready report is £25 +VAT, and only if you need it.

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