Soakaway Testing and Design: What Happens and Why It Matters

If you are planning a new build, extension or barn conversion, there is a good chance someone has told you that you need a soakaway, and a soakaway test to prove it will work. Councils and Building Control both ask for this evidence, and it cannot be desk-based guesswork.

Soakaways are the first option in the drainage hierarchy: national policy expects surface water to be infiltrated into the ground where possible, before any discharge to watercourses or sewers is considered.

This guide explains what soakaway testing involves, how the results feed into design, and what happens when the ground says no.

What is a soakaway?

A soakaway is an underground structure that stores rainwater runoff and lets it soak into the surrounding ground. Modern soakaways are usually built from plastic crates wrapped in geotextile, which store far more water for their size than the traditional rubble-filled pit.

They serve roofs, driveways and other hard surfaces, keeping runoff on your own land instead of loading sewers or watercourses.

What is a soakaway test?

A soakaway test, often called an infiltration or percolation test, measures how quickly water soaks into the ground at the depth and location of the proposed soakaway. The recognised method for surface water design is BRE Digest 365:

1. A trial pit is excavated to the anticipated soakaway depth.
2. The pit is filled with water and the rate at which it drains is recorded.
3. The fill is repeated, typically three times, to reflect saturated conditions.
4. The results give an infiltration rate used directly in the design calculations.

The test also reveals two deal-breakers that a desktop study can miss: high groundwater and impermeable soil. A soakaway that sits in the water table, or in heavy clay, simply will not drain.

How test results shape the design

The infiltration rate determines how large the soakaway needs to be to store and disperse the design storm, usually the 1 in 100 year event with an allowance for climate change. Slow ground means bigger storage; fast-draining ground means a compact system.

The design also has to respect separation distances. Building Regulations require soakaways to be at least 5 metres from buildings and boundaries, and they must be kept clear of drainage fields serving any sewage treatment system.

On a recent Mid Devon project involving two off-grid new builds, our soakaway testing confirmed the ground could accept infiltration, and underground crate soakaways were then sized to manage all roof and driveway runoff within each plot. The same tests informed the siting of the sewage treatment tanks and drainage fields. Read more in our rural new builds case study.

When is soakaway testing required?

• Planning applications: where a drainage strategy proposes infiltration, councils and Lead Local Flood Authorities increasingly expect test results, not assumptions
• Discharge of drainage conditions: detailed design without infiltration evidence is a common reason for refusal
• Building Regulations: Building Control will ask how surface water is disposed of, and soakaways must be justified
• Off-grid foul drainage: percolation testing to a related method is needed to design drainage fields for septic tanks and treatment plants

What if the ground fails the test?

A failed test is useful information, not a dead end. The design moves down the drainage hierarchy:

• Attenuation storage with a controlled discharge to a watercourse
• As a last resort, a restricted connection to a surface water sewer

Finding this out early is the whole point. Discovering mid-build that your soakaway will not drain leads to redesign, delay and sometimes retrospective planning issues.

Common mistakes

• Testing in the wrong place or at the wrong depth. Results only apply where they were taken.
• Testing in summer. Dry-season results can flatter the ground; designers apply judgement and safety factors for winter groundwater.
• Skipping the test entirely. Assumed infiltration rates in a submission are an easy target for LLFA objection.
• Forgetting maintenance. Crates and silt traps need occasional inspection, and councils increasingly ask for a maintenance plan.

Conclusion

Soakaway testing is a small investment that de-risks the whole drainage design. It proves whether infiltration will work, sizes the system correctly, and gives councils and Building Control the evidence they require.

If your project needs a soakaway test or a full infiltration drainage design, it pays to do it before your layout is fixed, not after.

FPS Environmental provides soakaway design, surface water drainage strategies and detailed drainage design across the UK, supported by site testing where required.