Infiltration Testing For Soakaway Design in the UK

When it comes to delivering a compliant and cost-effective drainage strategy, understanding how well your site absorbs water is essential. Infiltration testing  (often called a percolation test or soakaway test ) is the proven method for measuring the ground’s ability to manage surface water.

At Clear Environmental, we carry out BRE 365 infiltration testing across the UK, helping developers, civil engineers, drainage designers, and planners meet planning requirements and avoid costly redesigns or delays. Our approach is fully aligned with the National Standards for Sustainable Drainage Systems (SuDS) and relevant UK Building Standards, ensuring your drainage strategy is both compliant and future-proof.

What is Infiltration Testing?

Infiltration testing measures the rate at which water drains through the soil. The results confirm whether sustainable drainage systems (SuDS) such as soakaways, infiltration trenches, or permeable paving can be used on your site.

For planning and Building Control, infiltration testing is a key step in designing surface water drainage systems. The test data feeds directly into calculations for soakaway sizing and ensures compliance with:

• BRE Digest 365 – the established technical method recognised by UK planning authorities.
• National Standards for SuDS – the government’s performance requirements for drainage, including how infiltration should be assessed, designed, and maintained.
• UK Building Standards – statutory requirements in England, Wales, Scotland, and Northern Ireland that mandate safe, effective drainage solutions as part of building design and construction.

When Do You Need an Infiltration Test?

You’ll typically need an infiltration test in the UK if:

• Your drainage design includes a soakaway or infiltration system – Required by planning policy, Building Regulations, and now reinforced in the National SuDS Standards.
• You are seeking SuDS approval – Both BRE 365 and the National Standards require infiltration to be considered before discharging surface water to a sewer or watercourse.
• You need to satisfy Building Standards – Building Control will expect proof that the proposed drainage meets performance and safety criteria, particularly for infiltration-based systems.
• You want to avoid unexpected redesigns – Testing early in the project confirms whether infiltration is viable and reduces cost and programme risks. Alternatively, it provides the evidence that soakaway drainage is not suitable on your site and drainage design can determine what other options are available.

UK Standards – BRE Digest 365, National SuDS Standards, and Building Standards

In the UK, infiltration testing for drainage design is carried out in accordance with BRE Digest 365. This standard sets out the method for testing, recording, and interpreting infiltration rates, and is accepted by local planning authorities nationwide.

The updated National Standards for SuDS and UK Building Standards add emphasis on:

• Managing runoff as close to source as possible.
• Demonstrating that infiltration has been assessed before considering alternative drainage routes.
• Designing infiltration systems for long-term durability and safe operation.
• Providing adequate maintenance access to ensure ongoing compliance.

By combining BRE 365 methodology with the National SuDS Standards and statutory Building Standards, Clear Environmental ensures your infiltration strategy meets the technical, policy, and legal requirements for approval.

How an Infiltration Test is Carried Out

While specifics vary by site, the process generally involves:

1. Excavating Test Pits – Dug to the proposed depth of the soakaway or infiltration system, typically at least 1m deep, with straight sides. The location and depth should be selected to match the proposed system layout.
2. Assessing Groundwater Levels – Groundwater conditions must be checked to ensure no groundwater is entering the pit during the test. It’s also important to confirm that, under seasonal high-water conditions, groundwater will not rise above the base or the inlet/outlet level of the soakaway. This assessment should take account of seasonal variations, as groundwater can be significantly higher in wetter months.
3. Saturating the Soil – Filling the pit with water to allow the soil to reach typical wet-season conditions, ensuring results reflect the worst-case infiltration rate.
4. Measuring Drainage Performance – Recording how long it takes for the water to drain away, then calculating the infiltration rate in metres per second.
5. Reporting – Providing a council-ready report that includes infiltration rates, a comment on likely suitability for infiltration-based drainage, and confirmation that the testing meets BRE 365, National SuDS Standards, and Building Standards. These results should then be passed to a drainage engineer to finalise the system design.

‍Understanding the Results

The infiltration rate determines whether infiltration drainage is suitable:

• Acceptable rate – The design can proceed, with soakaway dimensions calculated in line with BRE 365, national SuDS requirements, and Building Standards.
• Too slow – Indicates impermeable or clay-rich soils; the standards require consideration of alternative SuDS features, such as swales, detention basins, or discharge to a sewer (with consent).
• Too fast – May require additional measures to ensure water is retained long enough for adequate treatment before infiltration.

Clear Environmental provides practical, commercially focused recommendations so your drainage strategy remains compliant and achievable under both the National Standards and Building Standards.

Why Choose Clear Environmental

• UK-Wide Service – Infiltration testing for projects of all sizes, nationwide.
• Standards Compliance – Testing meets BRE Digest 365 and the National Standards for SuDS.
• Fast Turnaround – Results delivered quickly to keep your project on track.

Ready to arrange infiltration testing?
Contact Clear Environmental today to discuss your site and book your test.