Soakaway Percolation Test (BRE 365)
Soakaway testing to BRE Digest 365 for extensions, new builds and planning drainage conditions, arranged through our specialist geotechnical consultants.
Will the Ground Take Your Rainwater?
If your rainwater is going into a soakaway, building control (and often the planning officer) will want proof that the ground can take it. A soakaway percolation test, done to BRE Digest 365, measures how quickly water drains into your soil.
One Stop Consult arranges soakaway testing as part of our site investigation service, through our specialist geotechnical consultants. You get a short report with the result, ready to send to building control.
When Do You Need a Soakaway Test?
You usually need one in any of these situations.
New Soakaway
You are building an extension, garage or new home and the roof or driveway water will go to a new soakaway.
Building Control Request
Building control has asked for percolation test results or soakaway calculations.
Planning Condition
Your planning approval has a drainage condition asking for infiltration testing, often linked to SuDS.
No Sewer Connection
There is no public surface water sewer nearby, or the water company won’t let you connect.
You may not need one if your rainwater goes to an existing surface water sewer with the water company’s agreement. If you’re not sure, send us your drawings.
What Happens on the Day
The test is done where the soakaway will go, so the result reflects the real ground.
It needs a clear area that a small digger can reach and a good supply of water, such as a bowser or a hose that can fill the pit quickly. Tell us what access and water you have on site when you ask for a quote.
The test usually takes a day. On slow-draining clay it can run into the next day.
Dig the Test Pit
A pit is dug at about the depth the soakaway will be built, often 1 to 2 metres.
Fill and Time
The pit is filled with water and the time it takes to drain is recorded.
Repeat Three Times
BRE 365 asks for three fills. The slowest result is used for the design.
Record the Ground
The soil is logged and any groundwater is noted.
What You Get
The report gives you what building control and your designer need to see.
Drainage Rate
The infiltration rate for your soil, worked out from the test.
Soil and Groundwater Notes
A record of the ground found in the pit and any groundwater.
Soakaway Sizing
The infiltration rate is what is used to size the soakaway. Tell us if you need the sizing too, and we will confirm what is included when we quote.
A Plain Answer
If the ground won’t take a soakaway, the result will show it, so you can look at other options early.
Where a Soakaway Can Go
Approved Document H of the Building Regulations in England gives the main rules. A soakaway should not be built:
What If the Ground Doesn’t Drain?
Heavy clay or a high water table can mean a soakaway won’t work. That isn’t the end of the job. Common answers are:
Send us the result and we can look at sustainable drainage support and utility applications for your site.
Other Services Your Project May Require
Soakaway testing is often coordinated alongside ground, drainage and approval services.
Important Soakaway Test Information
Results apply to the location and ground conditions at the time of the test. Whether a soakaway is acceptable is decided by your building control body and, where a planning condition applies, the planning authority. One Stop Consult provides consultancy and coordination support through its specialist geotechnical consultant network.
Soakaway Test FAQs
Is a percolation test the same as a soakaway test?
People use both names. For rainwater soakaways the test is done to BRE Digest 365. A different percolation test is used for septic tank and treatment plant drainage fields. Tell us which one building control has asked for.
Can I do the test myself?
Some building control officers may accept a self-done test on a small extension, but many want a professional report, and planning drainage conditions often do. A professional report avoids the risk of having to test again.
Does the weather matter?
Yes. Ground that has been very wet or frozen can give a poor result. We can advise on timing when we quote.
How deep is the test pit?
About the depth of the finished soakaway, often 1 to 2 metres. Known pipes and cables need to be avoided, and we can arrange a utility survey first if needed.
How long does the test take?
Usually a day on site. Slow-draining clay can take longer. We confirm the timescale for your site when we quote.
Do I need a soakaway test for planning or Building Control?
Either can ask. Planning asks when a drainage condition needs infiltration testing. Building control asks when it checks your drainage. One test and report can often cover both.
Request Your Free Soakaway Test Quote
Complete the form below with the site address and what the soakaway will drain. Tick Geotechnical Phase II Site Investigation and write “soakaway test” in the notes box. Upload your drawings and any planning or building control comments so our team can review the requirement and coordinate an appropriate quotation.