Percolation and soakaway tests
A percolation test measures how quickly water drains away into the ground on your site. Building control may ask for the result to show that a soakaway for rainwater, or a drainage field for a septic tank or small treatment plant, will work. The two uses are tested and judged differently, so it helps to know which one you need.
This guide covers the rules in England. See Wales, Scotland and Northern Ireland.
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Two different tests
Soakaway (infiltration) test
Checks whether the ground can take rainwater from roofs and hard surfaces, so a soakaway can be sized and sited. Part H of the Building Regulations asks that rainwater is dealt with without harming buildings.
Percolation test for a drainage field
Used where wastewater from a septic tank or small treatment plant is discharged to the ground. The result is an average percolation value, used to size the drainage field.
When is one likely to be needed?
- New homes or extensions with no surface water sewer. Approved Document H puts a soakaway first in the order of choices for rainwater where it is reasonably practicable, then a watercourse, then a sewer.
- New homes with no mains drains. A septic tank or treatment plant that discharges to the ground normally needs a percolation test to show the ground is suitable.
- Larger paved areas, new outbuildings and driveways that add enough run-off to need a proper soakaway or drainage design.
- A planning condition asking for a drainage strategy, which may include test results.
How a test works
A tester digs trial pits at the depth where the soakaway or drainage field would sit, fills them with water, and records how quickly the level falls. The result is an infiltration rate for the ground. It is then used to work out how big the soakaway or drainage field needs to be, so tests are done before the drainage is designed.
Tests are usually done in dry weather and, for a site with different soils, in more than one place. Rock, clay or a high water table can make a site unsuitable for infiltration, in which case the designer needs a different solution, such as discharge to a watercourse or sewer.
Things to check early
- Your building control body’s guidance on distances. Some councils’ guidance asks for soakaways to be a set minimum distance from buildings, so check yours.
- Whether you need other consents, such as from the Environment Agency for foul drainage that discharges to the ground, or from the sewerage undertaker if building near a public sewer.
- Whether the test result needs to be submitted with a full plans application or only made available to the building control officer on request.
In Wales, Scotland and Northern Ireland
This guide describes the rules in England. Building regulations are devolved, so here is how percolation and soakaway tests works in the rest of the UK.
Wales has its own Building Regulations Part H guidance, and new developments are subject to Welsh sustainable drainage rules. Check your local authority’s requirements for rainwater and foul drainage.
In Scotland, drainage is covered by Section 3 (Environment) of the Technical Handbook, and the SEPA authorisation regime applies to private sewage treatment. Check with your council’s building standards team.
In Northern Ireland, drainage is covered by Technical Booklet N, and discharges of private foul water to the ground may need consent from the Northern Ireland Environment Agency.
Common questions
What is a percolation test?
A field test that measures how quickly water soaks into the ground. The result shows whether a soakaway or septic tank drainage field is likely to work, and how big it needs to be.
Is a percolation test the same as a soakaway test?
They use a similar idea but are not the same. A soakaway test is for rainwater drainage. A percolation test, as the term is used for septic tanks, is for dispersing treated wastewater. Check which one your building control body or designer is asking for.
Do I need a percolation test for an extension?
Often not, if rainwater already goes to an existing sewer or soakaway that can take the extra run-off. You are more likely to need one if you have no sewer to connect to, or if the extension adds a lot of roof or paving.
What if the ground fails the test?
A failed test means water drains too slowly (or too quickly) for a standard design. Your designer may suggest a larger or different system, or another way of discharging water, such as to a watercourse.
How long does a test take?
The digging and testing is usually a short site visit, followed by a written result. Allow extra time for booking, for weather, and for the drainage to be designed from the result.
Who can do the test?
Drainage specialists, groundworks firms and ground investigation or environmental consultants offer it. Ask whether their report will be accepted by your building control body.
New to the jargon? See our glossary of terms, from air permeability to TrustMark.