Short answer: if you are putting water into the ground rather than into a sewer, somebody is going to want to know how fast the ground takes it. Here is how to tell whether that applies to you.
You almost certainly need one if…
- Your planning permission carries a drainage condition asking for infiltration testing, percolation testing, or a soakaway design.
- You are installing a soakaway for surface water from a roof, drive or new building.
- You are installing a septic tank or package treatment plant with a drainage field, and there is no mains foul connection.
- You are applying to a water authority under Section 106 to connect surface water to a public sewer — they will want evidence a soakaway was tested and ruled out first.
- Building control has asked for infiltration figures before signing off the drainage.
You probably do not need one if…
- The site already has an adequate connection to a public surface water sewer and the authority is content with it.
- The scheme discharges to a watercourse with the necessary consent already in place.
- There is no new hard surface, no new building and no change to the existing drainage at all.
If you are unsure, the condition wording usually gives it away. Terms like infiltration, soakaway, BRE 365, SuDS hierarchy or surface water drainage scheme all point at a test.
Which test, though?
This is where it goes wrong most often, and it is an expensive mistake to make.
- Surface water — rainwater from roofs and hard surfaces — is tested to BRE Digest 365, and the result sizes a soakaway.
- Foul water — from a septic tank or treatment plant — is tested to BS 6297, referenced by Part H of the Building Regulations, and the result sizes a drainage field.
They are different procedures and produce different numbers. A BRE 365 result will not satisfy a condition asking for foul infiltration testing, and the other way round. Plenty of rural sites need both, and they can be done on the same visit.
When to do it
Earlier than most people do. If infiltration turns out not to be viable, the whole drainage strategy has to change — attenuation, a discharge to a watercourse, or a different site layout. Finding that out after the groundworks have been priced, or worse after they have started, is expensive.
Ground conditions also matter to timing. Testing is possible year round, but a high winter water table can affect the result, because BRE 365 requires the base of a soakaway to sit clear of groundwater. A site can fail on that alone, however well the soil itself drains.
Not sure?
Send us the wording of your planning condition and the site postcode. We will tell you which test you need, or whether you need one at all, before you commit to anything. It takes two minutes and it saves wasted visits.