Septic Tanks
Percolation Test
What a percolation test involves, when you need one, and how the results determine your drainage field design.
In short: a percolation test measures how quickly water drains through your soil. The result — a “Vp” value — determines how large your drainage field needs to be, and sometimes whether a drainage field is a realistic option for your site at all.
When you need a percolation test
- Installing a new septic tank that will discharge to a drainage field
- Replacing a failed drainage field
- Assessing whether a site is suitable for private drainage before a purchase or development
You generally don’t need one if you’re installing a sewage treatment plant discharging under a permit or exemption directly to a watercourse, since there’s no drainage field to size.
How the test works
- Trial holes are dug at the proposed drainage field location, to a set depth below the invert level of the tank outlet.
- Water is used to pre-soak the holes, replicating saturated ground conditions.
- The time taken for water to drop a measured distance is recorded and averaged across multiple holes.
- The result is expressed as a Vp value (time in seconds per millimetre of drop), used to calculate the required drainage field area under BS 6297.
What the results mean
- Fast-draining soil (low Vp) can support a smaller drainage field, but may raise groundwater protection considerations
- Slow-draining soil (high Vp, typically heavy clay) needs a larger field, and beyond a certain point may rule out a drainage field entirely — pointing toward a sewage treatment plant instead
- Very fast or very slow results can both trigger a need for further assessment or an alternative design
If your soil drains too fast: engineered ground
Since an Environment Agency guidance update on 23 July 2026, a site whose natural soil drains too quickly can still meet the General Binding Rules by engineering the ground to slow the flow. The engineered ground must reach a Vp of at least 15, the site must not be in a floodplain, and the drainage field must otherwise follow BS 6297:2007. A percolation test on the engineered ground is how that Vp is confirmed.
See our septic tank vs sewage treatment plant comparison if your test suggests a drainage field isn’t viable.
Can I do a percolation test myself? + −
The physical digging can be done by hand, but interpreting results correctly against BS 6297 and using them to design a compliant drainage field is a specialist task, and most building control departments expect it to be carried out or verified by a professional.
How long does a percolation test take? + −
The on-site test itself typically takes a few hours, including pre-soak time, though scheduling around suitable weather (not testing after heavy rain, for instance) can affect timing.
What happens if my percolation test result is poor? + −
A poor result (very slow draining ground) usually means a larger field is needed, or that a drainage field isn't a practical option at all — in which case a sewage treatment plant is generally the alternative.