The short answer
A percolation test measures how fast water soaks into the ground where a drainage field would go. Holes are dug, filled with water and timed as the level drops; the average gives a percolation value, Vp, in seconds per millimetre. Building control uses it to decide whether a drainage field will work and how big it must be.
On this page
Every new septic tank or treatment plant that drains to the ground starts with the same question: will the soil take it? The percolation test is how that question is answered, and the number it produces decides both whether you can have a drainage field and how many square metres of garden it will take.
Why the test matters
A septic tank does not clean sewage; it settles it. The soil in the drainage field does the treating. Ground that drains too slowly will flood and surface effluent. Ground that drains too fast lets effluent reach groundwater before the soil has worked on it. Building control will not approve a drainage field without evidence that the ground sits between those extremes, and the Environment Agency’s general binding rules say the system must not cause pollution.
How the test is done
Approved Document H describes the method used in England and Wales.
- Trial pit first. A pit at least 1 metre deeper than the proposed trench level shows the soil layers and whether the winter water table is too high. If groundwater or rock is close to the surface, a standard field may not be possible at all.
- Dig the test holes. Holes 300 mm square are dug to 300 mm below the level at which the drainage pipes will be laid, spread across the area of the proposed field.
- Pre-soak. Each hole is filled with water and left to drain away, ideally overnight, so the surrounding soil is wet as it would be in use.
- Time the fall. Each hole is filled to 300 mm deep and the time taken for the water to fall from 75% full to 25% full (a drop of 150 mm) is recorded.
- Work out Vp. That time in seconds is divided by 150 to give seconds per millimetre. The test is repeated in each hole and the results averaged to give the percolation value, Vp.
Wet-season testing gives a truer picture. A test in a dry August can flatter clay ground that floods every winter.
What the result means
| Vp (seconds per mm) | What it usually means |
|---|---|
| Under about 15 | Very fast ground; effluent may reach groundwater too quickly. A treatment plant first, or another solution, may be needed |
| About 15 to 100 | Generally suitable for a drainage field, sized from the result |
| Over about 100 | Slow ground; a standard drainage field is unlikely to work |
NHBC’s standards use a similar scale: up to 100 suitable for field drains, 100 to 140 suitable only with underdrains, and over 140 unsuitable. Building control in your area makes the final call.
From Vp to the size of the field
Approved Document H turns the result into a floor area for the drainage trenches:
- after a septic tank: people served × Vp × 0.25 square metres;
- after a treatment plant: people served × Vp × 0.2 square metres.
For a house sized for six people on ground with a Vp of 40, that is 60 m² behind a septic tank or 48 m² behind a plant. Trenches are usually 300 to 900 mm wide, so the total trench length runs to tens of metres. Our cost estimator does the sum for your house.
The field also has to fit: council guidance on Part H puts it at least 15 metres from any building and 10 metres from any watercourse.
What a percolation test costs
Published UK guides put a site survey with a percolation test at about £250 to £500. Many installers include it in the price of a drainage field installation or a replacement, since they cannot design the job without it. You can dig and time the holes yourself, but building control will want to see how the test was done, and most owners prefer a result they will not have to repeat.
If the ground fails
A failed test is not the end of the road. The common options are:
- A sewage treatment plant discharging to a watercourse that flows all year, if there is one and the rules allow it.
- A drainage mound, a raised field built above the natural ground, allowed in England outside a floodplain and built to BS 6297.
- Engineered ground, where the Environment Agency’s updated rules now allow discharges to ground engineered to reduce percolation, with a minimum Vp of 15, outside a floodplain.
- A permit from the regulator for a non-standard solution.
- A cesspool as a last resort, with high emptying costs.
Which of these works depends on your plot. An installer who has seen the ground can tell you; the form below finds a septic tank installation company that works in your area.