Drainage field installation
A drainage field is a network of perforated pipes laid in gravel-filled trenches, so effluent from the tank seeps evenly into the soil, where bacteria finish treating it. It is not the same as an old soakaway pit or a rubble-filled hole, and the difference matters: in England a soakaway installed after December 2007 does not meet the general binding rules.
Most new fields cost £1,500 to £4,000 in published guides, with large or difficult sites reaching about £7,000. The size, and so the price, comes from a percolation test.
New drainage field£1,500 to £4,000a guide, not a quote
When a new drainage field is needed
- The old field is waterlogged, blocked with solids or root-bound, and effluent is surfacing.
- The existing outlet is a soakaway pit, well or borehole that cannot meet the binding rules.
- A septic tank currently discharges to a ditch or stream and has to be moved to ground.
- A new tank or plant is going in for a new build, a barn conversion or an extended house.
- The field is too close to the house, a boundary or a watercourse for current guidance.
What drainage fields typically cost
| Item | Typical price |
|---|---|
| Standard new drainage field Trenches, gravel, perforated pipe, distribution chamber | £1,500 to £4,000 |
| Large or complex field Slow ground, long runs, slopes or a mound | £4,000 to £7,000 |
| Replacing a failed field Old trenches dug out, new ones laid | £1,500 to £5,000 |
| Site survey and percolation test Test holes dug and timed, report for building control | £250 to £500 |
| Contaminated soil removal Only where a failed field has saturated the ground | about £40 per m³ |
Where our information comes from
Installed prices are typical ranges from published UK price guides, autumn 2026, summarised by us and read in November 2026. The guides rarely say whether VAT is included, so ask every installer for prices on the same basis. A guide for budgeting, not a quote.
How a drainage field is sized
The method is set out in Approved Document H. Your installer will run it, but it helps to know what the numbers mean.
The percolation test
Holes are dug 300 mm square and 300 mm deep below the level the trench pipes will sit at. Each is filled with water to 300 mm deep and the time taken to fall from 75% to 25% full (a drop of 150 mm) is recorded, then divided by 150 to give seconds per millimetre. The test is repeated and averaged across the holes to give the percolation value, Vp.
From Vp to floor area
Approved Document H gives the trench floor area as the number of people the system serves multiplied by Vp, then by 0.25 after a septic tank or 0.2 after a treatment plant. A four-bedroom house sized for six people on ground with a Vp of 40 needs about 60 m² of trench floor behind a septic tank, or 48 m² behind a plant. Our estimator runs the sum for you.
When the ground says no
Very slow ground (a high Vp) is generally unsuitable for a standard field; very fast ground can let effluent reach groundwater before it is treated. Neither should be guessed. A mound, a treatment plant to a watercourse or a permit may be the answer, and in England a site in Source Protection Zone 1 needs a permit whatever the soil.
Where it can go
Council guidance summarising Approved Document H puts a drainage field at least 15 metres from any building and 10 metres from any watercourse. It should be on land you control, away from driveways and not under anything that compacts or seals the ground.
Rules that apply to the field itself
From the Environment Agency's binding rules for discharges to ground in England.
- Built to BS 6297:2007
- New drainage fields and mounds should follow the British Standard.
- 2 cubic metres a day
- The most a system may put to ground under the rules without a permit. A normal home is far below it.
- No new soakaway pits
- Soakaways, wells and boreholes installed after December 2007 cannot meet the rules.
- Not in a floodplain
- A drainage mound is only allowed outside a floodplain, and must also follow BS 6297.
How a field is put in
Test the ground
Percolation holes dug and timed; trial pits show the water table and soil layers.
Design
Trench area and layout calculated from Vp and the design population, then sent to building control.
Dig and lay
Trenches dug level, gravel bed laid, perforated pipe set to a shallow fall, and a distribution box fitted.
Cover and record
Membrane, backfill and turf, then a plan of where the trenches run so nobody builds over them.
Effluent surfacing, or a field that never dries? Get a percolation test and a price.
Ask for a site visitQuestions about drainage fields
How much does a drainage field cost?
Most new drainage fields cost £1,500 to £4,000 in published UK guides, up to about £7,000 on large or difficult sites.
What is the difference between a soakaway and a drainage field?
A soakaway is usually a single pit filled with rubble or a crate. A drainage field spreads effluent through a network of trenches so the soil can treat it. Septic tank effluent needs a drainage field; in England a soakaway installed after 2007 does not meet the rules.
How big does a drainage field need to be?
Approved Document H sizes it from the design population and the percolation value: people × Vp × 0.25 after a septic tank, or × 0.2 after a treatment plant, in square metres of trench floor.
How long does a drainage field last?
A well designed field on suitable ground, behind a tank that is emptied on time, can last for decades. The usual killer is solids carried over from a tank that was not desludged.
Can I build or park over a drainage field?
No. Vehicles compact the soil and hard surfaces stop it breathing. Keep it as grass and keep trees well away.
Keep reading
- Septic tank replacement
When the old tank has to go, and what goes in its place.
- Sewage treatment plant installation
Powered treatment for a stream outlet or poor ground.
- Septic tank installation cost
Estimator with tank sizes and drainage field areas.
- Septic tank regulations
England, Scotland and Wales side by side.
Ask for a site visit for a drainage field
Three short steps: where the property is, what the system needs, and how to reach you. About two minutes.
A free matching service, not a off-mains drainage installer. We look for one who works in your area and name them to you first. Nothing is shared until you say yes.
Thanks, your enquiry is in.
- We look for an off-mains drainage installer who works in your area and takes on this kind of job, then email or call to tell you who it is.
- Your details reach that firm only if you say yes.
- They arrange a site visit, test the ground if needed and give you a written price. No marketing from us, ever.