Cost guide
Soakaway cost, and when you need one
What a drainage field or soakaway costs, when the rules require one, and why a percolation test comes before any price.
A soakaway is where water is handed back to the ground. The word covers two quite different things, and confusing them is the most common and most expensive mistake made on this subject.
A surface water soakaway takes clean rainwater from roofs and drives. A drainage field, which serves a septic tank or treatment plant, disperses treated effluent and is governed by the General Binding Rules, sized by a percolation test and subject to legal separation distances. They are not interchangeable, they cost different amounts, and effluent must never go to a rainwater soakaway.
This page covers both, with the drainage field treated as the main case, since that is the one with the rules and the larger bill attached.
What these figures are, and are not
These are typical market ranges for domestic systems in this part of the country, not quotes, and not prices this site can offer. They are gathered from generally published UK contractor price guidance and are intended to tell you whether a quote you receive is in the normal range. Real prices vary widely with access, ground conditions and how far a tanker or a digger has to travel. Anyone quoting you from a phone call alone, without seeing the site, is guessing.
Ranges last reviewed July 2026.
Typical ranges
| Work | Typical range |
|---|---|
| Percolation test to BS 6297, with written resultsTrial holes, a measured test and a written result. Non-negotiable before a drainage field is designed, and the thing that determines whether one is permitted at all. | £300 to £800 |
| Drainage field for a typical householdExcavation, distribution pipework, graded aggregate, geotextile and reinstatement, sized on the measured percolation rate and the number of bedrooms. Poor percolation means a larger field and a higher figure. | £3,000 to £8,000 |
| Localised drainage field repairRealistic where the fault is a crushed pipe, a root mass or a distribution box problem rather than the soil itself having sealed. A survey is what tells you which you have. | £500 to £2,000 |
| Surface water soakaway for roof and drive waterA smaller job than a drainage field, using crates or rubble, sized on roof area and rainfall. No effluent goes anywhere near it. | £800 to £3,000 |
| Engineered alternative where percolation failsA mound system or similar, or a switch to a treatment plant discharging to a watercourse. This is the branch that heavy clay frequently forces. | £8,000 and upwards |
What drives the variation
The percolation rate, which sets the size
The measured rate at which your soil accepts water feeds directly into the required area of drainage field. Slow ground needs a bigger field, and a bigger field costs more to dig and fill. This single measurement drives the price more than any other factor, which is why the test comes first.
Whether a field is permitted at all
If the test fails, the General Binding Rules do not allow a conventional drainage field, and no amount of money makes an unsuitable site suitable. At that point the honest options are a treatment plant discharging to a watercourse or an engineered system, and a contractor who offers to build a standard field anyway is selling you something that will fail.
Space and separation distances
A drainage field needs clearance from watercourses, boreholes and wells, buildings, boundaries and any area of hard standing. On a tight plot these distances, rather than the ground itself, are what rules a field out, and they are checkable before anyone digs.
Access for machinery and aggregate
A field is a substantial excavation needing an excavator in and many tonnes of graded aggregate delivered. Restricted access raises the cost of both, and spoil has to be spread or carted away.
What actually killed the old one
Most failed drainage fields were killed by the tank upstream, not by age. A tank left too long between empties carries sludge over and seals the soil. Building a new field without fixing that habit buys the same failure again, which makes this the one case where the cheapest useful advice is about maintenance rather than construction.
What a fair quote should itemise
Two quotes are only comparable when they cover the same work. Asking for these as separate lines is the quickest way to see which is genuinely cheaper, and it is a reasonable request that any decent contractor is used to.
- The percolation test, its measured result, and the field area that result requires
- Confirmation that separation distances are met on your plot
- Excavation, pipework, aggregate grade and geotextile, itemised
- Whether spoil is spread on site or removed
- Reinstatement of ground and any landscaping affected
- Investigation of the existing tank, where an old field has failed
- The alternative and its cost, if the percolation test fails
- VAT
In Shropshire specifically
Shropshire soils are the reason this page exists. The glacial clays and mosses across the north of the county are slow-draining, and percolation tests there fail often enough that a conventional drainage field cannot be assumed. Anyone quoting a drainage field on north Shropshire clay without a test result is guessing at the most important number in the job.
The south and west bring the opposite problem. Thin soils over hard rock on the border hills and over limestone on Wenlock Edge can mean there is simply not enough depth for a field, regardless of how well the top layer drains.
Slope matters across the hill country. Drainage fields need to be reasonably level to distribute effluent evenly, and steep ground either needs terracing or rules a field out. On valley-side plots around Church Stretton, Clun and the Marches this is a routine constraint rather than an unusual one.
Spring lines and a high winter water table are common in the border hills and on the moss-edge ground of the north. A test carried out in a dry summer can flatter a site that is saturated in February, so it is worth asking when the test was done and what allowance has been made for winter conditions.
Frequently asked questions
How much does a soakaway cost?
A drainage field serving a septic tank or treatment plant typically costs £3,000 to £8,000, plus £300 to £800 for the percolation test that must come first. A surface water soakaway taking only roof and drive water is smaller, usually £800 to £3,000. These are general market ranges rather than quotes.
Do I need a percolation test before getting a price?
Yes, in any meaningful sense. The test result determines whether a drainage field is legally permitted and how large it has to be, and the size is what sets the price. A quote given without one is an estimate resting on an assumption about the single most important variable.
What happens if my percolation test fails?
A conventional drainage field is then not permitted under the General Binding Rules. The recognised alternatives are a sewage treatment plant discharging to a watercourse, or an engineered solution such as a mound system. This is a common outcome on Shropshire clay and is not a dead end, but it does change the budget.
Can a failed soakaway be repaired instead of replaced?
Sometimes. If the fault is a crushed pipe, root ingress or a distribution box problem, a targeted repair is realistic and much cheaper. If the soil itself has sealed with sludge carried over from an under-maintained tank, replacement on fresh ground is usually the only durable fix. A survey tells you which case you are in before you spend anything.
Not sure which of these applies to you?
Describe the problem and we will tell you what you are most likely dealing with, and put you in touch with a local specialist where one covers your area. No charge for that, and no obligation.