The short answer
You usually need a soakaway when surface water (roof and paving runoff) cannot or should not go to a sewer, which is increasingly the default under SUDS and Part H — both favour soaking rainwater into the ground over piping it away. Whether a soakaway will work depends on your ground: it must be permeable enough, the water table low enough, and it must sit a safe distance from buildings (commonly at least 5m). This is confirmed by a percolation test. A typical domestic soakaway — a crate or rubble-filled pit sized to the area it drains — usually costs around £1,500–£4,000+ installed, more for large roofs, deep digs or poor-draining ground that needs a bigger or alternative solution. If the ground will not soak, you fall back to a watercourse or, as a last resort, a surface water sewer.
Soakaways are now the expected first option for rainwater on many extensions, but only if your ground will take the water. Here is how that is decided and what it costs.
Soakaways
- Used forSurface water (roof/paving)
- Suitability testPercolation test
- Distance from buildingUsually at least 5m
- Typical cost~£1,500–£4,000+
- RulePart H / SUDS hierarchy
When a soakaway is required
Under the Part H surface water hierarchy and SUDS policy, rainwater should be infiltrated into the ground first, used or stored second, sent to a watercourse third, and only piped to a sewer as a last resort. So for many extensions a soakaway is not optional — it is the expected solution unless the ground or water table rules it out. It is also commonly required where there is no surface water sewer to connect to, or where the water company will not accept extra surface water into an already-loaded system.
The percolation test that decides it
Whether a soakaway will work is settled by a percolation (infiltration) test: a pit is dug, filled with water, and the rate at which it drains is measured. Free-draining sand and gravel soak quickly and suit soakaways; heavy clay drains slowly and may not. The test result sizes the soakaway and confirms feasibility. A high water table can also rule one out, since a soakaway cannot discharge into already-saturated ground.
| Factor | Good for soakaway | Poor for soakaway |
|---|---|---|
| Soil type | Sand / gravel | Heavy clay |
| Water table | Low | High |
| Drained area | Modest roof/paving | Very large areas |
| Space | 5m+ from building | Tight plots |
Indicative suitability factors. A percolation test gives the actual answer. Sources: Planning Portal Part H; water company SUDS guidance.
What it costs and how it is built
A domestic soakaway is typically a buried crate (modular cellular) unit or a rubble-filled pit, wrapped in geotextile, sized from the percolation rate and the area it drains. Cost is driven by the volume needed, dig depth, ground conditions and distance from the building. Expect roughly £1,500–£4,000+ for a typical install, with large roofs, deep digs, or multiple soakaways costing more. Where infiltration is poor, an attenuation system (storing water and releasing it slowly) or a watercourse discharge may be used instead, which changes the cost. The design is checked under Building Regulations and may need local authority sign-off where SUDS approval applies.
Sizing, maintenance and the limits of a soakaway
A soakaway is sized so it can hold a design storm's worth of runoff and let it soak away before the next downpour. The volume comes from the area it drains (roof and paving), the rainfall design event, and the measured infiltration rate from the percolation test — free-draining ground needs a smaller pit than slow-draining ground for the same roof. Undersize it and it overflows in heavy rain; oversize it and you have paid for a bigger dig than you needed, which is why the percolation test is not a formality but the basis of the design.
- Position: at least around 5m from buildings (and a sensible distance from boundaries and other soakaways) so infiltrating water does not soften the ground around foundations.
- Pre-treatment: a silt trap or catchpit before the soakaway stops it clogging with grit and leaves, which is the main cause of soakaways failing over time.
- Maintenance: crate and rubble soakaways are not maintenance-free; the inlet, silt trap and any access need occasional checking and clearing to keep them working.
It is also worth being honest about when a soakaway simply will not do the job. Heavy clay, a high water table, contaminated ground, or very large drained areas can all make infiltration impractical or unsafe. In those cases you move down the surface water hierarchy to a watercourse discharge or, as a last resort, a restricted connection to a surface water sewer — frequently with an attenuation tank that stores the water and releases it slowly to avoid overloading the system. The right answer is whatever the ground and the rules allow, confirmed by the percolation test and the drainage design under Part H and any SUDS requirement, rather than assuming a soakaway is always possible.
How a percolation test is actually carried out
Because the percolation test decides whether a soakaway is viable and how big it must be, it is worth knowing what a proper test involves rather than treating it as a quick formality. The standard domestic method follows the approach set out in the relevant BRE guidance (Digest 365) and Approved Document H.
- Dig a trial pit at the proposed soakaway location, to roughly the depth the soakaway will sit at, so the test reflects the ground that will actually receive the water.
- Pre-soak the pit by filling it with water and letting it drain once, so the surrounding soil is wetted and the result is not flattered by dry ground absorbing the first fill.
- Measure the drop: refill to a set level and time how long the water takes to fall between two marked points. This is repeated, usually three times, and the slowest result is used to be safe.
- Calculate the infiltration rate from the readings, which then feeds the soakaway sizing.
A few practical points matter. The test should ideally be done in realistic conditions, not in the middle of a dry spell when the ground drains unusually well, because the soakaway has to cope in a wet winter too. A high water table found in the trial pit can rule a soakaway out regardless of soil type, since water cannot infiltrate into already-saturated ground. And the result is not just pass or fail — it sets the volume, so a slow-but-acceptable rate simply means a larger soakaway. Doing the test early, before levels and the extension footprint are fixed, means a viable soakaway position can be designed in rather than discovering after the roofs are on that the runoff has nowhere acceptable to go.
Frequently asked questions
How far from the house must a soakaway be?
Guidance commonly requires a soakaway to be at least 5 metres from buildings, to avoid water softening the ground around foundations. The exact distance depends on the design and ground conditions.
What if my ground fails the percolation test?
If the ground will not soak — typically heavy clay or a high water table — you move down the hierarchy to a watercourse, or as a last resort a surface water sewer, often with an attenuation system to limit the discharge rate.
Do I need approval for a soakaway?
The drainage design is checked under Building Regulations (Part H), and where SUDS approval applies the local authority may need to agree the surface water strategy. A percolation test usually supports the design.
Sources & further reading
- Planning Portal — Approved Document H (drainage)
- Checkatrade — soakaway cost guide
- GOV.UK — sustainable drainage systems (SUDS) guidance
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.