Guide
Soakaway drainage: the rules, the percolation test and what it costs
Updated
A soakaway is a pit, rubble-filled or built from plastic crates, that lets rainwater soak into the ground instead of a drain. The Building Regulations put it first in line for a new building's rainwater, keep it 5 metres from any building or road, and size it from a percolation test; two national price guides put a domestic soakaway at about £660 to £2,500.
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Where a soakaway fits
For a new building or extension, Approved Document H (2015 edition) sets the order rainwater must go in: an adequate soakaway or other infiltration system first, a watercourse only where that is not reasonably practicable, and a sewer last (requirement H3(3)). So building control will ask why not before it accepts a connection to the sewer. For a waterlogged garden the same logic applies without the paperwork: a land drain needs a lawful place to send the water, and a soakaway is often it (see land drains for gardens).
Where it cannot go
Approved Document H says an infiltration device should not be built (para 3.25):
- within 5 metres of a building or road, or on unstable land;
- where the water table reaches the bottom of it at any time of year;
- too close to drainage fields or other soakaways, so that together they exceed what the ground can take;
- where contaminated runoff could pollute groundwater.
The percolation test decides the size
Percolation tests are carried out to find how fast the soil takes water (para 3.28), and the soakaway is sized so its storage holds the difference between the rain coming in and the water soaking out. Small soakaways serving 25 m² or less can assume a design storm of 10 mm in 5 minutes; anything bigger is designed to a one-in-ten-year storm, using BRE Digest 365 or BS EN 752 (paras 3.27 and 3.30). Under 100 m² it is usually a square or round pit filled with rubble or lined with perforated rings or crates; larger areas take lined pits or trenches (para 3.26). In heavy clay the test often shows the ground cannot take the water at all, which is worth knowing before paying for a hole.
What it costs
| Type | MyBuilder | MyJobQuote |
|---|---|---|
| Rubble-filled soakaway | £750 to £1,600 | Small soakaways from £660 |
| Plastic crate soakaway | £1,500 to £2,500 | Average around £850, driveway soakaways often over £1,000 |
Sources: MyBuilder's drainage cost guide and MyJobQuote's soakaway cost guide (dated 6 October 2026). Few drainage contractors publish a soakaway price, because the size comes from the area drained and the test result. Ask each quote whether it includes the percolation test, the sizing calculation building control will want, the pipework from the house and putting the garden back.
Approved Document H applies in England; Wales has its own version. Read on gov.uk on 27 October 2026; nothing here replaces your building control body's view of your site.