The short answer, in England, is at least 5 m. Approved Document H of the Building Regulations says soakaways and other infiltration devices should not be built within 5 m of a building or road. A drainage field for a septic tank or treatment plant is different and needs to be further away. Here are the figures, where they come from, and why the distance is only the starting point.
Soakaways for rainwater: at least 5 m
Approved Document H, paragraph 3.25, says infiltration devices, which include soakaways, should not be built:
- within 5 m of a building or road, or in areas of unstable land
- in ground where the water table reaches the bottom of the soakaway at any time of year
- so close to other soakaways or drainage fields that the ground cannot take the combined flow
- where contamination in the run off could pollute groundwater
The 5 m rule keeps concentrated water away from foundations, where it can soften the ground they bear on. It applies to any building, so a neighbour’s house or garage counts as much as your own. Approved Document H does not give a separate distance from a boundary, but your building control officer may ask how the soakaway affects neighbouring land, so it is worth raising early.
Drainage fields for septic tanks and treatment plants
A drainage field takes treated effluent rather than rainwater, so the distances are greater. Approved Document H, paragraph 1.27, says a drainage field or mound serving a septic tank or treatment plant should be at least:
- 10 m from any watercourse or permeable drain
- 50 m from the point of abstraction of any groundwater supply, and not in a Zone 1 groundwater source protection zone
- 15 m from any building
It should also be far enough from other drainage fields and soakaways that the ground is not overloaded, downslope of groundwater sources, and clear of driveways, paved areas and other underground services.
In England the Environment Agency’s general binding rules also apply. Among other things, they say you cannot use the rules (and will need a permit) if the discharge is in a Zone 1 source protection zone, which includes anywhere within 50 m of a private water supply used for drinking. A new discharge must not be within 50 m of another system discharging under the rules. If a public foul sewer is within 30 m of the boundary of a single property you are expected to connect to it instead. A septic tank must not discharge to a watercourse, and the drainage field must be sited, designed and built in line with BS 6297. Our guide to the septic tank rules in England covers these in more detail.
Why ground testing still decides where it goes
Meeting the distances only tells you where a soakaway or drainage field is allowed. It does not tell you whether the ground there will take the water. That depends on the soil at the depth the soakaway or drainage pipes will sit, and the only way to find out is to test it in that position.
For a soakaway, that means a BRE 365 soakaway test. For a drainage field it means a BS 6297 foul infiltration test, and Approved Document H says a drainage field should only be used where the average Vp is between 12 and 100. Ground outside that range needs a different solution. The test result is what the drainage designer works from, so it is worth having it before the layout is fixed, not after.
When to call us
If you have a planning condition or a building control request for soakaway or percolation testing, or you are planning a septic tank or treatment plant, we can test the ground where you intend to put it and give you a report to BRE 365 or BS 6297.
or call 01902 475653, Monday to Friday, 8am to 6pm.