How Soakaways Work and When You Need One
A soakaway is the unsung infrastructure beneath thousands of London gardens, taking rainwater from roofs and paved areas and returning it to the ground rather than the public sewer. Most people only think about theirs when it stops working, which is usually evident as a soggy lawn, water pooling at a gully, or repeated flooding near a downpipe. This guide explains what a soakaway is, why current Building Regulations require them, how they are sized, the difference between traditional rubble and modern attenuation crates, and how to maintain or replace one in a London property.
What a soakaway actually is
A soakaway is an underground void designed to receive surface water runoff, store it briefly, and allow it to percolate slowly into the surrounding subsoil. It is part of the surface water drainage system and must be kept separate from any foul drain. The void can be filled with clean angular stone, modular plastic SuDS attenuation crates, or precast concrete chambers, depending on flow, ground conditions and depth.
The soakaway is not a tank. It does not hold water permanently. Its job is to provide enough void to buffer a peak storm and then dissipate the stored water into the surrounding soil within a defined recovery time, typically less than 24 hours so it is ready for the next storm.
Why a soakaway is often required
Surface water management is a statutory matter. Approved Document H3 of the Building Regulations sets out a hierarchy of acceptable surface water destinations:
- Infiltration to the ground (soakaway or other infiltration device).
- Discharge to a watercourse.
- Discharge to a surface water sewer.
- Discharge to a combined sewer, only with the sewerage undertaker’s consent.
Infiltration is at the top of the hierarchy because it most closely mimics natural drainage and reduces load on the public network. For new builds and many extensions, planning officers and building control will expect a soakaway unless infiltration has been demonstrated to be unfeasible. London has the additional driver of Thames Water’s combined sewer network in inner boroughs, where every disconnected downpipe reduces the chance of a combined sewer overflow.
The wider design principles are also captured in BS EN 752 for drain and sewer systems outside buildings and in BRE Digest 365, the longstanding UK reference for soakaway design and percolation testing.
Sizing: the percolation test
You cannot size a soakaway without knowing how fast the ground accepts water. The procedure, set out in BRE Digest 365, is a percolation test:
- A trial pit is excavated to the proposed soakaway depth, typically 1 to 2 metres below ground level.
- The pit is filled to a known depth with water and allowed to soak away once.
- It is refilled and the time taken for the water level to fall from 75 percent to 25 percent of effective depth is measured.
- The test is repeated three times and an average soil infiltration rate is calculated.
From this rate, the area of the contributing roof or paving, and a design rainfall figure for the location, an engineer calculates the required storage volume. London soils vary widely: alluvial gravels along the Thames will infiltrate readily, while large parts of inner North London sit on London Clay, where infiltration is very poor and a different solution may be needed.
Material options: gravel versus crate
Two formats dominate residential soakaways in 2026.
Traditional rubble or gravel soakaways
The classic design is a pit filled with clean angular stone, typically 40 mm to 75 mm, wrapped in a permeable geotextile membrane to stop fines migrating in. Void ratio is about 30 percent. Gravel soakaways are cheap to build and use familiar materials, but for the same storage volume they need to be roughly three times larger than a crate alternative, which matters in compact London gardens.
Modular plastic attenuation crates
Modern attenuation crates are interlocking plastic units with void ratios of 90 to 95 percent. They install in a small footprint, can be combined into large arrays, and provide structural strength to support light loadings above. They are now the default for most domestic applications in dense urban areas. Crate units should comply with the relevant manufacturer testing and be installed in accordance with the producer’s design tables and BS EN 752 performance principles.
Installation depth and the boundary distance rule
Whichever format is used, two siting rules tend to apply. The first is distance to the building. Approved Document H3 recommends that soakaways are positioned at least 5 metres from any building, and at least 2.5 metres from a boundary. The intent is to keep infiltrating water clear of foundations and to give a neighbour reasonable assurance that water is not undermining their land. The second rule is depth. The base of the soakaway needs to be above the seasonally high groundwater table, otherwise infiltration is permanently compromised. In parts of inner London close to the River Lea or the Thames floodplain, this is a real constraint that may push the design upwards or rule out infiltration entirely.
Maintenance and warning signs
Soakaways need very little active maintenance, but they do silt up over time. Warning signs that a soakaway is failing or full include:
- Standing water near the downpipe gully after rain.
- A persistently soggy patch of lawn directly above a known soakaway.
- Backflow up the gully during heavy rain.
- Cracking or settlement of paving above the soakaway location.
The first response is usually a CCTV drain survey down the lead-in pipe to confirm whether the issue is silt in the pipe, a blocked gully, or a soakaway that has reached the end of useful life. Routine cleaning of the upstream gully is the single most useful preventive action.
When a soakaway needs replacing
Soakaways typically last 20 to 40 years depending on soil and inflow quality. Replacement is warranted when:
- The original soakaway has filled with fines from years of unfiltered runoff.
- Ground conditions have changed (new neighbouring development, raised water table).
- The roof area discharging into it has been extended.
- The current installation is too small for current rainfall design figures.
Replacement is usually a one-to-two day excavation. Crate systems make this easier than digging out an old rubble pit, and they free up future garden use because the footprint is smaller.
What happens when ground conditions block infiltration
Large parts of inner North London sit on London Clay, where percolation rates are too low for a conventional infiltration soakaway to function effectively. In these conditions Approved Document H3 acknowledges that infiltration may not be feasible and that designers can move down the hierarchy to attenuated discharge: a sealed storage chamber that holds peak storm flow and releases it at a controlled, lower rate to the surface water sewer or watercourse. This is sometimes called a flow-restricted attenuation tank. The capacity calculations are similar to a soakaway, but the engineering brief shifts from “make this water disappear” to “release this water slowly”. The same modular crate units used in infiltration soakaways are commonly used as the storage component, wrapped in an impermeable membrane and fitted with a controlled outlet (typically a hydrobrake or fixed orifice).
Listed buildings and conservation considerations
Owners of listed buildings or properties in a conservation area face additional constraints. Any associated above-ground alteration (new gullies, visible downpipes, paving changes) may require consent. The soakaway itself, being below ground and invisible, is rarely the controlled element, but the connecting works often are. Always check with the local authority conservation officer before committing to a layout.
When to call a professional
If your garden floods, your downpipe gurgles or your soakaway is more than 30 years old, commission a survey before the next big storm. For maintenance, replacement or new installations, see drain repair services. Greater London Drainage carries out percolation testing, soakaway design and installation across North London, with experience in conservation area properties.
Final thoughts
A working soakaway is invisible. A failing one is everywhere: in waterlogged lawns, in flooding gullies, in damp foundations and in the slow erosion of confidence that the property is performing as it should. The good news is that the engineering is well understood, the regulations under Approved Document H3 are clear and supported by BS EN 752 performance principles, and modern attenuation crates make installation and replacement quicker and tidier than the traditional gravel approach ever was. In a city under increasing rainfall pressure, with Thames Water actively encouraging surface water disconnection where it can practically be achieved, an efficient soakaway is no longer optional infrastructure. It is the part of your property that quietly protects everything above it, every time it rains.
