If your architect or building control officer has mentioned surface water drainage, you have probably hit the point where a soakaway and an attenuation tank start sounding like the same thing. They are not. One gets rid of rainwater into the ground, the other holds it back and releases it slowly, and across much of Greater London the ground itself decides which one you are allowed to use.
A soakaway is a void in the ground, usually a stack of plastic crates wrapped in geotextile, that collects roof and hardstanding water and lets it soak away into the surrounding subsoil. Nothing leaves the site. It only works if the ground beneath you actually accepts water at a reasonable rate.
An attenuation tank looks almost identical underground, but it is wrapped in an impermeable membrane and has an outlet connected to the sewer or a watercourse, fitted with a flow control device such as a Hydro-Brake or an orifice plate. Rain fills the tank quickly and it trickles out at a restricted rate, often 2 to 5 litres per second, so the sewer is not overwhelmed during a downpour.
Large parts of London sit on London Clay: Camden, Islington, Hackney, Southwark, Wandsworth, much of Ealing and Barnet. Clay has a very low infiltration rate, and soakaways in it either fail outright or stay waterlogged for days. Parts of south west and north London sit on more forgiving river terrace gravels, Bagshot sands or Thames alluvium, where soakaways can work well.
You do not guess this. Building Regulations Part H requires a percolation test to BRE Digest 365 before a soakaway is designed. It involves digging a trial pit to the proposed soakaway depth, filling it with water three times and timing the drop from 75 per cent to 25 per cent full. A typical London test costs somewhere in the region of 400 to 900 pounds depending on access and depth. If the result is poor, the answer is attenuation, and the test is not wasted money because most water authorities want to see it before approving a sewer connection.
A soakaway must sit at least 5 metres from any building foundation and normally clear of the property boundary, which rules it out on a lot of terraced and semi detached plots in inner London. It also needs to be above the seasonal high water table, with a clear gap of around 1 metre beneath the base.
Attenuation tanks have no 5 metre rule in the same way, but they need a viable outfall. If you only have a combined sewer, you will need a Section 106 application to Thames Water, and they will specify a restricted discharge rate. Expect four to eight weeks for that approval, so start it early rather than after the groundworks are dug.
On a basement or a rear extension in a conservation area, the lead local flood authority at your borough may also want a drainage strategy showing greenfield runoff rates or a percentage betterment on the existing situation.
For a typical rear extension of 30 to 40 square metres, a crate soakaway installed complete with excavation, geotextile, silt trap and connection tends to land somewhere around 2,500 to 5,000 pounds. An attenuation tank of similar volume usually runs higher, roughly 4,000 to 9,000 pounds, because of the impermeable membrane, the flow control chamber, the inspection access and the sewer connection itself.
Both figures move a lot with depth, muck away costs, and whether a machine can reach the area or the dig has to be done by hand through a side return. Spoil disposal in London is a genuine cost line, not a rounding error. Anyone quoting a firm price before a percolation test and a drainage layout is guessing.
You can dig one, but Building Control will want evidence of a BRE 365 percolation test and a sized design before they sign off. A soakaway that is undersized or too close to foundations can cause subsidence, and that is not a risk worth taking with a London clay site.
The test result becomes the justification for an attenuation tank and a restricted discharge to the sewer, which is what Thames Water will expect to see with a Section 106 application. It is a normal outcome across most of inner London, not a setback.
Yes. The silt trap or catchpit upstream should be checked and cleaned roughly annually, and the flow control device inspected at the same time, because a blocked orifice turns the tank into a flood risk rather than a solution.
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