Can a septic tank go to a soakaway?

“Soakaway” is one of the most loosely used words in drainage. Estate agents use it for any outlet that goes into the ground, builders use it for rainwater crates, and many older septic tanks really do drain into a single rubble-filled pit. Under the current rules in England, the difference between a soakaway and a drainage field decides whether a system is legal.

Soakaway or drainage field?

A soakaway is usually a single pit or chamber, often filled with rubble, stone or a plastic crate, into which water is piped and left to soak away. It is the right thing for clean rainwater from a roof.

A drainage field (sometimes called a leach field or infiltration system) is a network of perforated pipes laid in shallow, gravel-filled trenches across a wide area. Effluent trickles out along the whole length and spreads into the topsoil and subsoil, where air and bacteria finish treating it.

The difference matters because septic tank effluent is not clean. A septic tank only settles solids. The treatment happens in the soil, and a single deep pit gives the effluent too little soil to pass through before it reaches groundwater.

What the rules say in England

The Environment Agency’s general binding rules for discharges to the ground say:

• a septic tank or small treatment plant must be followed by a drainage field;
• new drainage fields and drainage mounds should be designed and built to BS 6297:2007;
• soakaways, wells and boreholes installed after December 2007 are non-standard systems and cannot meet the rules;
• new discharges should avoid boreholes, shafts and concrete rings.

If yours is a newer soakaway pit, you must upgrade to a compliant drainage field, connect to the public sewer, or apply for a permit. If effluent reaches groundwater directly through a non-standard system, the Environment Agency says it will not grant a permit and the system must be replaced.

What about an old soakaway?

A soakaway installed before December 2007 is not automatically illegal. It can carry on under the general binding rules if it meets the others: domestic sewage only, no pollution, under 2 cubic metres a day, the tank emptied at least yearly, and so on. The catch is that old soakaway pits clog. Solids carried over from the tank seal the sides of the pit, the water level rises, and effluent backs up or surfaces. When that happens, repair usually means a new drainage field.

Scotland and Wales

SEPA in Scotland uses “soakaway” more broadly in its registration categories for discharges to land, which are set by the number of properties and the soil’s percolation value. In practice SEPA and Natural Resources Wales both expect a properly designed infiltration system, sized from a percolation test, rather than a pit.

How a drainage field is sized

Approved Document H sets out the method. A percolation test gives a value, Vp, in seconds per millimetre. The floor area of the drainage trenches is then:

So a house designed for six people on ground with a Vp of 40 needs about 60 m² of trench floor behind a septic tank. NHBC’s standards add practical detail: trenches separated by 2 metres of undisturbed ground, pipes laid at a shallow, even fall, and no field where the winter water table comes within 1 metre of the pipes.

Our cost estimator runs the sum for your bedrooms and Vp.

What it costs to replace a soakaway

Published UK guides put a new drainage field at about £1,500 to £4,000 for most homes, up to around £7,000 on large or difficult sites. A percolation test and site survey is typically £250 to £500. If the ground will not take a field at all, the alternative is usually a sewage treatment plant discharging to a watercourse, which our septic tank regulations page covers.

Selling a house with an old soakaway

Buyers’ surveyors increasingly ask what a septic tank drains into. An old soakaway that works is not a reason a sale should fail, but its age and type should be written down and passed on, as the general binding rules in England require. If the soakaway turns out to date from after 2007, or is clearly failing, expect the buyer to ask for a new drainage field or a price reduction to cover one.

Signs your soakaway is failing

• Toilets and sinks draining slowly, especially after rain.
• Wet, smelly or unusually green ground over the pit.
• The septic tank level staying high after it has been emptied.
• Effluent visible at the surface.

If you see these, a drainage field installation is the likely fix. Use the form below to ask for a site visit from an installer who works in your area.