Soakaways Explained: How They Work and When They Fail

A soakaway is an underground pit, filled with rubble or plastic crates, that collects rainwater from roofs and paving and lets it seep into the surrounding ground. It only works where the soil drains freely: BRE Digest 365 says a soakaway must empty from full to half-full within 24 hours and should not normally sit closer than 5 m to building foundations.

This guide explains how soakaways are designed and tested, why many struggle on clay, and what to do when one stops working.

What is a soakaway and what does it do?

A soakaway disposes of surface water (rain) on your own land instead of sending it into a sewer. Downpipes, patio drains or driveway channels feed a buried chamber, and the water soaks out through the sides and base of that chamber into the soil.

Soakaways are for rainwater only. Foul water from toilets, sinks, showers and washing machines must never go into one; it needs a connection to the public sewer or a properly designed treatment system. Approved Document H of the Building Regulations deals with foul drainage (Part H1) and rainwater drainage (Part H3) separately for exactly this reason.

What are the main types of soakaway?

Older soakaways were usually a hole filled with broken brick or hardcore. Modern ones typically use plastic cells (crates) wrapped in a geotextile membrane, which hold much more water for the same size of hole.

Whatever the type, a silt trap or catchpit upstream helps stop leaves and grit reaching the soakaway. Debris washing in is a common reason soakaways slowly fill up and stop working.

How is a soakaway designed and tested?

A soakaway should be sized from the rate at which water actually drains into the soil on your site, not from a rule of thumb. The standard method is BRE Digest 365, Soakaway design (2016 edition):

If the trial pit barely drains, that is your answer: a soakaway will not work there, however large you make it.

You may also come across the percolation test in Approved Document H. That one is for drainage fields serving septic tanks and treatment plants. A 300 mm square hole is filled, and the time the water takes to fall 150 mm is used to calculate a figure called Vp; the ground is suitable if the average Vp is between 12 and 100. It is a different test for a different purpose, so check your installer is using the right one.

Where can a soakaway go?

Distance matters because water soaking into the ground next to foundations can soften the soil beneath them. BRE 365 says a soakaway should not normally be built closer than 5 m to building foundations, and Approved Document H (para 3.25) says infiltration devices should not be built within 5 m of a building or road.

The guidance also points to ground where infiltration is a poor choice:

• clay subsoils, which let water through very slowly;
• a high water table, where the ground is already saturated for part of the year;
• unstable ground;
• plots where the 5 m distances cannot be achieved.

Clay is a real consideration in parts of our area. Around Radcliffe-on-Trent, for example, the red cliffs above the Trent are Mercia Mudstone, a clay-rich rock that limits how well soakaways drain. On ground like that, a test pit often shows the water needs to be stored and released slowly rather than soaked away.

Why do soakaways fail?

Most soakaways fail gradually. Silt, leaves and fine soil wash in over the years and coat the surfaces the water should drain through, until the chamber stays full between storms. Others were undersized from the start, built on clay, or crushed by vehicles driving over them.

Signs your soakaway is struggling:

• water backing up and overflowing from gullies or downpipe shoes during rain;
• standing water or a boggy patch on the lawn above or near the soakaway;
• downpipes that gurgle or spill at the joints in heavy rain;
• damp patches on walls close to where the rainwater pipe runs;
• problems getting worse after a wet spell, when the ground is already saturated.

A blocked or broken pipe between the house and the soakaway causes the same symptoms, so check that pipe before blaming the soakaway.

Can a failing soakaway be repaired?

Sometimes. If the system has an access point, high-pressure jetting can clear silt from the pipework and, with some designs, from the chamber itself; MyJobQuote's 2026 cost data puts soakaway jetting at roughly £100–£400. A CCTV drain survey of the pipe feeding it will show whether the real problem is a blockage, a collapse or tree roots.

If a rubble soakaway is completely clogged, or the ground simply does not drain, cleaning only buys time. The realistic options are to rebuild it larger or in a better position after a proper infiltration test, or to switch to attenuation storage that releases water slowly to an approved outlet. For a lawn that stays wet even without a soakaway nearby, see our guide to French drains for waterlogged gardens.

How much does a soakaway cost?

Prices depend on size, access, ground conditions and whether a machine can reach the spot. Published cost guides give these typical market ranges:

These are national ranges, not our prices. We quote every groundworks job after a site visit, and the price is agreed before any digging starts.

Do you need permission for a soakaway?

Drainage is a controlled service under the Building Regulations, so new drainage work, including a new soakaway for an extension, usually needs building control approval (LABC). Most like-for-like repairs do not (gov.uk). If the soakaway is part of a planning application, the council may also ask how surface water will be managed.

You may read that sustainable drainage (SuDS) approval under Schedule 3 of the Flood and Water Management Act 2010 is now compulsory. It is not: Schedule 3 has still not been commenced in England, and the government has favoured a planning-policy approach instead (House of Commons Library). Non-statutory National Standards for Sustainable Drainage Systems were published on 19 June 2025.

What are the alternatives to a soakaway?

Where the ground will not take water, or there is no room for the 5 m distances, other measures can reduce or slow the flow:

• water butts on downpipes to capture some roof water for the garden;
• permeable paving on drives and patios, so rain soaks through rather than running off;
• rain gardens or planted areas that hold water for a short time;
• attenuation tanks that store water and release it slowly to an approved outlet.

Whichever route you choose, get the ground tested first.

When to call a professional

Call a drainage contractor if water is backing up from your gullies, the lawn is waterlogged over the soakaway, or you are planning an extension that needs new surface-water drainage. We can jet and camera-inspect the existing pipework, carry out infiltration testing, and design or rebuild a soakaway through our groundworks service. If a run just needs clearing, see drain jetting.

We cover Nottinghamshire, Derbyshire, Leicestershire and Lincolnshire, with no call-out fee. Contact us or call 07300 600 950 to arrange a visit.

Sources

• BRE Digest 365: Soakaway design (2016)
• Approved Document H: Drainage and waste disposal (gov.uk)
• House of Commons Library: CBP-10483 on sustainable drainage
• gov.uk: National Standards for Sustainable Drainage Systems
• gov.uk: When you don't need building regulations approval
• MyJobQuote: Soakaway installation costs
• MyBuilder: Drainage system installation cost
• Checkatrade: Soakaway cost guide