What Is a Soakaway? How They Work, Cost & Installation (2026 UK Guide)
The Quick Answer
A soakaway is an underground pit or crate structure that collects surface water — usually rainwater from a roof or driveway — and lets it drain slowly into the surrounding ground instead of running into a sewer.
A modern crate soakaway for a typical house costs £1,000 to £2,500 installed in 2026. A simple rubble soakaway for a small area can be done for £100 to £400 in materials if you dig it yourself. The number moves on the size of the area you are draining, how deep you have to go, and — more than anything else — how well your ground actually soaks water away.
That last point is the whole game. In free-draining sand or gravel a soakaway works beautifully. In the clay soil that covers much of Norfolk, a badly designed soakaway fills up and stays full, and the water comes straight back to the surface. The rest of this guide is about getting it right the first time.
What Is a Soakaway?
A soakaway is a hole in the ground, filled with a material that holds open space, that stores rainwater temporarily and releases it into the surrounding soil. Rain hits your roof, runs down the gutter and downpipe, travels along an underground pipe, and empties into the soakaway. The water sits in the void space and then percolates — soaks — out through the sides and base into the earth.
It exists to solve one problem: what to do with surface water. Every roof and every hard-paved surface sheds a surprising volume of water in a downpour. That water has to go somewhere. Your options are a surface water sewer, a watercourse, or the ground itself. A soakaway is how you use the ground, and under Building Regulations it is the option you are expected to try first.
There are two things a soakaway is not. It is not a way to get rid of foul waste — that is a septic tank or a connection to the foul sewer, and putting foul water into a soakaway is illegal. And it is not a soakaway if it just moves water a few feet and lets it pool; a soakaway only works if the ground around it can actually accept the water.
How Does a Soakaway Work?
The mechanism is storage plus percolation.
Storage handles the peak. Rain does not arrive at a steady trickle — it arrives in bursts. The soakaway has to be big enough to hold the water from a heavy storm all at once, because the ground cannot swallow it that fast.
Percolation handles the emptying. Between storms, the stored water seeps out through the base and walls into the soil, so the soakaway is empty and ready for the next downpour.
If storage is too small, the soakaway overflows in heavy rain and floods the surface. If percolation is too slow — which is exactly what happens in clay — the soakaway never empties, so even a small shower overwhelms it. A working soakaway needs both: enough room to hold the storm, and ground that drains fast enough to clear it before the next one.
This is why you cannot size a soakaway by guessing. You size it against your soil's actual drainage rate, measured with a percolation test.
Types of Soakaway
There are three common builds, from oldest to best.
Rubble or hardcore soakaway. A pit filled with broken brick, stone or coarse gravel, wrapped in geotextile membrane to stop soil clogging the gaps. Cheap and simple, but the rubble itself fills most of the hole — you only get around 30% void space for water, so you need a much bigger hole for the same storage. It also silts up over the years.
Soakaway crates (the modern standard). Modular plastic cells — often called attenuation or soakaway crates — that stack together into a block and are wrapped in a permeable geotextile membrane. Crates are roughly 95% void, so they store more than three times the water of the same volume of rubble. They are quicker to install, last longer, and are what a groundworker will fit today unless there is a reason not to.
Soakaway chamber. A pre-formed perforated chamber, used on smaller domestic jobs or where access is tight.
For almost every new soakaway, crates are the right answer. The only real question is how many.
How Deep Should a Soakaway Be?
A domestic soakaway is typically between 1 and 2 metres deep. The exact figure depends on three things:
- You have to get below the topsoil. Topsoil holds water and drains poorly. The soakaway needs to sit in the permeable subsoil beneath it, which usually means starting at least 300–500mm down.
- You have to stay above the water table. The base of the soakaway should be at least 1 metre above the highest seasonal groundwater level. If the ground is already saturated, water cannot soak away into it. In low-lying parts of Norfolk with a high water table, this is a genuine constraint.
- Deeper ground often drains better. In many sites the subsoil a metre or two down is more permeable than the surface, so going deeper improves performance — up to the point where you meet the water table.
Depth alone does not fix a slow-draining site, though. A deep soakaway in solid clay is still a deep soakaway in solid clay.
How Many Soakaway Crates Do I Need?
This is the question people get wrong, and it comes down to a percolation test and a bit of arithmetic.
Step one: run a percolation test. Dig a test pit at the proposed soakaway location, fill it with water, let it drain once to wet the sides, then fill it again and time how long the water takes to fall a set amount. The standard method is set out in BRE Digest 365, the recognised UK guidance for soakaway design. The result gives you the soil infiltration rate — how fast your ground actually accepts water.
Step two: work out the storage volume. You need enough crate volume to hold the design storm for the area you are draining, given how quickly your soil empties. As a rough domestic rule of thumb, a soakaway is often sized at around 1 cubic metre of storage for every 50 square metres of roof or hard surface on free-draining ground — but on slow ground you need considerably more, and the percolation test is what tells you.
Step three: convert to crates. A standard soakaway crate holds a known volume (commonly a little under 200 litres each, at ~95% void). Divide your required storage by the crate volume to get the count. A modest house roof on decent ground might need six to ten crates; a large roof plus a driveway on clay could need several times that.
If you take one thing from this section: do the percolation test. Sizing a soakaway without one is guessing, and on clay the guess is almost always too small.
Soakaways and Building Regulations
Surface water drainage is covered by Part H3 of the Building Regulations, and it sets a clear order of preference: use a soakaway or other infiltration first; only if that is not reasonably practicable do you go to a watercourse, and only after that to a sewer. So a soakaway is not just allowed — it is the expected first choice.
The practical rules that catch people out:
- Distance from buildings. A soakaway should be at least 5 metres from any building or structure, so the water you are discharging does not undermine foundations. This is the single most common mistake — a soakaway dug too close to the house, softening the ground under the very foundations it should protect.
- Distance from boundaries. Keep it clear of the boundary — commonly a minimum of 2.5 metres — so you are not discharging onto a neighbour's land.
- Not for foul water. Soakaways take clean surface water only. Foul drainage is a separate system.
- Septic tank soakaways are different. A soakaway serving a septic tank (a drainage field) is governed by separate rules and needs the treated effluent and correct sizing under the general binding rules — do not confuse the two.
On a new build or an extension, the soakaway design usually has to be signed off, and the percolation test is part of that evidence.
Soakaways in Clay Soil: Why Norfolk Is Different
Much of Norfolk sits on clay-rich ground, and clay is the enemy of the soakaway. Clay has tiny pore spaces and holds water rather than releasing it, so its infiltration rate is very low. Drop a standard soakaway into heavy clay and it behaves like a bucket — it fills, it stays full, and the water resurfaces.
This is the same clay that drives Norfolk's seasonal ground movement and, in dry-then-wet cycles, contributes to local subsidence. Water management in this ground is not a detail; it is central.
If your ground is clay, you have a few honest options:
- Go bigger and deeper, reaching down to any more permeable layer beneath the clay, sized against a proper percolation test rather than a rule of thumb.
- Use attenuation instead of pure infiltration — store the water in crates and release it slowly to a watercourse or, with permission, the surface water sewer.
- Reconsider the location — permeability can vary across a single plot, and a test pit a few metres away sometimes drains far better.
What you should not do is install a textbook soakaway in clay and hope. It will fail, and the failure shows up as a permanently boggy patch or water coming back up around the crates. This is precisely the sort of judgement call where local ground knowledge earns its keep.
How to Build a Soakaway
The principle is simple; doing it properly is where the value is. In outline:
- Test the ground. Percolation test first, so you know the soakaway will work and how big it needs to be.
- Locate it correctly. At least 5m from the building, clear of the boundary, downhill of the area you are draining, and away from trees whose roots will find it.
- Excavate. Dig the pit to the calculated size and depth, getting below topsoil into subsoil and staying above the water table.
- Line with geotextile membrane. This lets water through but keeps fine soil out, which is what stops the soakaway silting up and failing early.
- Install the crates (or fill with clean hardcore). Assemble and stack the crates, then wrap the membrane fully around the block.
- Connect the inlet pipe from the downpipe or channel drain, ideally through a silt trap or catchpit so grit is caught before it reaches the crates.
- Backfill and reinstate the ground above.
The membrane and the silt trap are the two things DIY jobs most often skip, and they are the two things that decide whether the soakaway is still working in ten years. If the soakaway sits under a driveway or anywhere a vehicle will cross, the crates must be load-rated for it — standard garden crates will collapse under a car.
How Much Does a Soakaway Cost?
Guide prices for 2026:
| Job | Typical cost |
|---|---|
| DIY rubble soakaway (materials, small area) | £100 - £400 |
| Crate soakaway, single downpipe, good ground | £1,000 - £1,800 |
| Crate soakaway, whole roof or larger property | £1,800 - £2,500 |
| Large or deep soakaway, clay ground, driveway load | £2,500 - £4,000+ |
| Percolation test on its own | £150 - £400 |
| Unblocking or rebuilding a failed soakaway | £200 - £900 |
What moves the price: the volume of storage you need (which the percolation test decides), how deep you have to dig, access for a mini-digger, whether spoil has to be carted away, and whether the crates need to be load-rated. Clay ground pushes every one of these upward, because you need more storage and often a deeper dig.
A "cheap" soakaway that is undersized for the ground is not cheap — it fails, and you pay again to put it right.
Blocked Soakaway: Signs and Fixes
A soakaway does not last forever, and a blocked or failed one announces itself:
- Standing water or a permanently boggy patch above or near the soakaway
- Water backing up in gullies, downpipes or a lower drain
- A soakaway that used to cope and no longer does, especially after years of silt build-up
The usual causes are silt clogging the media or membrane, roots invading the structure, the soakaway being undersized from the start, or the ground around it becoming saturated. Fixes range from jetting and clearing a silt trap, through to excavating and rebuilding the soakaway properly — often the honest answer when the original was undersized or built without a membrane. Unblocking runs from a couple of hundred pounds for a clear-out to several hundred for a rebuild.
Do You Even Have a Soakaway?
On many older properties nobody is quite sure. If your downpipes disappear into the ground and there is no obvious connection to a sewer, you probably have a soakaway somewhere. A boggy patch of lawn in one spot after rain often marks it. If you are planning an extension, a driveway, or you have unexplained damp or standing water, it is worth locating and checking it — a CCTV drainage survey can trace where your surface water actually goes.
FAQ: Soakaways
What is a soakaway?
A soakaway is an underground pit or crate structure that collects surface water — usually rainwater from a roof or driveway — and lets it drain slowly into the surrounding ground rather than into a sewer. It stores the water during a storm and releases it into the soil by percolation between storms. It handles clean surface water only, never foul waste.
How does a soakaway work?
It works by storage plus percolation. The soakaway stores rainwater in a void space (crates or rubble) during a downpour, because the ground cannot accept water as fast as a storm delivers it. Between storms, the stored water seeps out through the base and sides into the surrounding soil, emptying the soakaway ready for the next rain. It only works if the ground drains fast enough to clear the water in time.
How deep should a soakaway be?
A domestic soakaway is typically 1 to 2 metres deep. It needs to sit below the topsoil in permeable subsoil, and its base should stay at least 1 metre above the highest seasonal water table. Deeper subsoil often drains better, but depth alone will not fix slow-draining clay ground.
How many soakaway crates do I need?
It depends on the area you are draining and how fast your soil accepts water, which you find with a percolation test to BRE Digest 365. As a rough guide on free-draining ground, allow around 1 cubic metre of storage per 50 square metres of roof or hard surface, then divide by the crate volume (commonly just under 200 litres each). Slow or clay ground needs considerably more, so always base the count on a percolation test rather than a rule of thumb.
How far should a soakaway be from a house?
A soakaway should be at least 5 metres from any building or structure, so the discharged water does not undermine foundations. It should also be kept clear of boundaries, commonly a minimum of 2.5 metres, so water is not discharged onto neighbouring land. Placing a soakaway too close to the house is the most common and most damaging mistake.
Do soakaways work in clay soil?
Often poorly, without special design. Clay has a very low infiltration rate, so a standard soakaway in heavy clay fills and stays full, and the water resurfaces. In clay ground — common across Norfolk — you typically need a larger, deeper soakaway reaching any more permeable layer, or an attenuation approach that stores water and releases it slowly to a watercourse or sewer with permission. A percolation test is essential before building.
How much does a soakaway cost?
A crate soakaway for a typical house costs around £1,000 to £2,500 installed in 2026. A simple DIY rubble soakaway for a small area can be £100 to £400 in materials. Larger, deeper soakaways, clay ground, or load-rated crates under a driveway push the figure to £2,500 to £4,000 or more. A standalone percolation test is usually £150 to £400.
Do I need Building Regulations approval for a soakaway?
Surface water drainage is covered by Part H3 of the Building Regulations, which requires you to use a soakaway or other infiltration first where reasonably practicable. On a new build or extension the soakaway design, including the percolation test, usually needs to be evidenced and signed off. A soakaway must be at least 5 metres from buildings and take clean surface water only.
Why is my soakaway not working?
The usual causes are that it is undersized for the ground, silt has clogged the media or membrane, roots have invaded it, or the surrounding ground is saturated, often because it is clay with a low drainage rate. Signs include standing water or a boggy patch above it, and water backing up in gullies or downpipes. The fix ranges from jetting and clearing a silt trap to excavating and rebuilding a properly sized soakaway with a geotextile membrane.
Can a soakaway go under a driveway?
Yes, but only with load-rated crates designed to take vehicle weight — standard garden soakaway crates will collapse under a car. It also still needs to meet the usual rules: sized to a percolation test, at least 5 metres from the house, and wrapped in geotextile membrane with a silt trap on the inlet.
What is the difference between a soakaway and a septic tank soakaway?
A standard soakaway disposes of clean surface water (rain from roofs and paving). A septic tank soakaway, properly called a drainage field, disposes of treated effluent from a septic tank and is governed by separate rules and sizing under the general binding rules. The two must never be confused or connected — surface water and foul drainage are entirely separate systems.
The Bottom Line
A soakaway is a storage-and-percolation structure that lets surface water drain into the ground instead of a sewer, and under Building Regulations it is the first option you are expected to use. Modern crate soakaways store far more than old rubble pits, sit 1 to 2 metres deep, and must be at least 5 metres from the house.
Everything hinges on the ground. A soakaway is only as good as the soil's ability to accept water, which is why a percolation test — not a rule of thumb — should decide its size. In the clay that covers much of Norfolk, this matters enormously: an undersized soakaway in clay does not work, it just floods, and clay is also the ground that drives the region's seasonal movement and subsidence.
Get the test done, size it honestly, keep it 5 metres from the building, wrap it in membrane, and put a silt trap on the inlet. Do that and a soakaway will handle your surface water quietly for decades. Skip it and you will be digging it up again — which is the more expensive way to end up in the same place.
Surface water problem or planning a soakaway in Norfolk? KiKK brings groundworks and clay ground-condition expertise to drainage, soakaways and surface water across Norwich and Norfolk. Contact KiKK →
This article provides general guidance and is not a substitute for site-specific design. Soakaways serving new builds or extensions should be designed to BRE Digest 365 and comply with Part H of the Building Regulations. KiKK Groundworks provides groundworks, foundations, drainage, driveways, and civil engineering across Norwich and Norfolk. NRSWA accredited, fully insured, owner-led.