Soakaway for a Garden Room: Size, Siting and Build

A garden room roof sheds a lot of rain, and that water has to go somewhere. A soakaway is the simplest answer: an underground pit that takes the water from your gutters and lets it drain slowly into the soil. It works well for a garden room as long as your soil drains properly, the pit is big enough and it sits far enough from buildings and boundaries. This guide walks you through each step.

TL;DR

• A soakaway takes roof water from your gutters and lets it seep into the ground. It’s for rainwater only, never anything from a toilet or sink.
• Do a simple soil test first. If water is still standing after two hours, your ground probably won’t cope.
• Keep the pit at least 5m from any building and 2.5m from any boundary, with the water table well below its base.
• Size it from your roof area. The council method is roof area x 0.040 for the volume in cubic metres.
• Clear your gutters often and expect to renew a rubble-filled soakaway roughly every ten years.

Table of Contents

• What a soakaway does for a garden room
• Do you need permission?
• Step 1: Test your soil first
• Step 2: Pick the spot
• Step 3: Work out the size
• Step 4: Choose what goes in the hole
• Keep it working
• Our view on drainage and garden rooms
• Plan the cabin and drainage together
• FAQ
• Sources

What a soakaway does for a garden room

A soakaway is a hole in the ground, filled with something that leaves gaps for water (rubble, or plastic crates), and wrapped so soil can’t wash in. Rain runs off the cabin roof, down the gutter and downpipe, into the pit. From there it soaks out into the surrounding soil over hours rather than minutes.

Why bother? Without a plan, all that water drops beside your cabin walls. It pools, turns the ground soft and can splash up the cladding. Over a wet winter that’s no good for timber or the base it sits on.

The rules put soakaways first in the queue. Part H3 of the Building Regulations says rainwater from a roof should go to a soakaway or other infiltration system if that’s reasonably practical. If it isn’t, it goes to a watercourse, and failing that to a sewer. You can read the order of priority in the regulations themselves. So a soakaway isn’t a fancy extra. It’s the default choice for most gardens.

One point to keep clear: a soakaway is for clean roof water. Waste water from a garden room toilet or kitchenette is a different job with different rules. If you’re planning plumbing, our guide on planning a garden room toilet with your supplier is worth a read before you dig anything.

Do you need permission?

For the drainage itself, the rules are the Building Regulations, not a planning application. Part H asks for adequate provision to carry rainwater away from roofs, and the soakaway is the first-choice way to do it. The exact rules for your cabin, such as its size and how close it sits to your boundary, are a separate question. We cover that in our guide to garden room boundary distances.

If you’re unsure how your local council treats a small soakaway, a quick call to building control is free and takes five minutes. They’ve seen it all before.

Scotland has its own building standards rather than Approved Document H, so if you’re north of the border, check locally.

Step 1: Test your soil first

This is the step people skip, and it’s the one that matters most. A soakaway only works if the ground can take the water. Heavy clay won’t, and a pit dug in clay can end up as a very expensive puddle.

A basic test, as Homebuilding describes it, goes like this:

1. Dig a square hole about 300mm x 300mm and 300mm deep, where you’d like the soakaway.
2. Fill it with water and let it drain away once, so the soil is wet.
3. Fill it again and time how long it takes to empty.
4. If the soil is clay or the water is still standing after two hours, the ground is likely to be a poor match.

This is a quick check, not a full design test. Stratford-on-Avon District Council also lists the same simple 300mm pit method. For a larger roof or a tricky site, building control may want a proper trial pit, and the full method is set out in BRE Digest 365.

Safety matters here. The BRE guidance says a trial pit needs a health and safety risk assessment and that nobody should climb into one, according to the BRE Digest 365 guide. Deeper holes can collapse without warning. If your test needs a hole deeper than knee height, get a groundworker in.

What if the soil fails?

Don’t panic. You still have choices:

• Send the water to a water butt first, then overflow to a drain.
• Connect to an existing surface water drain, if the house has one and you have permission.
• Ask a drainage firm about a larger or deeper soakaway, or a different layout.

The Part H order of priority means a soakaway isn’t the only legal outcome. It’s just the first one to try.

Step 2: Pick the spot

Two sets of distances decide where the pit can go.

The first two come from Eden District Council’s soakaway guidance, which also says the pit must not sit where the water table reaches its base at any time of year. The 1m gap is set out in Hastings Borough Council’s advice note.

Remember that your garden room counts as a building. That means the soakaway needs to be 5m from the cabin too, not just from the house. A neighbour’s garage or extension counts as well. Treat the numbers as a minimum and give yourself extra if the garden allows.

The reason is simple. Water seeping out of a pit too close to a foundation can soften the ground under it. Our guide to garden building foundations explains why a firm, dry base matters so much for a timber cabin.

Working out the water table

You don’t need a survey for most gardens. Dig your test hole after a wet spell, then check it. If water seeps in at the bottom of the hole, the water table is high, and a soakaway there won’t work. Low spots, spots next to ponds and gardens at the bottom of a slope are the usual trouble.

Also look out for:

• Pipes and cables running through the garden.
• Tree roots, which can block a pit over time.
• Slopes that would send the overflow towards a neighbour.

Step 3: Work out the size

The maths is friendlier than it sounds. Horsham District Council’s guidance gives the basic sum. Work out the design area, which depends on your roof type and pitch, then assume a maximum of 40mm of rain in a day. The volume you need to store is the design area x 0.040, in cubic metres.

A worked example

Say your cabin has a roof of about 12 square metres, like a 4m x 3m cabin with a small overhang. Using the council method:

12 x 0.040 = 0.48 cubic metres

That’s the water the pit needs to hold on a very wet day. Remember that rubble or crates don’t fill the whole hole. Rubble leaves roughly a third of the space as gaps, so the hole has to be bigger than the water volume. Crate systems have a stated void ratio, usually much higher, so check the maker’s figures when you buy.

For a bigger cabin, or a cabin with a long roof run, the number goes up in line with the area. A multi-room layout has more roof to drain than a single room. If you’re planning one, take a look at our multi-room log cabins and work out the roof area before you plan the pit.

An apex roof gives you two slopes, but the design area is based on the roof as the council guidance sets out, not on the ridge height. Our apex roof cabins page shows the common shapes if you’re still choosing.

How deep?

There isn’t one magic depth. It depends on the volume you need, your soil and where the water table sits. BRE Digest 365 uses trial pits 1m to 1.5m below the drain level for smaller roof areas, and deeper ones for larger areas. For a small garden room, most pits end up shallow and wide rather than deep, which also keeps them well above the water table. Your building control officer or drainage contractor can confirm the final depth.

Step 4: Choose what goes in the hole

There are a few standard ways to build one. Horsham’s guidance lists hardcore-filled pits, perforated concrete rings, honeycombed brickwork and storage cells.

For most garden rooms, crates are the tidy choice. Homebuilding notes that a membrane around the crates keeps soil out while letting water through, and that a silt trap helps stop leaves and debris blocking the system.

Costs are generally low to medium compared with other drainage, and larger soakaways cost more because of the extra labour and materials, as the BRE guidance puts it. Ask for two or three quotes if you’re hiring someone, and make sure each one says how big the pit will be.

The build in short

1. Mark out the position with pegs and string, keeping to the distances above.
2. Dig the hole to the size you’ve worked out, with a stable, safe edge.
3. Line the sides with a geotextile membrane.
4. Add a layer of clean gravel in the base, then your crates or rubble.
5. Fit a silt trap on the pipe coming in, so leaves and grit don’t reach the pit.
6. Run the pipe from the downpipe with a gentle fall towards the pit.
7. Wrap the top of the membrane over, backfill and cover with soil or turf.

If any of that feels like more than you fancy, a local groundworker can do it in a day for most small cabins.

Keep it working

A soakaway isn’t a fit-and-forget job, but it’s close. A few habits go a long way.

• Clear the gutters. Leaves and moss are what block a soakaway first. Keeping gutters clean and clear stops debris reaching the pipes in the first place, and it’s a quick job twice a year, usually in autumn and spring.
• Empty the silt trap. The BRE guidance lists removing sediment and debris from pretreatment devices, cleaning gutters or downpipe filters, and removing roots that cause blockages as the main upkeep tasks, as set out in the BRE Digest 365 guide.
• Watch for puddles. If water starts to sit above the pit after rain, or the downpipe backs up, it’s slowing down.
• Keep trees away. Roots find their way into pits and pipes.

How long does one last? It varies with soil and care. Eden District Council says a domestic rubble-filled soakaway may need renewing about every ten years.

One last tip: draw the position of your soakaway on a plan and keep it with your house papers. The BRE guidance points out that finding them later is a common problem, and a simple sketch saves a lot of digging.

Our view on drainage and garden rooms

We talk to customers about garden rooms every day, and drainage is the thing they most often leave until last. That’s understandable. You’ve picked the size, the doors, the windows and the finish, and the pit in the ground doesn’t feel like the fun part.

But it’s worth thinking about early, because it affects where the cabin sits. If you decide on the position first, then find out the only spot for a soakaway is 3m away, you’re stuck. Plan the cabin, the base and the drainage together, and everything fits.

This is where a bespoke build helps. With a made-to-measure cabin you can choose the roof shape, the overhang and where the gutters and downpipes fall. That means you can put the downpipe on the side nearest your soakaway, or split the roof water between two points if the garden is awkward. A standard size doesn’t give you that choice. Good timber needs a dry base and dry walls, and sorting out rainwater is part of looking after your investment.

Our honest advice: test the soil before you order, not after. It costs nothing and tells you which way to go.

— Martin

Plan the cabin and drainage together

If you’re still choosing your garden room, we can help with the layout as well as the look. Our bespoke log cabin service lets you choose the size, roof, doors and windows to suit your garden, including where the gutters and downpipes sit. You can also browse our standard garden log cabins and ask us to adjust one to fit. Pop us a message with your garden measurements and we’ll be pleased to help you work out what’s possible. We deliver across the UK mainland.

A cabin that’s well planned from the start stays drier and lasts longer. Our piece on weatherproof garden rooms explains more about how that works.

FAQ

How deep should my soakaway be?

It depends on how much water you need to store, your soil and the water table. For a small garden room it’s usually a shallow, wide pit rather than a deep one. The base must stay at least 1m above the water table at all times of the year, so check that before you dig.

Can I put a soakaway close to my garden room?

No. Council guidance says a soakaway should be at least 5m from any building and 2.5m from any boundary. Your garden room counts as a building, so the pit needs to sit that far away from it too.

How long does a garden soakaway last?

It varies with your soil, the size of the pit and how well you look after it. A rubble-filled domestic soakaway may need renewing about every ten years. Clearing the gutters and emptying the silt trap helps it last longer.

What if my soil is clay?

Clay drains very slowly, so a standard soakaway may not work. If water is still standing in your test hole after two hours, talk to a drainage contractor or your building control team. Other options include a water butt with an overflow to a drain.

Sources

• The Building Regulations 2010 schedule, Part H drainage requirements
• What is a soakaway? What you need to know, Homebuilding
• Percolation Testing, Stratford-on-Avon District Council
• Rainwater soakaway design guidance, Eden District Council
• Soakaway advice note, Hastings Borough Council
• Technical guidance for soakaway design, Horsham District Council
• Soakaway design, BRE Digest 365

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