blogTL;DR: A rainwater soakaway in the UK is normally built 1 to 2 metres deep, measured below the incoming pipe, with 1.2 to 1.5 metres common for a typical home. The correct depth depends on your soil, the height of the water table, and how much water drains into it. This guide covers the depth rules, why Cornish ground makes them harder to hit, and when to bring in a professional.
Dig a soakaway too shallow and it floods your garden in the first heavy storm. Dig it too deep and it sits in groundwater, doing nothing. So how deep should a soakaway be? The honest answer is that depth is not a fixed number you can copy from a neighbour’s garden. It is a calculation based on your soil, your ground, and the volume of water you need to move.
Get it right and a soakaway quietly drains your roof and driveway for decades. Get it wrong and you are left with standing water, damp near the foundations, or a system that fails its building control inspection.
This guide walks through the standard depth range, the factors that set the correct figure, how Cornwall’s clay and granite change the sums, and the separate rules that apply to foul drainage.
How Deep Should a Soakaway Be?
A rainwater soakaway is usually built between 1 and 2 metres deep, measured below the invert (the base of the incoming pipe). Most domestic soakaways sit around 1.2 to 1.5 metres deep. The exact figure depends on how quickly your soil absorbs water, how high the water table sits, and the roof or hard surface area feeding into it.
Depth on its own does not size a soakaway. What matters is the storage volume: the empty space that holds water while it soaks into the surrounding soil. You reach that volume through a combination of depth, width, and the void ratio of what fills the pit. Modern plastic crates hold far more water per cubic metre than rubble, so a crate soakaway can be shallower and narrower for the same capacity.
As a rule of thumb, a home needs roughly 1 cubic metre of storage for every 30 to 50 square metres of roof draining into it. Councils will usually accept soakaways down to about 2.5 metres deep for a house, and deeper pits are discouraged because of the risk to nearby foundations if they ever need excavating. Below that band, you are into engineered territory that needs a proper design.
What Decides the Right Depth for Your Soakaway?
Four things set the right depth: how fast your soil absorbs water, how high the water table sits, the area of roof or paving draining into it, and how far you can dig before hitting rock. A percolation test measures the soil’s drainage rate and confirms whether a soakaway will work at all. Skip that test and you are guessing.
Soil Type and Drainage Rate
A percolation test is a simple on-site check that times how fast water drains out of a pit dug at the proposed soakaway location. The standard method is BRE Digest 365, referenced in the Building Regulations. If water drains too slowly, a soakaway will overflow and the design has to change.
Free-draining sandy or gravel soils take water quickly, so a compact pit works well. Clay holds water and often fails the test outright. Where percolation is slow, you may need a wider, shallower design or a completely different drainage route.
The Height of the Water Table
The water table is the level below ground where the soil is fully saturated with water. A soakaway has to sit above it all year round, including in a wet Cornish winter. If the base of your pit dips into groundwater, there is nowhere for the rainwater to go.
This single factor caps how deep you can safely dig on many sites. A high water table often forces a shallower, wider soakaway instead of a deep one.
The Area Draining Into It
More roof and paving means more water, which means more storage. A large detached house with extensive hard standing needs far more capacity than a small terraced cottage. Once you know the catchment area and the soil’s drainage rate, the volume calculation tells you the depth and width you need.
Soakaway Depth in Cornwall’s Ground Conditions
Cornwall’s ground rarely gives you a free hand with depth. Clay subsoils, shallow granite and shillet bedrock, and high coastal water tables all limit how far down you can dig and how well the soil will drain. Local ground conditions are the single biggest reason a textbook depth figure often does not apply here.
Three patterns come up again and again on Cornish sites:
- Heavy clay: Large parts of the county sit on clay subsoil, which drains slowly and frequently fails a percolation test. A deep pit in clay simply fills and stays full.
- Shallow rock: Granite and shillet (the local weathered slate and shale) can sit close to the surface, especially inland and on higher ground. You cannot dig a 1.5 metre pit if you hit solid rock at 800 millimetres.
- High water tables: Coastal plots, valley bottoms, and low-lying ground near estuaries often have groundwater close to the surface, which caps usable depth and rules out deep soakaways.
Add Cornwall’s high annual rainfall and the sizing has to cope with real downpours, not average figures. This is why we always test the ground before committing to a depth. When a soakaway is not viable, we look at alternatives such as garden drainage systems, connection to a surface water sewer, or a controlled outfall to a watercourse. A percolation test costs far less than digging a pit that never works.
Do Foul Soakaways Follow Different Depth Rules?
Yes, and the difference is significant. Since January 2015, a traditional soakaway can no longer be used for foul water from a septic tank. Sewage effluent must discharge to a drainage field, where perforated pipes sit shallow, at around 700 to 800 millimetres deep, so bacteria in the upper soil can treat the effluent before it reaches groundwater.
A drainage field is a network of perforated pipes laid in shallow gravel-filled trenches that lets treated effluent trickle out and soak into the soil. It is deliberately shallow. A deep pit would bury the effluent below the active soil layer where the treatment happens. The Environment Agency general binding rules confirm a rainwater soakaway cannot be used for sewage, and drainage fields must be designed to British Standard BS 6297.
The depth logic is the reverse of a rainwater soakaway:
- A rainwater soakaway goes deeper to gain storage volume for surface water.
- A foul drainage field stays shallow so the soil can treat the effluent.
Before any drainage field goes in, a deep trial hole is dug to check that the water table or bedrock never rises to within 1.5 metres of the base of the pipes. If it does, the site fails and a different solution is needed. If you are dealing with a septic system rather than rainwater, our Septic Tank Maintenance service covers the testing and compliance side properly.
How Far Should a Soakaway Be From Your House?
A soakaway should sit at least 5 metres from any building or road and at least 2.5 metres from a boundary, under Approved Document H of the Building Regulations. These distances stop infiltrating water from softening the ground beneath foundations. On tight Cornish plots, available space often decides where, and how deep, the soakaway can go.
The 5 metre figure is a well-established guideline rather than an absolute limit. Where space is short, a shorter distance can be acceptable if geotechnical data shows it is safe. A shallow, wide soakaway or a run of linked shallow pits has a smaller zone of influence on foundations than one deep pit, which is often the answer on a small garden.
Depth and distance work together. A deeper pit close to a wall carries more risk than a shallow one at the same distance. Getting both right takes a look at the actual site, not a formula applied blind.
What Happens When the Depth Is Wrong
A soakaway at the wrong depth fails in predictable ways. Too shallow and it lacks the storage volume to hold a storm, so water backs up and pools on the surface. Too deep and it drops into the water table or hits rock, leaving the water with nowhere to drain. Either way, you end up back where you started, often after paying to dig the thing twice.
The common failures we are called out to fix include:
- Garden flooding during heavy rain because the soakaway is undersized or too shallow.
- Persistent standing water because the base sits in groundwater or impermeable clay.
- Silting, where sediment washes in and clogs the surrounding soil over time.
- Damp problems near foundations from a soakaway dug too close to the house.
A silted soakaway can sometimes be cleared with high-pressure jetting rather than replaced, which is worth checking before you commit to a rebuild. Where a collapsed or displaced pipe is feeding the problem, a CCTV Drain Survey pinpoints the fault, and options such as drain relining can repair it without digging up the garden. We handle these issues and blocked drains across Cornwall every day.
The right soakaway depth is the one your ground can actually support. For most Cornish homes that means a rainwater pit between 1 and 2 metres deep, sized to the roof area, tested against the soil, and kept above the water table. Foul drainage follows the opposite logic and stays shallow.
Two things settle it every time: a proper percolation test, and honest local knowledge of what the ground is likely to do. Guesswork is what fills gardens with standing water.
If you are planning a soakaway, or a recent one is not draining, call our team on 01872 222555 for a fixed-price quote with no call-out fee. We are on site anywhere in Cornwall within 1 to 2 hours, 24 hours a day, and every repair carries our 5-Year Guarantee. Call our emergency team right now if water is already pooling.
Frequently Asked Questions
What Is the Minimum Depth for a Soakaway?
There is no single legal minimum, but the invert of the incoming pipe should generally sit at least 600 millimetres below the surface, and most rainwater soakaways are at least 1 metre deep to hold enough storage. The true minimum is whatever depth gives you the storage volume the calculation requires for your roof area and soil type.
Can a Soakaway Be Too Deep?
Yes. A soakaway dug too deep can drop into the water table, where saturated ground stops it draining, or reach less permeable soil layers. Very deep pits also carry a higher risk to nearby foundations, which is why councils generally accept domestic soakaways only down to about 2.5 metres.
How Deep Should a Soakaway Be in Clay Soil?
Depth is not the fix for clay. Clay drains slowly and often fails a percolation test regardless of how deep you dig, so a deeper pit in clay simply fills and stays full. If a percolation test fails, you need an alternative such as a surface water connection or a watercourse outfall rather than a bigger hole.
How Deep Is a Soakaway for a Driveway?
A driveway soakaway follows the same rules as a roof soakaway, typically 1 to 2 metres deep, sized to the paved area draining into it. Because paving sheds almost all the rain that lands on it, driveway catchments often need more storage than you would expect, so the sizing calculation matters.
Do I Need a Percolation Test Before Installing a Soakaway?
Yes, for any new soakaway tied to building work, and it is strongly advised even for standalone projects. Building control will ask for the results as part of drainage approval. The test confirms the ground drains fast enough and sets the size and depth, so a reputable installer will always carry one out first.
Can I Dig a Soakaway Myself?
You can, but the design has to be right or it will fail. Percolation testing, storage sizing, the 5 metre distance rule, and the water table check all need to line up. On Cornish ground with clay, shallow rock, or a high water table, a professional assessment usually saves money by getting the depth right the first time.
