Soakaway design for a householder extension

Rainwater from a new extension roof has to go somewhere, and the soakaway extension building regulations question is answered by Part H of the Building Regulations rather than by planning. Approved Document H sets a hierarchy: get the water into the ground first, use a watercourse second, and only use a sewer if neither of the first two works. It also sets the separation everyone remembers, which is that a soakaway should sit at least 5 m from building foundations.

That single figure causes more redesigns than anything else on a small plot. Measure it before the drawings are finished.

When a new soakaway is needed

Any new roof area needs its own disposal route, and an extension usually means either a new soakaway or a proven connection into an existing system with spare capacity. Connecting a new downpipe into an existing gully that already floods in heavy rain is not a solution, and building control will not accept it on the basis that it has always drained that way.

You need a new soakaway where there is no surface water sewer within reach, where the existing drainage is combined and the water company will not accept the extra flow, or where the existing soakaway is already at capacity.

You do not need one where the roof discharges to a permeable surface that can take it, or where the water company confirms a connection to a separate surface water sewer. Under section 106 of the Water Industry Act 1991 there is a right to communicate with a public sewer, but the undertaker can refuse a surface water connection where the sewer is combined or overloaded, and that answer comes in writing.

Planning rarely controls any of this for a householder extension. What planning can do is attach a condition requiring drainage details to be approved, and on a site in Flood Zone 2 or 3 the surface water question feeds into the Flood Risk Assessment instead.

Separation from buildings and boundaries

Approved Document H gives 5 m from building foundations. That distance applies to your own house and to the neighbour's, and to the extension itself, so on a narrow urban plot the only compliant position is often the far end of the garden.

Boundaries are handled differently. There is no single national figure, but the water leaving a soakaway spreads sideways as well as down, and a soakaway tight against a boundary saturates ground under next door's wall or shed. Agree the offset with building control rather than guessing, and keep the unit clear of drains, service trenches and tree root protection areas.

Three other constraints bite on small plots:

• The base of the soakaway should sit above the seasonal high water table, with a margin. A pit that stands full of groundwater in winter has no storage.
• Sloping sites concentrate flow. A soakaway at the bottom of a garden that already receives overland run-off from uphill will be doing two jobs.
• Where the ground is contaminated or the site sits in a groundwater source protection zone, infiltration may not be acceptable at all.

Ground that stays wet for weeks after rain is telling you something. Our note on evidencing groundwater flooding covers how that gets recorded properly.

Sizing from roof area and the infiltration rate

Sizing is a calculation, not a rule of thumb, and it needs two inputs: the impermeable area draining to the unit, and the soil infiltration rate measured on your own plot.

Approved Document H points to BRE Digest 365 for soakaway design, and requires design for a storm with a return period of once in ten years. The BRE method works like this:

1. Measure the plan area of the new roof, plus any paving draining to the same point.
2. Establish the infiltration rate from percolation tests in a trial pit at the proposed depth.
3. Choose a trial soakaway size and calculate the volume of water arriving during a storm of a given duration.
4. Subtract the volume that soaks away through the sides of the unit during that same storm.
5. Repeat for a range of storm durations, from short intense events to longer ones, and take the duration that demands the most storage.

Step five is the one people skip. A short cloudburst and a long steady soaking produce different answers, and the worst case is not always obvious. Where a crate or ring system is used, remember that the storage volume is the void volume, not the excavated volume, so granular fill at around a third voids means a much bigger hole than the calculation first suggests.

Testing on a small plot without heavy plant

You do not need a rig to do this properly. A trial pit of roughly 0.3 m by 0.3 m in plan, dug to the depth the soakaway will sit at, is what the BRE method uses.

Fill it with water to at least three quarters of its effective depth, then record the time the level takes to fall from 75 per cent full to 25 per cent full. Do it three times in succession, because the first test wets up the soil and the later results are the ones that matter. Use the slowest of the three.

Two practical points. Test in the position the soakaway will actually occupy, because garden soils vary over a few metres, and photograph the pit face with a tape in shot so building control can see the strata. Made ground and clay look very different from sand in a photograph, and that photograph saves an argument.

If the pit holds water overnight without dropping, stop. The ground will not infiltrate and you need a different strategy.

Alternatives where the ground will not take water

Clay sites, high water tables and shallow rock all defeat infiltration. The options then run in roughly this order.

An outfall into a ditch needs consent in its own right, which our piece on land drainage consent for works to an ordinary watercourse sets out. On larger schemes the same logic is written up as a Drainage Strategy, and foul drainage follows a separate route covered in foul drainage with no mains sewer.

What building control asks to see

Keep the submission short and complete. A drainage layout at a readable scale showing the extension, the downpipes, the run to the soakaway and the 5 m separation dimensioned. The percolation test results with dates, depths and the three timed runs. The sizing calculation with the design return period stated. A section through the soakaway showing depth, void ratio, geotextile wrap and the invert of the incoming pipe. Cover levels and a note on access for maintenance.

Inspectors want to see the hole before it is backfilled. Book that visit rather than covering it and hoping.

Common installation mistakes

The ones that come back to haunt people are dull and repeatable. Crates installed without a geotextile wrap, so fines migrate in and the voids silt up within a few years. No catchpit or silt trap upstream, which guarantees the same outcome more slowly. Granular fill contaminated with site spoil during backfilling. A unit placed at a low spot that collects overland flow as well as roof water.

And the classic: a soakaway built 3 m from the house because that is where there was room. It works for two winters. Then the wall starts moving.

This is general guidance rather than legal advice, and drainage design depends on your ground conditions. Check the position with your building control body and, where a condition requires it, with your local planning authority before relying on any of it.

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