Do I need a soakaway for my drive?

You need a soakaway when a front drive of more than 5m² has a surface that water cannot pass through and there is no lawn or border on your land for the water to run onto. A permeable drive usually does not need one. Where a soakaway is needed, building regulations apply, and it is sized from a percolation test.

That covers most front drives laid in tarmac, imprinted concrete or standard block paving. Resin-bound on a porous base, gravel and permeable block paving let the rain soak through where it falls. Clay soil, the distance from the house and the size of the paved area then decide whether a soakaway will work and how big it has to be.

What is a soakaway?

A soakaway is a pit or trench in the ground that collects rainwater and lets it soak slowly into the surrounding soil. The government's guidance on paving front gardens describes it as an underground chamber or gravel-filled trench that collects water from the driveway and lets it soak away. Approved Document H, the official guidance on drainage, says soakaways for areas under 100m² are generally square or circular pits, filled with rubble or lined with dry-jointed masonry or perforated rings. Many installers now use plastic crates wrapped in a membrane, which hold more water for the same size of hole.

On a drive, water usually reaches the soakaway from a channel drain across the drive or along its low edge, through a pipe run to the pit.

When does a drive need a soakaway?

In England, a new or replaced hard surface of more than 5m² in a front garden needs planning permission unless it is permeable, or the water runs to a permeable area within your own boundary. Letting it run onto the pavement or into the road does not count. Our guide to the 5m² front-garden rule explains the rule in full.

So a soakaway is one of three ways to meet the rule, not the only one:

• A permeable surface, such as resin-bound on a porous base, gravel or permeable block paving, which needs no soakaway at all in most cases.
• A fall to a lawn or border. Approved Document H says paths, driveways and other narrow areas of paving should drain freely to a pervious area such as grass, as long as the water is not discharged next to buildings and the ground is not overloaded.
• A channel drain and soakaway, where the drive covers most of the garden and there is nowhere else for the water to go.

Drives behind the house are not covered by the 5m² rule, but the water still has to go somewhere that will not harm the house or the neighbours.

Will a soakaway work in clay soil?

Often not well. Approved Document H lists sandy clay, silty clay and clay as soils with poor percolation, and sand, gravel, chalk and sandy loam as good. Heavy clay is common across Birmingham and the Black Country.

The government's front-garden guidance says the soil below a soakaway or rain garden must be sandy or gravelly, not clay, or a connection to the drains may be needed. On very clayey sites it suggests a pipe in the sub-base that takes water to the drains carrying the roof water, and it gives a rule of thumb: a pipe is needed if a 300mm by 300mm by 300mm pit filled with water takes more than 11 hours to empty. Any connection to a public sewer needs the water company's consent, which in most of the West Midlands means Severn Trent.

How is a percolation test done?

Approved Document H says percolation tests should be carried out to find the capacity of the soil, and refers to its own test method:

1. Dig a hole 300mm square, to the depth the soakaway will work at.
2. Fill it with at least 300mm of water and leave it to soak away overnight.
3. Next day, fill it again and time how long the water takes to fall from 75% full to 25% full.
4. Repeat at least three times, with at least two trial holes, and take the average.

The test should not be done in heavy rain, severe frost or drought. For soakaways serving larger areas, Approved Document H points to BRE Digest 365, Soakaway design, instead. A local specialist or the council's building control team can tell you which test they want to see.

How far from the house must a soakaway be?

Approved Document H says a soakaway should not be built within 5m of a building or a road, in unstable ground, where the water table reaches the bottom of it at any time of year, or so close to other soakaways that the ground cannot take the water. The reason is simple: water released next to a house can soften the ground under its foundations.

The government's front-garden guidance says that for small driveways a minimum of 3m may be suitable, depending on the ground, and to ask your council's building control department if you are unsure. On a small front garden, a 5m gap from both the house and the road can be hard to find, which is one reason permeable surfaces are popular at the front.

How big does a soakaway need to be?

There is no standard size. It depends on the paved area, how hard it rains and how fast your ground drains, so the honest answer only comes from a percolation test. Approved Document H says soakaways should be designed for a storm expected about once in ten years, and that for small soakaways serving 25m² or less, a design rainfall of 10mm in 5 minutes may be assumed. Where the ground is marginal, an overflow drain can be acceptable.

Do I need building regulations or planning permission for a soakaway?

The Planning Portal says a soakaway does not normally need planning permission, as long as it deals with surface water and does not discharge into a public sewer. Extra rules may apply in a conservation area or within the boundary of a listed building.

It also says building regulations apply to the creation of a soakaway, under Part H, which covers where it can go and how big it is. Check with your council's building control team before work starts, and keep any paperwork for when you sell.

What does a soakaway cost?

On a new two-car block-paved drive at the front, a soakaway can take the total from £3,550–£5,730 up to about £6,930. The price depends on the dig, the ground and how far the pipe has to run. See drainage and soakaways for what is included, or the driveway cost per m² guide for the whole drive.