Soakaway or Attenuation Tank for Your Ealing Driveway?

Start with the planning rule

Since 2008, paving more than 5 square metres of a front garden with an impermeable surface needs planning permission, unless the rainwater is directed to a permeable area within your boundary. Ealing Council applies this rule, and it is the main reason drainage comes up on driveway projects in the borough.

In practice you have three routes: use a permeable surface so water soaks through, lay a traditional surface that falls towards a lawn, border or rain garden, or collect the runoff and deal with it underground. Only that last route involves a soakaway or an attenuation tank. You cannot simply run a new driveway into the road gully or the public sewer without permission.

Why Ealing's ground makes a difference

Much of the borough, including large parts of Ealing, Acton, Hanwell and Greenford, sits on London Clay. Clay drains very slowly, so a soakaway that would work well on sandy or gravelly ground can sit full of water for days here. Some pockets, particularly areas closer to the River Brent and parts of Southall and Northolt, have terrace gravels over the clay and drain much better.

You cannot tell from the postcode alone. A proper infiltration test to BRE Digest 365 involves digging a trial pit, filling it with water and timing how quickly it drains, usually three times. It typically takes a day and costs a few hundred pounds, and it tells you whether a soakaway is realistic before you commit to one.

When a soakaway makes sense

A soakaway is a pit filled with plastic crates or clean stone, wrapped in geotextile, which holds rainwater and lets it seep into the surrounding ground. It suits properties where the test shows reasonable infiltration and there is enough space.

• It should sit at least 5 metres from the house or any other building, and away from boundaries and trees.
• Under a driveway, the crates must be rated for vehicle loads, not garden grade.
• Size depends on the paved area and the test result. A typical 40 to 60 square metre driveway might need anywhere from 1 to 3 cubic metres of storage.
• Installed costs commonly run from around £1,500 to £4,000, depending on size, access and how much spoil needs removing.

When an attenuation tank is the answer

If the ground is heavy clay, the front garden is too small to keep a soakaway 5 metres from the house, or there is a high water table, an attenuation tank is often the fallback. It stores rainwater during a storm and releases it slowly through a flow control into a surface water sewer.

This route needs more paperwork. Connecting to the public sewer requires consent from Thames Water, and you must connect to a surface water sewer, not a foul one. Many older Ealing streets have combined sewers, which can complicate things. Attenuation systems usually cost more than a soakaway because of the tank, the control chamber and the connection, often from £3,000 to £8,000 or more for a domestic driveway.

Often, you need neither

For many Ealing driveways, the simplest and cheapest solution is a permeable surface. Permeable block paving laid on an open graded sub base, resin bound surfacing on a permeable base, or reinforced gravel all let water soak through where it lands. On clay, the sub base is usually built deeper so it can hold water while it drains away slowly.

Another option is to lay block paving or tarmac with falls towards a planted border, lawn or rain garden. This works well where there is still some soft landscaping in the front garden.

If you also need a dropped kerb, that is a separate application to Ealing Council, and they will want to see that your drainage is dealt with. It is worth sorting both at the same time.