Do I need a soakaway for a kitchen extension?

If extension increases impermeable area more than 100m² OR if Thames Water refuses additional surface water flow to sewer, yes. Most London extensions either route to existing rainwater downpipe (no soakaway needed if downpipe already discharges to combined sewer + Thames Water OK) or new SUDS solution if Thames Water refuses.

Can I install a soakaway myself?

Construction yes; sizing + percolation test by competent ground engineer. Undersized soakaway = backflow + waterlogged garden in heavy rain. Building Control sign-off requires percolation evidence + sizing calculation.

What if London clay won't percolate?

Most south-east London is clay (Wandsworth, Lambeth, Lewisham, Southwark) — soakaways often fail. Use rain garden + attenuation tank with controlled discharge to combined sewer (Thames Water BOA). Cost similar to soakaway when soakaway works.

Soakaway Cost London 2026 | Builderr

How Much Does a Soakaway Cost in a London Garden?

London soakaway cost £850–£3,250 supplied + dug + connected. Single Polycrub/Aquacell crate (240L) £850–£1,450 for 10m² catchment; double crate (480L) £1,450–£1,850 for 20m²; large multi-crate (1,000L+) £2,250–£3,250. Sized via BRE Digest 365 percolation test. London clay often fails — alternative SUDS (rain garden, swale, attenuation tank) required.

Why soakaways matter in London

Soakaway = below-ground void receiving rainwater run-off from impermeable surfaces (drives, patios, roof downpipes) and allowing it to percolate into surrounding soil. Required if you want impermeable paving over 5m² in a front garden (SUDS rule). Also required for new extensions where Thames Water refuses additional surface water discharge to combined sewer (increasingly common — they want to reduce CSO discharges). Soakaway must be located min. 5m from any habitable building, 2.5m from any boundary or path, sized to BRE Digest 365 method.

Sizing + percolation test

BRE Digest 365 percolation test: dig 1m × 1m × 1m test pit at proposed soakaway location, fill with water, measure time for water level to drop 25%-75%. Calculate soil infiltration rate (mm/hour). Soakaway volume = 10-year 30-minute design storm × impermeable area ÷ infiltration rate. London clay typical infiltration 1–10mm/hour — soakaway volume often 1,000–2,500L for a 25m² drive — large excavation. Sandy soils (Hounslow, Richmond gravel terraces) 50–200mm/hour — small soakaway 240–480L adequate. Test cost £350–£550 by ground engineer.

Construction + alternatives if clay fails

Build-up: dig pit 1.5–2.5m deep to depth below seasonal water table, line with non-woven geotextile, install Polycrub/Wavin AquaCell crates (95% void ratio), backfill with 20mm clean angular stone, top with geotextile, cover with 600mm topsoil. Connect via 110mm uPVC pipe to drive surface drain or downpipe. London-clay alternatives if soakaway fails percolation: (1) rain garden — shallow depression planted with water-tolerant species (irises, sedges) — £450–£1,250; (2) green roof attenuation — sedum mat over impermeable build-up; (3) attenuation tank with restricted discharge to combined sewer (Thames Water Build-Over Agreement required, £980–£1,650 fee) — controlled flow rate 1–5 L/s.