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Build a soakaway.
Crate or rubble soakaway sized to roof area — for surface-water drainage compliant with Building Regs Part H.
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Undersized soakaway
Cheap quotes use a "rule of thumb" 1m³ without a percolation test. Clay soils need 2–3m³ for the same roof area.
Crate crushed
Driven over by a car, the cheap crates collapse. Use crates rated for the load above (some are car-rated, some not).
Geotextile missing
Without the wrap, silt clogs the crate within 3–5 years. The membrane is non-negotiable.
Too close to building
Soakaways within 5m of a wall can cause subsidence in clay soils. The 5m minimum is non-negotiable.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Percolation testOne-day site visit
£100–£250
Domestic soakaway crate (1m³)Stormcrate or equivalent
£80–£180
Geotextile membrane3–6m² depending on size
£30–£80
Excavation (1.5–2m deep)Mini-digger or hand-dig
£200–£500
Connection pipe & inspection chamber110mm pipe, rodding access
£120–£300
Backfill & reinstatementTopsoil, grass or paving
£80–£300
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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A soakaway disperses rainwater into the surrounding subsoil instead of going to the public drain. They are required where no surface-water drain is available, or where SuDS principles take priority. Expect to pay £500–£2,500 for a typical UK domestic soakaway, depending on size and access.
What Does the Work Involve?
The installer digs a pit at least 5m from the building and 2.5m from any boundary. The pit is sized based on a percolation test (a quick infiltration test) and the connected roof area. Modern soakaways use plastic crates (Stormcrate, Hydrocrate, Polypipe Polystorm) wrapped in geotextile membrane — providing 95% void ratio in a 1m³ space. Older soakaways used loose gravel (30% void). The downpipe is connected to the soakaway via a buried pipe at least 1m below ground level. The pit is backfilled with topsoil or grass.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Percolation test | 100 | 250 | One-day site visit |
| Domestic soakaway crate (1m³) | 80 | 180 | Stormcrate or equivalent |
| Geotextile membrane | 30 | 80 | 3–6m² depending on size |
| Excavation (1.5–2m deep) | 200 | 500 | Mini-digger or hand-dig |
| Connection pipe & inspection chamber | 120 | 300 | 110mm pipe, rodding access |
| Backfill & reinstatement | 80 | 300 | Topsoil, grass or paving |
| Total domestic soakaway | 500 | 2,500 | Depends on size and access |
How Long Does It Take?
Percolation test: half a day, plus 1–2 days lag while soil saturates. Excavation and install: 1–2 days. Reinstatement: 1 day. Total project: 4–5 days from booking.
DIY or Professional?
A confident DIYer with a hired mini-digger can install a domestic soakaway. The percolation test result and sizing calculation are the most important step — get the size wrong and the soakaway overflows. Most insurers want a professional contractor for warranty on drainage works.
Choosing the Right Tradesperson
A drainage contractor or general groundworker. Ask whether they do their own percolation test or hire a specialist. Confirm crate brand — Stormcrate, Polypipe and Hydrocrate are the UK market leaders. Get a 25-year warranty on the buried crate system.
UK Regulations
Approved Document Part H covers soakaway design. BRE Digest 365 is the sizing guide. Soakaways must be at least 5m from any building, 2.5m from a boundary, and not in ground prone to subsidence. SuDS (Sustainable Drainage Systems) policy favours soakaways over piped connections. Notification to building control may be needed if part of a larger build.
Common Problems
- Undersized soakaway: Cheap quotes use a "rule of thumb" 1m³ without a percolation test. Clay soils need 2–3m³ for the same roof area.
- Crate crushed: Driven over by a car, the cheap crates collapse. Use crates rated for the load above (some are car-rated, some not).
- Geotextile missing: Without the wrap, silt clogs the crate within 3–5 years. The membrane is non-negotiable.
- Too close to building: Soakaways within 5m of a wall can cause subsidence in clay soils. The 5m minimum is non-negotiable.
Frequently Asked Questions
When do I need a soakaway?
When your property has no public surface-water drain available, or you're extending and the existing drain is overloaded. Increasingly, new builds use soakaways under SuDS policy regardless.
How big should my soakaway be?
Sized from the percolation test. Rule of thumb: 0.5–1.5 m³ per 25 m² of roof area in average UK clay soils. Sandy soils can be smaller; heavy clay much bigger.
Crate or gravel soakaway?
Crates — 95% void ratio in a small footprint. Gravel was the old method (30% void) but is now considered obsolete except in very small applications.
Can I drive over a soakaway?
Yes, if the crate is rated for vehicle load — Polystorm Lite is foot-traffic only; Polystorm Access is car-rated; Polystorm Inspect is HGV-rated. Specify the right one.
How long does a soakaway last?
25+ years on the crate (warrantied). Wrapped in geotextile, no silt entry, lifetime is essentially the crate's UV exposure (none, when buried).
Do I need a percolation test?
For sizing, yes — without it the soakaway is guesswork. Some contractors skip it on small projects but they're guessing. A £150 test pays for itself in correct sizing.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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