What is the difference between a soakaway and a French drain?

A soakaway is a pit (filled with crates or rubble) that collects water and allows it to soak into the ground. A French drain is a trench with a perforated pipe in gravel that collects and diverts water along its length, discharging to a soakaway, watercourse or drain. A soakaway is the endpoint; a French drain is the transport system.

Do I need building regulations approval for a soakaway?

A soakaway for surface water (gutters, driveways) does not usually need Building Regulations approval unless it is connected to a new drainage system as part of an extension. A soakaway must be at least 5m from a building and not in clay soil. Building Control may require a percolation test (BRE 365) to confirm the ground can absorb water.

Do soakaways work in clay soil?

Poorly or not at all. Clay has a very low percolation rate — water takes days to drain away. A BRE 365 percolation test will confirm whether a soakaway is viable. If the soil is heavy clay, you will need an alternative: a French drain discharging to a watercourse, a surface water sewer connection (with water company permission), or a rainwater harvesting system.

How big does a soakaway need to be?

Soakaway size depends on the roof/surface area draining to it and the soil percolation rate. A typical domestic soakaway for a 3-bed house (80-100 m² roof) is about 1m x 1m x 1m using soakaway crates (storage volume ~0.8 m³). Our soakaway sizing calculator can give you an exact size based on BRE 365 test results.

How long does a French drain last?

A well-installed French drain with geotextile membrane wrapping the gravel lasts 15-25 years. Without membrane, the gravel clogs with silt much faster (5-10 years). The perforated pipe itself lasts 30+ years. Flushing the pipe every few years extends the life. If the gravel becomes clogged, the trench needs re-digging.

Soakaway vs French Drain — UK Drainage Guide

Side-by-Side Comparison

Property	Soakaway	French Drain
Purpose	Water disposal (soaks into ground)	Water collection and diversion
How it works	Pit stores water, percolates into soil	Perforated pipe in gravel collects/redirects water
Typical size	1m x 1m x 1m pit	300mm wide x 500mm deep trench
Installation cost	£400-1,000	£30-60 per linear metre
Soil requirement	Free-draining (not clay)	Any soil type
Distance from building	Min. 5m from foundations	Can run alongside building
Maintenance	Very low once installed	Occasional pipe flushing
Lifespan	20-30+ years (crates)	15-25 years (with membrane)
Discharge point	Into surrounding soil	Soakaway, watercourse or drain

UK Cost Comparison (2026 Prices)

Component	Soakaway (1m³)	French Drain (10m)
Excavation	£150-300	£200-400
Soakaway crates / perforated pipe	£80-150	£40-80
Gravel (20mm clean)	£50-80	£100-200
Geotextile membrane	£20-40	£30-60
Connecting pipework	£50-100	£30-60
Total (DIY / professional)	£350-670 / £600-1,000	£400-800 / £600-1,200

When to Choose Each

Choose a Soakaway When

Soil drains well (sandy, chalky, loam)
Disposing of roof rainwater
No nearby watercourse or drain
BRE 365 test confirms suitability
Space available 5m+ from buildings

Choose a French Drain When

Intercepting groundwater before it reaches a building
Draining a waterlogged lawn or garden
Clay soil where soakaways fail
Diverting water to a specific discharge point
Along the base of a retaining wall

Building Regulations

Under Approved Document H (Drainage), surface water should discharge in the following hierarchy: (1) soakaway or infiltration system, (2) watercourse, (3) surface water sewer, (4) combined sewer. You must demonstrate that each option is not feasible before moving to the next. A BRE 365 percolation test is the standard method for proving whether a soakaway is suitable.

Related Calculators

Soakaway Sizing Calculator — size a soakaway from BRE 365 test results
French Drain Calculator — calculate gravel, pipe and membrane for French drains

Soakaway vs French Drain — UK Drainage Guide

Side-by-Side Comparison

UK Cost Comparison (2026 Prices)

When to Choose Each

Building Regulations

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Frequently Asked Questions

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