French Drain vs Soakaway: Which Do You Need?

Summary

"French drain" and "soakaway" get used almost interchangeably on site, and the confusion costs money — either a soakaway gets specified where a simple cut-off drain would have done the job for a third of the price, or a French drain gets installed with no outfall and the customer is back on the phone within a year asking why the garden is still wet.

The core distinction is what happens to the water at the end of the run. A French drain is a conveyance system: a gravel-wrapped perforated pipe laid to a fall, collecting water along its length and carrying it somewhere else — a soakaway, a surface water sewer, a ditch, or a lower part of the garden. It doesn't dispose of water on its own; it needs an outfall. A soakaway is a disposal system: an underground void (crate-filled or rubble-filled chamber) that receives water and lets it infiltrate into the surrounding subsoil over time. The two are frequently used together — a French drain collecting water from a problem area and discharging it into a soakaway is one of the most common combined details in UK groundworks and landscaping.

Getting the choice wrong is expensive in both directions. Installing a soakaway on clay ground with no percolation test wastes the crate cost and the labour, because the water simply won't infiltrate — it sits there and eventually resurfaces. Installing a French drain with no downstream disposal point just relocates the puddle a few metres. This article sets out when each is the right call, how to combine them, and the regulatory framework that governs both under Building Regulations Part H.

Key Facts

• French drain function — conveys water along a perforated pipe in a gravel-filled trench to an outfall; does not dispose of water itself
• Soakaway function — stores and disposes of water by infiltration into the surrounding subsoil
• Governing regulation — Building Regulations Approved Document H3 (rainwater drainage and SuDS); surface water must not discharge to the foul sewer under Part H3
• SuDS hierarchy (Approved Document H3, in order of preference) — infiltration to ground (soakaway) first, then attenuation/discharge to watercourse, then discharge to surface water sewer, then combined sewer as last resort
• Percolation test — BRE Digest 365 method, 300mm × 300mm test pit, required before sizing any soakaway; not required for a French drain unless it discharges to a soakaway
• Soakaway minimum distance from building — 5m (Approved Document H3, paragraph 3.27); French drains have no equivalent statutory setback but should still avoid undermining foundations
• Soakaway minimum distance from boundary — 2.5m typical; consult the local authority for site-specific requirements
• French drain typical fall — minimum 1:80 for 100mm pipe (12.5mm per metre); 1:150 acceptable for 150mm pipe
• French drain trench depth — 450–600mm for surface water interception; 750–1200mm for foundation relief or below-DPC damp management
• Soakaway invert depth — minimum 1m below ground level, must sit above the seasonal high water table
• Combined system — a French drain intercepting water and discharging into a soakaway is standard practice; the soakaway must still be sized from a percolation test on the ground it sits in
• Ground suitability — clay and heavy silt generally rule out or severely limit soakaway feasibility; French drains work regardless of infiltration rate because they only convey, not dispose
• BRE Digest 365 — the reference method for percolation testing and soakaway sizing (still current, no direct BS EN replacement for the field test procedure)
• BS EN 752 — Drain and Sewer Systems Outside Buildings; broader European standard covering system design principles referenced alongside Part H
• Land Drainage Act 1991 — prohibits obstructing or discharging water in a way that causes flooding to a neighbouring property or watercourse without consent
• Building Control approval — required for both soakaways and land drainage systems that form part of the site drainage strategy, particularly where they interact with a building's foundations
• Failure mode — French drain — installed without an outfall; water backs up and the trench itself becomes a soakaway by accident, usually undersized for that purpose
• Failure mode — soakaway — undersized because no percolation test was carried out, or sited on ground that doesn't drain

Quick Reference Table

Detailed Guidance

When a French drain is the right answer

A French drain is the correct specification when the problem is localised wet ground that needs intercepting and redirecting, not a whole-property surface water disposal strategy. Classic cases: a lawn that stays waterlogged after rain because of a perched water table or compacted subsoil; a boundary wall or retaining structure showing damp or hydrostatic pressure signs; a driveway or patio shedding water toward a building. In each case the drain's job is to intercept water before it reaches the problem area and carry it somewhere it can be dealt with — typically a soakaway, but sometimes a surface water gully, ditch, or lower point in the garden with landowner permission.

French drains do not require a percolation test unless their outfall is a soakaway (in which case the soakaway itself needs the test). This makes them cheaper to design and specify when a suitable gravity outfall already exists.

When a soakaway is the right answer

A soakaway is required wherever surface water needs permanent disposal and mains sewer connection is either unavailable or, under the SuDS hierarchy in Approved Document H3, deliberately avoided in favour of infiltration. This covers roof rainwater, paved area runoff, and driveway drainage — anywhere Building Control expects a documented disposal route for water that would otherwise run off the property.

A soakaway is only appropriate where the BRE Digest 365 percolation test confirms the ground will actually accept the design volume of water within a reasonable time. Skipping this test is the single most common cause of soakaway failure — see soakaway design for the full sizing calculation and test method.

Combining the two systems

The most robust and common domestic detail is a French drain collecting water from a defined problem area (a wet lawn corner, the base of a retaining wall, a driveway edge) and discharging via a rodding-accessible junction into a soakaway sized for that catchment. This gives the conveyance flexibility of a French drain with the guaranteed disposal of a soakaway, and lets the installer size the soakaway precisely to the catchment area rather than guessing at a whole-garden volume.

Where this goes wrong on site is when the French drain trench is dug first, the "soakaway" at the end is just an enlarged pit with no percolation test and no calculated void volume, and it fails within a season because it was never actually sized. Treat the two as separate design decisions even when they're built as one continuous job.

Ground conditions that decide the choice

Clay subsoil, a high water table, or contaminated land generally rule out a soakaway, or at minimum require oversizing, a deep bored soakaway, or geotechnical input that pushes cost up sharply. In these conditions, a French drain conveying water to a surface water sewer connection or a watercourse (with the necessary consents) is often the only practical route — see groundworks near trees and soil types and bearing capacity for how ground type affects both drainage and foundation decisions on the same site.

Regulatory route and Building Control

Both systems sit under Approved Document H3 and, where they interact with foundations or form part of a wider site drainage strategy, may need Building Control sign-off. Discharging water onto a neighbouring property or the highway without consent is an offence under the Land Drainage Act 1991, regardless of whether the system is a French drain or a soakaway — always confirm the outfall point is legitimate before pricing the job.

Frequently Asked Questions

Can I just dig a bigger French drain instead of a proper soakaway?

No — not reliably. An oversized trench with no percolation test and no defined void volume behaves unpredictably: it may work adequately in free-draining sand and gravel, but on anything less permeable it will simply fill up and stay full, defeating the point. If the design intent is permanent disposal by infiltration, size it as a soakaway using BRE Digest 365, even if it's built inside what looks like a long trench.

Does a French drain need Building Control approval?

Not always as a standalone item, but if it discharges into a soakaway, connects to the surface water drainage system, or is part of works near a building's foundations, it can fall under Building Control's remit via Approved Document H. Always check with the local authority before assuming it's exempt, particularly on jobs close to a building or a boundary.

My garden is on clay — can I still use a soakaway?

Only with caution. Clay has very low permeability, and a standard percolation test will usually show unacceptably slow infiltration rates. Options include a much larger soakaway, a deep bored soakaway below the clay layer (if permeable strata exist beneath it — needs geotechnical investigation), or abandoning the soakaway route entirely in favour of attenuation and controlled discharge to a sewer or watercourse. A French drain conveying water off-site to a legitimate outfall is often the more realistic solution on clay.

What's the difference in cost between the two?

A domestic French drain typically runs £60–£120 per linear metre for a standard garden-depth trench — see french drain installation pricing guide for the full cost breakdown. A soakaway's cost is driven by excavation volume and whether crates or rubble are used, and scales with the catchment area it serves rather than a linear rate. On a typical combined job, expect the soakaway itself to be the larger single cost component once it's sized beyond a small garden catchment.

Regulations & Standards

• Building Regulations Approved Document H3 (Rainwater Drainage and Rainwater Reuse) — governs the SuDS hierarchy, soakaway design principles, and disposal route selection

• BS EN 752 — Drain and Sewer Systems Outside Buildings; general design principles for drainage systems

• BRE Digest 365 — Soakaway Design; the standard percolation test method and sizing calculation

• Land Drainage Act 1991 — prohibits discharge or obstruction of water flow causing flooding to third-party land or watercourses without consent

• BS EN 1401 — specification for unplasticized PVC-U pipe systems used in buried drainage, relevant to French drain pipe selection

• CDM Regulations 2015 — apply to excavation work on both French drains and soakaways where relevant thresholds are met

• Planning Portal — Rainwater Drainage (Approved Document H) — official guidance on Part H requirements

• BRE — Digest 365: Soakaway Design — the primary reference for percolation testing and soakaway sizing

• gov.uk — Sustainable Drainage Systems — SuDS hierarchy and infiltration-first principle

• soakaway design — full BS EN 752 / BRE Digest 365 sizing guide for soakaways

• french drain installation — installation technique, pipe sizing and fall for French drains

• building regs part h drainage — the wider Part H framework covering foul and surface water drainage

• french drain installation pricing guide — labour and materials pricing for French drain jobs

• soakaway installation pricing guide — pricing for soakaway installation