Land drain for a garden: French drains, clay soil and real fixes

How a garden land drain works

A land drain (French drain) is a trench holding a perforated pipe surrounded by clean gravel, usually wrapped in a geotextile membrane to keep soil fines out. Water in the soil finds the gravel, enters the pipe, and flows by gravity to an outfall. On larger lawns, several runs feed a main in a herringbone pattern. Installed cost is typically £50–£100 per metre (typical 2026 quotes); see the cost guide for what moves the figure.

Why clay gardens flood

• Clay barely lets water through, so rain sits on or near the surface instead of soaking away; drains in clay do real work and are dug at the top of the price range.
• New-build gardens often add compacted subsoil under a thin layer of topsoil, which behaves like clay even where the native soil isn't.
• Soakaways struggle in clay: a soakaway is a hole water is supposed to leave, and in heavy clay it often can't. A percolation test tells the truth before money is spent.
• Levels rule everything: water only flows downhill, so a garden lower than its surroundings may need a different answer, including accepting a wet corner and planting for it.

Where the water can go

1. A soakaway, if the ground percolates: for drainage serving a new building this falls under Building Regulations Part H, and sizing follows a percolation test.
2. An existing surface water drain, where one exists and has capacity; connecting garden drainage to the foul sewer is not the answer.
3. A ditch or watercourse: possible, but discharging into an ordinary watercourse or altering it can need consent, see consents and responsibilities.

DIY is realistic for a short run in decent soil if you can dig to a fall and have somewhere for the water to go; the hard half of the job is the outfall, not the trench. For clay, big lawns, or anywhere near buildings, get contractors to quote it, misplaced drains are expensive to do twice.