Drainage

1. 1

   Work out where the water goes

   Identify the legal outfall first: a soakaway, a surface water sewer or a watercourse. Never connect surface water to a foul drain.

2. 2

   Test the ground

   Dig a trial hole and run a BRE 365 percolation test before sizing a soakaway, because clay may rule it out.

3. 3

   Set the falls

   Set levels so the paved surface falls at roughly 1:60 to 1:80 toward the channel or gully, always away from the house.

4. 4

   Dig the runs

   Excavate trenches to a steady gradient, around 1:80 for a 100 mm pipe, and bed on pea shingle.

5. 5

   Install the drainage

   Lay channel drains on a concrete haunch level with the finished surface, connect pipework with proper fittings and fit rodding access.

6. 6

   Build the soakaway

   Site it at least 5 m from any building, wrap crates or clean stone in geotextile and backfill in layers.

7. 7

   Test it

   Flood the system with a hose before the surface goes down and watch every joint and outlet.

Surface water law in the UK is simpler than people think: rain that lands on your new paving is your problem, and the rules dictate the order in which you may get rid of it.

• Surface water must not be connected to the foul sewer. Soakaway, watercourse or surface-water sewer only, in that order of preference under the SuDS hierarchy. ^[1]
• Soakaway design and percolation testing follow BRE Digest 365. ^[2]
• A soakaway must sit at least 5 m from any building and 2.5 m from a boundary, closer and you risk softening the ground under foundations, yours or your neighbour's. ^[1]
• Discharging onto the public highway or a neighbour's land is an actionable nuisance, paving falls must keep run-off inside your own curtilage until it reaches a legal outfall. ^[1]
• Front gardens over 5 m² laid impermeably need planning permission unless run-off goes to a permeable area on your own land, the drainage detail is what makes a driveway legal. ^[3]

Drainage is the part of a landscaping job that fails silently for two winters, then fails loudly. These are the details that decide which way yours goes.

• Before trenching a single metre, CAT & Genny the line (HSE HSG47). Drain runs invariably cross incoming gas, water, electric and BT. A CAT alone won't find plastic mains; the Genny in Induction mode will. Full method in the Safety guide.
• Channel-drain (ACO) fall: minimum 1:200 to the outlet. Confirm with manufacturer for shallow runs.
• Soakaway rule of thumb (London clay): 1 m³ of void per 50 m² of impermeable surface, but always percolation-test before specifying.
• On sandy / loam soils budget ~1 m³ of void per 80 m² and still percolation-test.
• Cover soakaways with geotextile to stop fines clogging the void.
• If existing house drains take run-off, confirm capacity with the water authority before adding more area, overloading is illegal under Building Reg H3. ^[1]
• Drain-to-lawn or border counts as a permeable area under GPDO Class F, keep at least 1 m² of receiving ground per 5 m² of paving.
• Land drains behind retaining walls: 100 mm perforated pipe in a permeable backfill, exiting to a free outfall.
• Fall paving away from the house at minimum 1:80; where paving meets the wall, an ACO channel at the door threshold catches what the fall misses.
• Percolation test: dig a 300×300×300 mm hole, fill with water, time the drop. If it drains slower than roughly 1 hour per 100 mm twice running, a soakaway won't work on that plot. ^[2]

The SuDS hierarchy is not guidance, it is the order building control and water authorities expect you to follow. Work down it and stop at the first option that genuinely works.

• 1. Soakaway: the default answer. A hole or crate system wrapped in geotextile that lets water disperse into the ground. Works on any free-draining soil, fails on saturated clay. ^[2]
• 2. Watercourse: if a ditch, stream or culvert crosses or borders the plot you may be able to discharge to it, usually needing the consent of the landowner or lead local flood authority.
• 3. Surface-water sewer: connection needs the water authority's approval and they will ask why options 1 and 2 failed. Connection charges and a Section 106-style approval apply. ^[1]
• 4. Permeable area: for front gardens, draining to your own lawn or planting counts as managing run-off on site, and is what keeps a big driveway within permitted development. ^[3]

• Soakaway under the patio: it has to be 5 m from the house, but also not directly beneath the paving it drains. Saturated ground under a sub-base pumps and settles.
• No percolation test on clay: a crate soakaway dug into London clay fills in the first storm and stays full. The test costs an afternoon; the failure costs a re-lay.
• Channel drain with no fall: an ACO laid dead level holds a static line of water and silt. Even 5 mm over a 10 m run makes it self-cleansing.
• Piping gutters into a small soakaway already taking a new patio: add the roof area to your sizing, it is usually the bigger contributor.
• Soakaway filled with rubble: broken brick holds around 30 % voids versus 95 % for a crate. Rubble soakaways are a third of the size they look.

Typical 2026 supply-and-fit ranges for garden drainage in the UK. Access and spoil disposal move these more than the parts do.

• Channel drain (ACO type) with a polymer body and galvanised grate: roughly £60 to £120 per metre installed, more for a heel-safe or stainless grate.
• Crate soakaway, dug, wrapped in geotextile and connected: roughly £900 to £2,000 for a typical 2 to 3 m3 domestic unit, driven mostly by dig and muck-away.
• Land drain (perforated pipe in a stone-filled trench): roughly £40 to £70 per metre for a 450 mm deep trench in accessible ground.
• Percolation test to BRE 365: roughly £250 to £500 if you pay someone, or an afternoon of your own time and a hosepipe.
• Linear gully connection into an existing surface-water manhole: roughly £300 to £600 where the manhole is close and the invert works.
• Spoil disposal is the hidden cost: a 3 m3 soakaway pit produces around 5 tonnes of muck-away at £80 to £120 per tonne.