A soakaway — a below-ground pit or crate system that lets rainwater soak into the surrounding ground instead of running to a sewer or watercourse — typically costs £600–£1,200 in 2026 for a single downpipe or a small roof area, rising to £1,500–£2,500 for one sized to take all the rainwater off an ordinary house roof. Build a larger one to serve a driveway, a patio, or the drainage strategy for an extension, and the fee typically moves to £2,500–£4,500 or more.
None of those figures is worth fixing on before you know one thing: how quickly your own ground drains. A soakaway isn't a fixed product ordered off a shelf — it's sized against your soil using a formula in BRE Digest 365, the standard method every competent installer works to. Get the percolation rate wrong, or skip testing it altogether, and you can end up with a hole that floods the garden after every downpour, or one sited too close to the house that quietly saturates the ground under a wall it was never supposed to go near.
I'm a building surveyor, not a groundworker — I don't dig or size soakaways myself. What follows is 2026 pricing cross-checked against current UK drainage and groundworks firms' published rates, what a percolation test actually involves, the distance rules from buildings and boundaries that exist for a genuine structural reason, and when infiltration isn't the right answer at all.
The short answer
Figures below were checked on 20 September 2026 against several UK drainage and groundworks cost guides, cross-referenced against each other. As with the rest of this series, most firms quote against a site visit rather than a phone estimate, because the ground itself decides the job.
| Job type | Typical cost (2026) | What's included |
|---|---|---|
| Single downpipe or small roof area (crate) | £600–£1,200 | One or two soakaway crates, short pipe run, backfill |
| Whole-house soakaway (all roof water) | £1,500–£2,500 | Sized to BRE 365, several crates or a larger rubble-filled pit |
| Driveway, patio or extension drainage strategy | £2,500–£4,500+ | Larger volume, load-rated crates, possible attenuation |
| Percolation test alone (BRE Digest 365) | £150–£400 | Trial pit, two timed readings a day apart, written result |
One cost in this guide is a genuinely wide range rather than a firm figure, and worth flagging as such: labour, usually charged as a day rate for a groundworker with a mini-digger, runs from around £150 a day at the low end to £350 or more, depending on region and whether the digger hire is bundled into the quote. It's the single biggest source of variation between two quotes for what looks like the same job.
What drives the price
Soil type. This matters more than anything else on this page. A percolation test measures how many seconds it takes water to drop 150mm in a test hole, expressed as a Vp value in seconds per millimetre. Free-draining sand or gravel often tests well within the roughly 12–100 s/mm range BRE Digest 365 treats as suitable for infiltration. Heavy clay routinely tests above 100 s/mm, sometimes far above it — and past that threshold, a soakaway either needs building much larger than most gardens have room for, or isn't workable at all.
Size, calculated rather than guessed. BRE Digest 365 gives a formula based on the roof or paved area draining into it, local rainfall intensity for a storm of a given return period, and your own percolation result — not a rule-of-thumb table. A bigger catchment area or a slower-draining soil both push the required storage volume up, which is really what you're paying for once the job's bigger than a single downpipe.
Crate system or rubble-filled pit. Modern plastic soakaway crates, roughly £35–£60 supplied per 190-litre unit, wrap in a geotextile membrane and assemble quickly, so a small job can work out similarly priced to a traditional rubble-filled pit despite costing more in materials — you're trading materials cost for a shorter, cleaner dig. A larger job usually still favours crates, because moving and disposing of the volume of hardcore an equivalent rubble pit would need gets expensive fast.
Depth, access and groundwater. A soakaway typically sits at least a metre below ground, and design guidance generally keeps its base around a metre above the highest groundwater level the site sees across the year. Where access is tight — a narrow side return, no way in for a mini-digger — spoil has to be barrowed out rather than dug and loaded directly, and both plant and labour take longer.
Where it's allowed to go. Covered in full below, but it affects price directly: a small plot with buildings and boundaries close on every side may not leave anywhere the required separation distances allow a soakaway to sit, forcing a rethink of the whole drainage strategy before a single crate is ordered.
Where a soakaway can and can't go
The distance rules here exist for a reason worth taking seriously, not just a box to tick. Three separate bodies of guidance — BS 8301:1985, BRE Digest 365, and NHBC's own technical standards for new-build warranties — independently arrive at the same figure: a soakaway shouldn't be built closer than about 5 metres to a building's foundations. Guidance commonly cited under Building Regulations Approved Document H puts the minimum distance from a boundary at 2.5 metres. Three separate standards, developed for different purposes, converging on the same 5-metre figure tells you it isn't an arbitrary planning nicety.
The reasoning is straightforward. A soakaway's whole job is to get water into the ground, and ground deliberately kept wet right next to a foundation is exactly the kind of ground clay shrinks and swells in as it wets and dries through the year. I haven't seen a formal industry study putting a number on how often this goes wrong, but I have seen a soakaway sited a metre or two off a wall on a tight plot — the kind of shortcut a small garden tempts a contractor into when there's genuinely nowhere else obvious to put it. If a property I'm surveying has an existing soakaway close to the building, that's something I'll flag, not wave through.
Where the plot is too small, or the soil's percolation too poor, to make infiltration work at all, the fallback is an attenuation tank with a flow restrictor discharging to a sewer or watercourse, or — where there's a genuine use for the water — a rainwater harvesting system. Both cost more than a straightforward soakaway: an attenuation-based domestic scheme is commonly quoted from around £1,500 up to £4,000 or more, and rainwater harvesting typically runs £2,500–£6,000 installed. Neither is something most people choose over infiltration; they're what's left once infiltration genuinely isn't an option.
Do you actually need one?
Two situations bring most people to this page. The first is an existing house with a blocked, collapsed or non-existent soakaway that a downpipe has clearly been discharging onto the lawn for years — worth fixing, though rarely urgent unless it's causing damp against a nearby wall.
The second, and more common now, is a driveway or a hard-standing extension. Since a rule introduced in October 2008 and now consolidated into the Town and Country Planning (General Permitted Development) (England) Order 2015, any hard surface bigger than 5 square metres laid between the front of a house and the road loses its permitted development rights — meaning a planning application is needed — unless it's either made of permeable material or built so run-off drains to a soakaway or another permeable area within the property. This is the rule behind almost every "do I need a soakaway for my new driveway" search, and it's a planning trigger rather than a Building Regulations one: get it wrong and the exposure is enforcement action over an unauthorised change to the land, not a query from Building Control.
For roof water specifically, Approved Document H sets a different, Building Regulations-level order of priority: rainwater should go to an adequate soakaway or infiltration system first; only where that's not reasonably practicable does a watercourse become the next option; and only where neither is practicable should it go to a sewer at all. In practice, that means an extension can't simply plumb its new guttering into the existing surface water drain because it's the easy option — Building Control will expect to see that infiltration has genuinely been ruled out, normally with a percolation test.
For anything larger than an ordinary domestic job — a scheme big enough to need planning permission in its own right — a separate, five-tier hierarchy under Defra's National Standards for Sustainable Drainage Systems, updated in July 2025, now ranks collecting rainwater for reuse ahead of infiltration, before working down through discharge to a watercourse or sewer. It mainly bites on larger developments assessed through planning policy rather than a single house's soakaway, but worth knowing the wording has moved on if you're dealing with a bigger site.
What I'd watch for in a quote
- Confirm a percolation test has actually been done, or ask why one hasn't if the soil elsewhere on the plot is known to be heavy clay.
- Ask to see the BRE 365 sizing calculation, not just a stated number of crates — the volume should trace back to your roof or paved area and your own test result.
- Check the design keeps the required distance from the house, the boundary, and any existing soakaway on the plot.
- Ask how existing services are handled — gas, water and drainage runs located before digging, not discovered mid-excavation.
- Get plant hire and spoil disposal priced into the fixed fee, not added once the digger's already on site.
Questions I get asked
How much does a soakaway cost in 2026? A soakaway for a single downpipe or a small roof area typically costs £600–£1,200. One sized to take all the rainwater off an ordinary house roof runs £1,500–£2,500. A larger soakaway for a driveway, patio or extension drainage strategy typically costs £2,500–£4,500 or more. A percolation test to size it correctly, priced separately from installation, typically costs £150–£400.
Do I need planning permission for a soakaway? Installing a soakaway itself doesn't normally need planning permission. Where it does become a planning question is a front-garden driveway or hard-standing: since a rule introduced in October 2008, now in the Town and Country Planning (General Permitted Development) (England) Order 2015, a hard surface over 5 square metres between the house and the road loses its permitted development rights unless it's permeable or drains to a soakaway or another permeable area within the property.
How close can a soakaway be to my house or the boundary? Guidance commonly cited under Approved Document H, together with BS 8301:1985 and BRE Digest 365, puts the minimum distance from a soakaway to a building's foundations at around 5 metres, and 2.5 metres from a boundary. NHBC's own technical standards for new-build warranties independently reach the same 5-metre figure. The reason is structural: water encouraged to soak into the ground right next to a foundation can affect clay soil's behaviour under it.
What happens if a percolation test shows my soil won't drain? BRE Digest 365 treats a percolation value of roughly 12–100 seconds per millimetre as suitable for infiltration. Heavy clay commonly tests well above that, at which point a soakaway either has to be built far larger than most gardens allow, or isn't workable at all. The fallback is an attenuation tank with a flow restrictor discharging to a sewer or watercourse, typically £1,500–£4,000 or more, or a rainwater harvesting system, typically £2,500–£6,000 installed. Both cost more than a straightforward soakaway.
A soakaway sited too close to a wall, or one that's clearly not coping with the water it's asked to handle, is exactly the kind of thing a proper building survey should pick up and flag before you buy, not after you own the problem. Where the concern is a defect in the pipework feeding it, rather than the soakaway itself, that's covered by a drainage survey instead. Call 07946 618203 or get in touch for an independent read on a property before you commit to any of it.
Small print. This guide is general information, not advice on a specific site, and it doesn’t replace a proper percolation test, a BRE Digest 365 sizing calculation, or a written quote for your own property. Prices were researched and correct to the best of my knowledge on 20 September 2026; costs move and vary by region, soil conditions and access. Always get at least three written quotes, and confirm the design has been sized against your own soil, not an assumed one.