Cold plunge installation: base, electrics, drainage and what it costs

Installing a home cold plunge comes down to four practical problems: a base that will carry over 500 kg, a permanent outdoor electrical supply for the chiller, somewhere for a few hundred litres of water to drain, and a position where the chiller can reject heat without recirculating it. Solve those four and the rest is finishing work.

Budget £900–£3,000 for installation on top of the tub for a straightforward free-standing plunge on a new base with a new garden circuit. Decking or a sunken installation takes it considerably higher. The cold plunge cost calculator will put a range on your specific setup.

1. The base and the load

This is the part most often underestimated. Water weighs 1 kg per litre, so:

Component	Typical weight
400 litres of water	400 kg
Shell (rotomoulded, GRP or timber-clad)	40–120 kg
Occupant	70–110 kg
Total	510–630 kg

Spread over roughly a square metre, that is around 5 kN/m². Domestic floors and decks are typically designed for about 1.5 kN/m². A standard garden deck will not take a cold plunge without being specifically built for it.

What works:

• Paving slabs on a compacted sub-base. 50 mm sharp sand over 100 mm of compacted MOT Type 1, with slabs laid to a level within a few millimetres. Adequate for most free-standing tubs and the cheapest good option.
• A concrete pad. Typically 100 mm of concrete over 100 mm compacted hardcore, sometimes with mesh reinforcement. The default for heavier or permanent installations. Allow three to seven days to cure before loading it.
• Purpose-built decking. Joists at 400 mm centres or closer beneath the tub, posts on individual concrete pads, and joist hangers rather than skew nails. Leave an access hatch for the chiller pipework and a non-slip finish on the boards — wet decking beside a cold plunge is a genuine slip hazard.

Level matters more than people expect. An out-of-level shell puts uneven stress on the structure, and a visibly tilted waterline is very hard to ignore once it is filled.

2. Siting

Get this wrong and the tub gets used for three months. Work through:

• Distance from the back door. In January, five metres of wet gravel between the tub and a towel is a real obstacle. Closer is better, every time.
• Cable run to the consumer unit. The single biggest variable in the electrical cost. Twenty metres of trenched armoured cable costs meaningfully more than five.
• Shade. Direct afternoon sun in July makes the chiller work continuously. A little shading, or a light-coloured shell, cuts running costs.
• Neighbours and bedroom windows. Chillers are not silent. See below.
• Access for delivery. Measure the tightest point of the route. A 1.6 m shell will not go through a 0.8 m side gate, and craning over a house is a £600–£1,500 line item.
• Drainage and a hose tap within reach.
• Privacy and somewhere dry for a towel.

If you are weighing a garage or utility room against the garden, our indoor vs garden cold plunge page covers the trade-offs — the short version is that indoors solves the winter walk and creates a condensation and drainage problem instead.

3. Electrical supply

A domestic chiller and circulation pump together typically draw 0.5–1.5 kW while pulling temperature down — roughly 400–600 W for a 1/2 HP unit and 900–1,200 W for a 1 HP, plus 100–300 W for the pump. That is a continuous load on a permanent connection, not something to run from an extension lead through a window.

What a compliant UK installation looks like:

• A dedicated radial circuit, commonly 16 A or 20 A in 2.5 mm² SWA cable, buried at least 450 mm deep with warning tape above it, or run in conduit.
• 30 mA RCD protection, which is not optional for an outdoor circuit near water.
• A weatherproof enclosure, IP65 or better, and a local isolator within sight of the chiller so it can be worked on safely.
• Often a small garden consumer unit if you are also feeding sauna, lighting or a socket — usually cheaper than running separate circuits later.

In England and Wales, installing a new circuit is notifiable work under Part P of the Building Regulations. That means either a registered competent-person electrician who self-certifies the work (NICEIC, NAPIT, ELECSA and similar), or a building control application. Scotland operates a building warrant regime and Northern Ireland has its own regulations. Either way, you should end up with an Electrical Installation Certificate and, where applicable, a Building Regulations compliance certificate. Keep both — they matter when you sell the house.

Indicative cost for a garden supply: £400–£1,200, rising with cable length, trenching through hard landscaping, and any consumer unit work. Our planning permission and electrics page goes into where planning rules bite as well.

4. Water and drainage

Filling is almost always done by hose from an outside tap — there is rarely a case for plumbing a plunge in permanently. If a supplier does propose a permanent mains connection, it needs appropriate backflow protection under the Water Supply (Water Fittings) Regulations, because a vessel people sit in is a contamination risk to the mains. A hose fill with an air gap sidesteps the whole question.

Draining needs more thought than people give it:

• You will empty 300–1,000 litres several times a year, plus filter backwash.
• Discharge to a foul drain via an external gully, not to a surface water drain — surface water often runs to a watercourse, and sanitised water should not go there.
• If you are draining to lawn or a soakaway, let chlorinated or brominated water stand uncovered for a few days first so the residual dissipates.
• A submersible pump (£30–£60) or a plumbed drain valve makes this a five-minute job instead of an afternoon.

5. Chiller placement and ventilation

A chiller is an air-cooled refrigeration unit. It takes heat out of the water and dumps it into the air around it, so:

• Do not box it in. Allow at least 300–500 mm of clear space on the intake and discharge faces. An enclosure needs open sides or generous louvres.
• Do not let it recirculate its own exhaust. Blowing hot air at a wall a foot away makes it work harder, cost more and fail sooner.
• Keep the pipe run short — typically within 2–3 m of the tub — to limit heat gain and pressure loss.
• Raise it off the ground on a plinth or feet, clear of standing water and leaf litter.
• Winterise it. Exposed pipework and pumps can freeze in a hard UK winter; ask the installer what the drain-down procedure is.

Noise is the neighbour issue. Domestic chillers typically sit around 45–60 dB(A) at one metre, cycling on and off. That is a fridge you can hear from the patio, and it is enough to matter under a bedroom window at 6 a.m. Excessive noise from domestic plant can be pursued as a statutory nuisance by a local authority, so site it away from boundaries, consider an acoustic screen with an open back, and avoid running the compressor overnight if the tub holds temperature without it.

6. Sunken vs free-standing

	Free-standing	Sunken
Install cost on top of tub	£900–£3,000	£3,000–£9,000
Groundwork	Base only	Excavation, spoil removal, retaining structure
Entry and exit	Steps needed	Easier, especially at height
Servicing access	Straightforward	Needs a designed access pit or hatch
Movable later	Yes	No
Extra risks	—	Groundwater buoyancy on an empty shell; drainage

The buoyancy point catches people out: an empty shell in a wet hole can lift. A sunken installation needs a land drain or sump and, usually, physical anchoring. Sunken also starts to look like a built structure, which is where permitted development limits can come into play — free-standing tubs almost never raise a planning question.

Timeline

Stage	Typical duration
Site survey and quotation	1–2 weeks
Base construction	1 day, plus 3–7 days cure for concrete
Electrical work	1 day; 2 with a long trenched run
Delivery and positioning	Half a day; longer if craned
Commissioning and first pull-down	12–36 hours to reach temperature
Order to first plunge	2–6 weeks

What an installer should be quoting for

An itemised quote should show, separately:

• Base construction — materials, spoil removal, and the specification (thickness, sub-base)
• Electrical work — cable size and length, circuit rating, RCD, isolator, certification
• Delivery, access route and positioning, including any craning
• Chiller siting, pipework, and any acoustic or ventilation provision
• Drainage arrangement
• Commissioning, water balancing and a handover walkthrough
• Making good — reinstating paving, turf or fencing

Questions worth asking

• Who certifies the electrical work, and what certificate do I receive?
• What is the chiller sized at, and what temperature will it hold in a 30°C ambient? Ask for the rating at your target water temperature, not the headline figure.
• What is the noise rating at one metre?
• How is the warranty split between shell, chiller and installation, and who do I call?
• What is the winterisation procedure?
• Is the access route confirmed, and who is liable if the tub will not fit?
• What happens to the spoil and the old paving?

Get two or three quotes and compare the base and electrical specifications, not just the total. If you are still deciding whether you need a chiller at all, chiller vs ice works through the crossover, and ice baths at home covers the no-installation route.