A built-in cold plunge pool that looks as if it was always part of the terrace

What you see, and what you do not

The visible part is modest. A basin of 1.4 m, round or square, in mosaic, tile or stone, set flush with the paving or raised to a seat-height rim, with two steps you can find with your feet. A small plain display on a short post shows a single number: 4. Everything else is somewhere else. The chiller, the circulation pump and the filter sit in a ventilated niche, a plant room or a buried chamber, joined to the pool by insulated pipes.

That separation is the whole design. A cold pool that has to look good in a hotel courtyard cannot also be a machine room, and a machine that has to breathe in 38 °C air cannot be bricked in behind a pool wall. We draw the two apart first and connect them afterwards.

Shapes, sizes and the seat you did not know you needed

• Round, 1.4 m across (about 1,100 litres). The easiest shape to line in small mosaic, because the tile follows the curve without cutting, and it fits where a hot tub would not.
• Square, 1.4 × 1.4 m with a seat-height coping (about 1,200 litres). The shape that suits a straight-lined terrace. The broad coping on one side is the bench people sit on before they lower themselves in.
• Long and narrow, drawn to fit. The old irrigation tank of a finca (the alberca in Andalusia, the safareig in Mallorca) can make a very good plunge pool at 1.0 × 3.0 m with the steps at one end: about 2,400 litres and two 4 HP chillers.
• Corner and step-down variants for terraces with an awkward corner or a change of level: send the plan.

The water is about 0.9 m at the deepest point, shoulder-deep for most adults sitting on the lower step. The steps take up part of the volume, which is how a 1.4 m pool comes to about 1,100 litres.

Sizing the chiller for a 38 °C afternoon

The water is the easy part: 1,100 litres at 4 °C is a modest load. The air is the hard part. A chiller throws heat into the air around it, and on a terrace in Seville or Limassol that air can be 38–40 °C. We design the Mediterranean pools for that ambient, not for a showroom in Helsinki, and we size by the litres in the basin:

The first fill is the biggest job. Taking 1,100 kg of water from a 24 °C hose down to 4 °C means removing about 26 kWh of heat (1,100 kg × 4.19 kJ per kg per kelvin × 20 K), so the first pull-down is overnight work. After that the chiller only holds the temperature, cycling on and off, and a cover or a shade sail stops the afternoon sun doing the chiller's job in reverse.

Finishes that live with salt, sun and wet feet

Glass mosaic reads as water: sea-glass green sets the tone in a Mediterranean courtyard, slate grey suits Atlantic light. Porcelain tile is quieter and easier to replace. Stone coping (travertine, limestone, basalt) needs a non-slip finish, and the steps should grip at least as well as the coping: ask your builder which slip-resistance class applies to wet barefoot areas where you live. Near the sea, every metal part that touches the water or the salt air, such as the handrail, the anchors and the drain grate, should be 316 stainless steel.

Home pool or hotel pool: two different jobs

A private plunge pool is used by a household. A pool in a boutique hotel, a padel club or a clinic is used by strangers, in numbers that set the rota, and becomes a hygiene question as much as a cooling question. The shell can look identical; the plant behind it cannot. It needs filtration sized to the number of bathers, disinfection you can document, a plan for water changes and a cleaning rota. Read hygiene for commercial plunge pools before you write the brief for a hotel.

What we need from you before we draw anything

• Photos of the terrace from two corners, and a plan with dimensions. A phone sketch is enough.
• Where the water can drain to, and the level of the nearest drain.
• Where the machinery can go: a niche or room as close as possible to the pool, with air coming in and going out. Every metre of pipe is heat gained.
• Whether the supply is single-phase or three-phase, and the amps left at the board. A 4 HP chiller draws about 3.6 kW, roughly 16 A on a 230 V single phase.
• Access for the crates: door widths, stairs, whether a crane is possible.
• Any rules from your community, town hall or hotel brand.

The firm quote follows from the layout, not the other way round.

To put a number on it, the price list has the round and the square pool with their chillers, and the kit builder adds a cabin and shows the range for the whole circuit.