The private spa at home: what it takes to build one in Marbella, what it costs to run, and whether it pays

For twenty years, the luxury Marbella villa had a fairly standard specification at the top of the market: pool, garage for several cars, staff quarters, cinema room, and somewhere in the basement a wet area with a sauna, a steam room and an indoor pool nobody swam in.

That last part has changed. Not the presence of the spa, which is now close to standard at a certain level, but what it is for. The current generation of buyers is not asking for a marble hall with an indoor pool at the centre. They are asking for something more specific and considerably more used: a room where you can get hot, get cold, and recover. The pool has become optional. The sauna and the plunge have become the point.

This has consequences for anyone building, renovating or buying. A spa designed to the old brief is expensive to run and often unloved. A spa designed to the new one can be smaller, cheaper and better used. And in both cases, the difference between a space that works for twenty years and one that quietly destroys the structure around it comes down to engineering decisions that almost nobody discusses at the design stage.

This guide covers what a private spa actually involves, without reference to any particular property, because the questions are the same whichever house you are looking at.

The term covers everything from a sauna cabin in a bathroom to a 300-square-metre lower floor. In practice, most serious installations draw from the same set of components.

Heat. A traditional Finnish sauna, running at 70 to 100 degrees with low humidity, or an infrared cabin, which operates far cooler and heats the body rather than the air. They are different experiences and serve different users. Infrared is easier to install, cheaper to run, quicker to heat and easier to fit into a bathroom; traditional sauna is what most people mean by sauna and delivers a more intense session.

Steam. A hammam or steam room, at around 45 degrees and close to saturated humidity. Structurally the most demanding element of any spa, because everything in it is permanently wet.

Cold. The component that has changed the picture. A plunge pool, chilled bath or dedicated cold tub, ideally chiller-fed rather than ice-filled. Cold immersion has moved from athletes' recovery into mainstream residential design, and it is now the element buyers ask about most.

Water. An indoor pool, whirlpool or experience shower. The indoor pool is the most expensive item on this list by a wide margin, in build cost, running cost and engineering complexity.

Recovery and treatment. A relaxation area, a treatment room for massage, sometimes red light panels or a salt room. This is where the trend has moved: dedicated recovery space rather than more wet area.

Support. Changing, showers, a WC, towel storage, and a plant room. The plant room is the component most often undersized, and it is the one that determines whether the rest can be maintained.

You do not need all of it. The most-used private spas tend to be the ones that chose three or four elements and executed them properly, rather than the ones that installed everything.

The shift is not a Marbella phenomenon; it is global, and it is well documented. Consumer interest in cold plunge activity has risen sharply since 2022. Ninety percent of luxury travellers named wellness a key factor in booking decisions in 2025, up from eighty percent the year before. Recovery-focused membership clubs have opened worldwide, built around contrast therapy suites rather than around pools, and private premium recovery suites for two to four people have become a distinct category in high-end fitness and spa design.

Residential design has followed. Infrared saunas and cold plunges are being built into primary suites and home gyms rather than into distant basements. At the very top of the international market, developments are marketing snow rooms, red-light-enhanced thermal grottos, halotherapy salt caverns and cryotherapy as headline amenities.

The practical implication for a Marbella house is that a spa no longer has to be large to be desirable. A well-designed sauna, a chilled plunge, a good shower and somewhere comfortable to sit afterwards will be used several times a week. A 200-square-metre wet floor with an indoor pool may be used monthly, and will cost considerably more to keep at temperature. When advising on renovation, we increasingly suggest the smaller, warmer, better-connected version.

This is the section that saves money, and it is almost entirely absent from articles about home spas.

Humidity, not heat, is the problem. An indoor pool is a large open water surface evaporating continuously into an enclosed space. That moisture has to go somewhere. If the room is not properly dehumidified, it condenses on every cool surface it can find: inside wall build-ups, behind tiling, on structural elements, in the ceiling void. The result is mould, corroded fixings, blown plaster, failed finishes and, over years, structural damage that is expensive to reach and repair.

The correct answer is a dedicated dehumidification and air-handling system sized to the pool surface area, not simply air conditioning. Relative humidity in a pool hall wants to be controlled in the region of 50 to 65 percent, and the air temperature should sit slightly above the water temperature, typically by a degree or two, because warmer air reduces evaporation. Get that relationship the wrong way round and the room fogs, the glass streams and the evaporation rate climbs.

The building fabric has to be designed for it. Continuous vapour barriers, correctly detailed junctions, insulation positioned to keep surfaces above dew point, and tanking to wet-area standards throughout. Retrofitting a pool into a room that was not built for one is where most failures originate, because the fabric behind the finishes was never designed to handle vapour.

The plant room is not an afterthought. Filtration, heating, dehumidification, chemical dosing, sauna control, plunge chiller and often a heat recovery unit all need space, access and servicing clearance. A cramped plant room means deferred maintenance, and deferred maintenance in a wet environment escalates quickly.

One diagnostic worth knowing. If a spa smells strongly of chlorine, the ventilation is inadequate. That smell is chloramines, the by-product of chlorine reacting with organic matter, and in a well-ventilated pool hall it is barely detectable. A strong chemical smell on a viewing is not a sign of cleanliness. It is a sign of an air-handling problem.

Marbella's sloping plots make basements natural, and the majority of private spas here sit on a lower ground floor. This suits the engineering: the structure is heavy, the thermal mass is stable, the noise is isolated, and the excavation is happening anyway.

Two conditions determine whether that space gets used or becomes storage.

Daylight. A windowless basement spa is a room people intend to use and don't. On a sloping plot, the lower floor can usually open directly to garden level on one side, or take light through a courtyard, light well or glazed stairwell. Natural light is the single strongest predictor of whether a spa is used regularly.

Sequence. Heat and cold need to be adjacent. The experience is a cycle, and if the cold plunge is across a corridor from the sauna, the cycle breaks and people stop bothering. Plan the route: sauna, shower, plunge, rest, repeat, with the whole loop walkable barefoot and wet, over drained, non-slip surfaces, without crossing anywhere you would not want water.

The related question is proximity to the rest of the house. A spa attached to the gym gets used after training. A spa three floors below the bedrooms gets used when guests come.

Most Marbella villas of a certain age were not built with a spa, and a great many now have one. The usual candidates are an underused basement, an oversized garage, a games room or a generous bathroom.

What is feasible depends on three things: ceiling height, drainage, and whether you are introducing an indoor pool. A sauna, an infrared cabin, a plunge and a good shower can be retrofitted into a surprisingly modest footprint, provided drainage and power are reachable. An indoor pool is a different order of project: structure, excavation, tanking, air handling and plant, typically only sensible as part of a wider renovation.

Bear in mind that works of this kind require the appropriate municipal licence, and that whether the project counts as minor or major works affects both the timeline and the paperwork. Anything involving structure, excavation or a change to the building's volume will be treated as major. Marbella's planning framework has been through significant change, and our guide to the new PGOM sets out what it alters for owners considering works. Establish the position before you commission drawings, not after.

A private spa is a permanent operating cost, and the range between a well-designed installation and a poorly designed one is very wide.

The main draws are heating the pool water, heating the pool hall, running dehumidification, heating the sauna, and chilling the plunge. Of these, an indoor pool at year-round temperature is by far the largest, and the one most worth engineering carefully.

What makes the difference:

• Heat pump or aerothermal systems rather than direct electric or gas. In this climate, air-source heat pumps operate efficiently for most of the year and are the standard specification on new installations.
• A pool cover, which is unglamorous and enormously effective. Most heat loss from an indoor pool is evaporative, and covering the surface when the pool is not in use cuts both the heating load and the dehumidification load at the same time.
• Heat recovery on the air-handling system, recapturing warmth from extracted air.
• Zoning and scheduling, so the sauna and the pool are not held at temperature continuously when the house is empty. Remote control is now standard and genuinely useful for owners who are not resident year-round.
• Solar generation, which pairs well with a load that is largely daytime and thermal.

The honest summary is that an indoor pool kept warm all year in a poorly insulated hall is one of the most expensive things you can have in a house, and that the same pool, covered, heat-pumped and properly zoned, is a manageable cost.

This is the question owners ask least and should ask first.

As a rule, a private spa does not return its build cost on resale. A serious installation with an indoor pool represents a substantial six-figure investment once structure, tanking, plant, air handling and finishes are counted, and the uplift in sale price is generally a fraction of that. Buyers value it, but they rarely value it at replacement cost.

Three qualifications matter, though.

At the top of the market, absence is a problem. Above a certain price level, buyers expect a wellness area, and a house without one is competing at a disadvantage. Here the spa may not add its cost, but its absence subtracts.

Quality of execution changes everything. A well-designed, well-maintained spa is an asset. A tired one with fogged glass, dated tiling and a chloramine smell is a liability, because the buyer is pricing in the cost of fixing it. There is very little middle ground.

Buying one already built is usually the better trade. If you want a spa, the economics favour buying a house where a previous owner absorbed the depreciation, rather than building your own. This is one of the clearer arbitrages in the prime market, and it argues for making a spa a search criterion rather than a renovation project.

Build one because you will use it several times a week for years. That is a good reason. Building one to add value is usually not.

• The plant room: size, access, ventilation, the age and make of equipment, and whether there is servicing clearance around it.
• The air-handling system: whether there is dedicated dehumidification sized to the pool, or whether someone has installed air conditioning and hoped.
• The smell: strong chlorine indicates inadequate ventilation, not cleanliness.
• The glass and the ceiling: condensation staining, corroded fixings, blown or discoloured plaster, and any tide-line on wall finishes.
• The tiling and grout: movement cracks, hollow-sounding areas, failed grout at junctions, all signs of substrate or waterproofing problems.
• The sauna: heater age, timber condition, and whether the cabin has been used recently or has become a cupboard.
• Service records: for pool plant, dehumidification and water treatment. Their absence is itself informative.
• The running-cost history: ask what it actually costs to keep at temperature, and compare that against the size and specification of the plant.
• Licences: confirm that any spa built after the house was completed was properly licensed and reflected in the property's documentation.

• No natural light, which is the most reliable way to build a spa nobody uses.
• Heat and cold too far apart, breaking the contrast cycle.
• An undersized plant room, which guarantees maintenance problems later.
• Dehumidification treated as an add-on rather than as the primary system.
• No cold element at all, which is now the component buyers most want.
• Treatment rooms with no water supply, which limits what a therapist can actually do.
• Hard acoustics, so a relaxation room echoes like a swimming pool, because it is one.
• Materials chosen for a dry room: untreated metals, unsuitable timbers and unsealed stone all fail quickly in this environment.

Is an indoor pool necessary for a home spa?

No, and increasingly it is not the priority. Many of the most used private spas combine a sauna, a cold plunge, a good shower and a relaxation area, with the swimming pool left outdoors. That configuration costs far less to build and to run.

What is the difference between a traditional and an infrared sauna?

A traditional sauna heats the air to around 70 to 100 degrees; an infrared cabin operates much cooler and warms the body directly. Infrared is easier to install, cheaper to run and simpler to fit into an existing bathroom. Traditional sauna gives the more intense, familiar experience.

What is the most common technical failure in a home spa?

Inadequate humidity control around an indoor pool. It damages finishes, fixings and eventually structure, and because it happens behind surfaces it is often well advanced before it is visible.

Can a spa be added to an existing villa?

Often yes. A sauna, plunge and shower suite can be retrofitted into a modest footprint where drainage and power allow. Adding an indoor pool is a much larger project involving structure, tanking and air handling, and is usually only sensible within a wider renovation. Municipal licensing applies.

How much does a home spa cost to run?

It depends almost entirely on whether there is a heated indoor pool and how it is engineered. A sauna-and-plunge suite is a modest cost. A year-round heated indoor pool without a cover, heat recovery or heat pump is among the most expensive features a house can have.

Does a home spa add to resale value?

Usually not at its build cost. It is valued by buyers, and at the top of the market its absence counts against a property, but building one purely as an investment rarely makes financial sense. Buying a house that already has a good one generally does.

What is contrast therapy?

Alternating heat and cold exposure, typically sauna followed by cold immersion, repeated in cycles. It is the organising idea behind most current home spa design, which is why the proximity of sauna and plunge matters more than the size of either.

The private spa has quietly become a better product than it was. The version that dominated a decade ago, a large tiled hall with an indoor pool, was often a room built to be shown rather than used, and it carried running costs to match. The current version is smaller, warmer, closer to the gym and the garden, built around a cycle of heat and cold that people actually repeat several times a week.

Where we would counsel care is in the assumption that more equals better. The most impressive spa we see on viewings is rarely the most used one. A sauna with a window, a cold plunge two steps away, a shower between them and a comfortable place to sit will beat a marble hall on any measure that matters after the first month.

And if the motivation is value rather than use, the answer is usually to buy rather than build. Someone else has already paid for the excavation, the tanking and the plant, and the market will not fully reimburse them for it. That is an unromantic way to look at a wellness space, but it is the accurate one.

The questions worth asking about a home spa are practical rather than aesthetic. Is there daylight. Are the hot and cold elements adjacent. Is there a dehumidification system sized to the water. Is the plant room big enough to service. Does the room connect to how the household actually lives, or does it sit at the bottom of the house waiting for visitors.

Answer those and the finishes will look after themselves. Get them wrong and no amount of stone and lighting will produce a room anyone uses, or one that lasts.

If a private spa is on your list, it is worth being specific about which version you want, because it changes the search considerably. A full wet floor with an indoor pool is a different set of houses, and a different running cost, from a sauna-and-recovery suite off a gym.