How to Build a Sauna in the UK: A Realistic DIY Guide

Building a sauna is a timber-framed, insulated box lined with untreated softwood, heated by a stove sized to the room's volume, with two vents to keep the air fresh. Most DIYers can handle the framing, insulation and panelling over several weekends. What you can't do yourself in the UK is the electrical supply for an electric heater: that is notifiable work under Part P of the Building Regulations, so a registered electrician does it. Get the volume, insulation and heater sizing right and the rest is careful carpentry.

Indoor or garden: decide first

The location decides your budget, your timeline and which rules apply, so settle it before you buy a single length of timber.

An indoor sauna goes into a spare room, a basement, a loft or a corner of a garage. You already have a roof, walls and (usually) a power supply nearby, so the build is mostly an insulated inner box. The catch is moisture: a sauna sheds damp warm air after every session, and that air needs somewhere to go other than into your plasterboard. Our indoor sauna guide covers the ready-made options if your space is awkward.

A garden sauna is a full building project: a level base, a weatherproof shell, a roof, and an armoured cable run from the house. It's more work, but it keeps the heat and humidity out of your home and planning rules rarely get in the way (more on that below). See our garden sauna guide for the barrel, cabin and pod formats.

Sizing: work in cubic metres

Heaters are rated by room volume, so every decision about size feeds straight into which stove you'll need and how much power it draws. Harvia's installation manual for its Cilindro heaters says a sauna room is usually 2,100 to 2,300mm high, and that the gap between the top bench and the ceiling shouldn't exceed 1,200mm. Heat rises and pools, so a tall room with low benches wastes energy and leaves your feet cold.

For seating, measure bodies rather than guessing. A bench around 1.5m long seats two adults side by side, and if you want to lie flat you need roughly 2m. A two-person room of 2m x 1.5m x 2.1m works out at 6.3 cubic metres, which sits right on the boundary between Harvia's small and mid-sized heaters. Keep it compact; every extra cubic metre means more heater, more warm-up time and a bigger electricity bill.

Timber: what to use inside the hot room

Everything inside the hot room should be untreated softwood with no glues, paints or varnishes, because these can give off fumes or degrade at 80°C and above. Harvia's manual specifies low-mass panel boards 12 to 16mm thick for the walls and ceiling.

• Nordic spruce. The standard, affordable cladding for sauna walls. It's light and pale and does the job, though it can weep resin from knots when hot, so pick clear boards for benches.
• Thermowood (thermally modified spruce, pine or aspen). Heat-treated to make it more stable and resistant to moisture. A sensible upgrade for garden saunas and for the exterior cladding.
• Western red cedar. The premium option, with a strong aroma and good natural rot resistance. It costs noticeably more, and a few people find the smell overpowering in a small room.

For benches, people often choose aspen or abachi because they stay cooler to the touch, but any knot-free, untreated softwood works.

Insulation and the foil vapour barrier

This is the layer that separates a sauna that reaches 80°C in 40 minutes from one that never gets there. Harvia's installation manual sets out the wall build-up from the outside in:

1. Insulation wool, 50 to 100mm thick, between the studs. Harvia says the room "must be insulated carefully" so the heater output can stay moderate. Put the thicker end of that range in the ceiling, where the heat collects.
2. Aluminium foil vapour barrier, glossy side facing into the sauna, with every seam taped using aluminium tape. Any gap lets moisture into the insulation, where it condenses and rots the frame over time.
3. A vent gap of about 10mm between the foil and the panelling (battens give you this).
4. The timber panelling. Harvia also recommends a 3mm gap between the wall and ceiling panels.

Don't use polythene as a vapour barrier in a sauna, and don't fix heat-protection boards directly onto the wall behind the stove; Harvia warns that light protective covers fixed straight to the wall or ceiling may be a fire risk. Follow the clearance distances in your specific heater's manual, because they vary by model.

Ventilation: two vents, placed properly

A sealed hot box gets stuffy fast. Harvia's manual says the air in the sauna room should change six times an hour, and lays out where the vents go for an electric heater:

• Supply vent: below or next to the heater if you're relying on natural (gravity) ventilation, or above the heater if you have mechanical extraction. The pipe should be 50 to 100mm in diameter.
• Exhaust vent: near the floor, as far from the heater as possible, with a pipe twice the diameter of the supply.

Keep the supply vent away from the temperature sensor: Harvia specifies at least 1,000mm for an omnidirectional vent, because cool incoming air fools the sensor and can cause the heater to overheat. An optional third vent high up, kept closed while you're bathing, helps the room dry out afterwards. Leaving the door open does the same job.

Choosing and sizing the heater

Pick your heater from the room volume, then adjust for any surfaces that soak up heat. Harvia's tables give a feel for the scale: its Qubic 4.5kW heater covers 3 to 6 cubic metres, the 6.8kW Cilindro 6 to 10, and the 9kW Cilindro 8 to 14. That works out at roughly 1kW for every 1 to 1.5 cubic metres of well-insulated room.

Two adjustments from Harvia's Cilindro installation manual matter more than most DIY guides admit:

• Uninsulated surfaces: add 1.2 cubic metres to your volume for each square metre of glass, brick, glass block, concrete or tile. Harvia's example is a 10 cubic metre room with a glass door, which needs the heater output of a roughly 12 cubic metre room.
• Log walls: multiply the volume by 1.5.

That glass door you fancied can push you up a heater size. Check the minimum ceiling height too: the Cilindro models need at least 1,900mm.

If you want a stove you can buy now, the Harvia Cilindro PC90XE (9kW, built-in controls, for 8 to 14 cubic metres) was £990 on Amazon UK at the time of writing. Stones aren't included, so budget for those separately.

Check the Harvia Cilindro 9kW heater price on Amazon

Wood-burning stoves skip the electrician but bring a flue, a chimney, fire clearances and log storage instead, and in a smoke control area you'll need to check what you're allowed to burn.

Electrics: this part is not DIY

Here the guide gets firm. Under Approved Document P, the government's guidance on electrical safety in dwellings in England, installing a new circuit is notifiable work. The same document defines a room containing a sauna heater as a "special location", which makes any addition or alteration to existing circuits in that room notifiable too.

In practice, use an electrician registered with a competent person scheme, who can self-certify the work and give you the certificate. Notifying building control yourself and paying for inspection is technically an option, but it doesn't make DIY wiring of a 20A to 50A heater circuit a good idea. A 9kW Cilindro on single phase needs a 50A fuse and 10mm² cable, a load comparable to an electric shower, and your main fuse or consumer unit may not cope. Book the electrician before you finalise the heater, and if they say your supply can't take 9kW, build a smaller room.

Planning permission for a garden sauna

Most garden saunas in England fall under permitted development as outbuildings, so they don't need planning permission. The Planning Portal's outbuilding rules set the limits:

• Single storey, with a maximum eaves height of 2.5m and an overall height of 4m for a dual-pitched roof or 3m for any other roof.
• No more than 2.5m high if it's within 2m of a boundary.
• Not forward of the principal elevation (the front of the house).
• Outbuildings and extensions together can't cover more than half the land around the original house.
• Anything within the grounds of a listed building needs planning permission, and designated land (conservation areas, National Parks and similar) adds restrictions.

These are England's rules; Wales, Scotland and Northern Ireland run their own systems. Building regulations approval for the structure is a separate question, covered in our garden sauna guide.

Cost and time

We won't invent a price for a DIY build, because timber, insulation and electrician quotes swing too much by region and size. Relative to a kit, though, a scratch build saves money mainly on the shell; the heater, controller, stones and the electrician cost the same whichever route you take. For real price ranges on kits, cabins and garden saunas, see how much a home sauna costs.

On time, think in weekends rather than days. An indoor box inside an existing room is the quickest job, while a garden build adds the base, roof and weatherproofing, and you'll be waiting on the electrician's diary as well.

When a kit or ready-made sauna makes more sense

Building from scratch is worth it if you enjoy carpentry, have an odd-shaped space, or want a room built to your exact size. Buy instead if:

• You want infrared, not Finnish heat. A one-person cabin like the SaunaMed infrared sauna plugs into a 13A socket, so there's no new circuit at all. Infrared runs cooler, though; our sauna temperature guide explains the difference.
• You don't trust your vapour barrier work. A pre-insulated kit takes the most failure-prone step out of your hands.
• You're not sure you'll use it. A tent or blanket costs a small fraction of a build and tells you within a month whether the habit sticks. Our best home sauna roundup covers the options.

Common mistakes

• Building too big. Extra volume means a bigger heater, a longer warm-up and possibly a supply upgrade.
• Untaped or reversed foil. Glossy side in, every seam taped, or damp gets into the walls.
• Forgetting glass in the heater sum. A glass door or window adds 1.2 cubic metres per square metre.
• Benches too low. Keep the top bench within 1,200mm of the ceiling so you sit in the heat.
• Treated timber or varnish inside. Leave the interior wood bare.
• Leaving the electrician until last. Their verdict on your supply should decide the heater, not the other way round.

Recommended reads

1. Garden sauna UK: types, costs and planning rules
2. Indoor sauna UK: types, space, power and costs
3. How much does a home sauna cost?
4. The best home sauna in the UK
5. Sauna temperature: how hot is a sauna?