A sauna in bathroom is a sensible pairing for an obvious reason: the shower you want between rounds is already there, and so is the extract fan and often a floor that can take water. What limits it is almost always floor area, and what people underestimate is the interaction between two humid rooms sharing one extract.
Why it is a good site
The cool rinse is a step away, the floor is already tiled or vinyl, there is a drain if anything spills, and a bathroom is already a room you are comfortable being undressed in. In a renovation the extra cost over a bare room is small, and a cabin against one wall of a large bathroom looks deliberate rather than parked.
Ventilation, for both rooms
The cabin needs its own air path in and out, and the bathroom needs to clear the humidity both the shower and the cabin produce. One small extract fan struggling with both is how bathrooms with saunas end up with condensation on the ceiling. Size the extract for the room and run it after sessions, not only during.
The circuit
Bathroom electrical rules are stricter than elsewhere in a house, with zones around the bath and shower governing what may be installed where. A sauna heater's supply and its control have to respect those zones as well as the heater's own requirements, so this is a licensed job with two sets of rules, not one.
Where the space runs out
A one or two seat cabin needs roughly the footprint of a large shower enclosure plus clearance and a door that swings out. In a typical family bathroom that is the space the bath occupies, which turns the project into a choice rather than an addition. Measure the cabin plus the door swing before assuming it fits.
Questions people ask about sauna in bathroom
Can you put a sauna in a bathroom?
Yes, and it is a good site: the rinse is a step away and the floor already handles water. Space and the electrical zones are the constraints.
Will it make the bathroom damp?
Only if the extract is undersized for both the shower and the cabin. Size it for the room and run it after sessions, not only during.
Are the electrical rules different?
Yes. Bathroom zones govern what may be installed where, and they apply on top of the heater's own requirements. It is a licensed job.