A garage sauna uses the one piece of square footage most American houses have spare. It takes the heat, the moisture and the noise out of the living space, it has a hard floor and usually a subpanel nearby, and it is the site where a big cabin becomes affordable. It also has one drawback that decides how often you use it.
Why the garage suits it
Concrete floor, room for a full size cabin, tolerance for drips and sawdust, a door wide enough to get panels through, and often a subpanel on the wall. You are not renovating a finished room, so the build is the cabin plus a circuit rather than a construction project. A bay is enough for a four seat cabin and a bench to change on.
The cold weather problem
An unheated garage in winter is a cold room to undress in and a cold walk back. That, more than anything technical, decides whether a garage sauna gets used from November to March. The fixes are a small heater for the changing area, an insulated garage door, or accepting it. Think about this before you build, not after.
Insulating the cabin, not the garage
You do not need to insulate the whole garage. The cabin is its own insulated box and will heat perfectly well in a cold bay, just using a bit more energy on the way up. What you do need is the full wall build-up on the cabin itself, with the foil vapour barrier sealed, because the temperature difference across that wall is larger than indoors.
The floor and the fumes
A concrete floor is ideal, with a duckboard or mat inside the cabin. Keep the cabin off the slab on bearers if the slab gets damp. One genuine caution: do not run a vehicle in a closed garage with anyone in the cabin, and if the garage stores fuel or solvents, move them, because a sauna is a heat source in a room you have just made airtight.
Questions people ask about garage sauna
Does the garage need insulating?
No. The cabin is its own insulated box. A cold bay costs a little more energy on heat-up and nothing else.
What is the real drawback?
Undressing in a cold garage in winter. It decides usage more than any technical factor, so plan a warm changing corner.
Is a concrete floor fine?
Ideal. Use a duckboard or mat inside the cabin, and keep the cabin on bearers if the slab is ever damp.