A DIY outdoor sauna is one of the more satisfying things you can build in a back yard and one of the easier structures to get wrong, because it is a small building, a wet room and a heating appliance at once. Taken in the right order it is a long weekend or two rather than a project.
The base comes first and decides everything
Level, drained and able to carry the loaded weight, staying level through frost. Compacted gravel with pavers under bearers is the usual answer; a small slab is better and more work. Get this wrong and the frame racks, the door stops closing and no amount of good carpentry above fixes it. Check setbacks from boundaries before you dig.
The shell
A conventional stud frame on treated bearers, sheathed and clad outside, insulated with mineral wool, then the foil vapour barrier on the warm side with an air gap and the interior cladding. A low ceiling over a high bench. Air in low by the heater, out on the opposite wall. A door that opens outward with no lock. None of that is difficult; all of it is specified.
Electric or wood
This is the fork. Electric means a trench, a cable and a licensed electrician, and then you press a button. Wood means a chimney, clearances, a hearth, a permit in most places and a conversation with your insurer, and then you get the heat and the ritual people build these for. Decide before you frame, because the two want different wall details.
What people underestimate
The roof, the door and the cost of good interior timber, in that order. A cheap door leaks heat and warps. Knotty bench boards are unpleasant to sit on. And the total materials bill for a well built small cabin lands closer to a barrel kit than most self builders expect, so build for the pleasure and the fit, not to save money.
Questions people ask about diy outdoor sauna
Do I need a permit?
Often for the structure and usually for a wood stove, plus setbacks from boundaries. Ask locally before the base goes down, not after.
Concrete slab or gravel?
Gravel with pavers under bearers is common and works if compacted and drained. A slab is better and more work. Either must stay level through frost.
Is building cheaper than a kit?
Less than people hope. Good interior timber, a decent door and the roof add up close to a barrel kit. Build for the fit and the doing.