DIY sauna plans are easy to find and mostly unusable, because a drawing that shows a box with a bench answers none of the questions that decide whether the finished room works. Four things separate plans worth building from plans worth looking at, and they are the four an amateur build gets wrong.
The wall build-up, layer by layer
Usable plans specify the whole sandwich: stud size, insulation type and depth, the foil vapour barrier on the warm side, the air gap, and the cladding. A drawing that shows a wall as one line is telling you nothing about the part that fails. Ask specifically where the vapour barrier sits and how it is sealed at every penetration.
Bench geometry
Bench height, depth and the gap to the wall decide comfort more than anything else in the room. The upper bench needs to be high enough to sit in the hot layer and deep enough to lie on if that matters to you, with headroom above. Plans should give the dimension from the top bench to the ceiling, not just a floor plan.
Ventilation
A sauna needs air in low near the heater and out high or low on the opposite side, and the drawing should place both. Without it the room stratifies badly, the lower bench stays cold and the cabin never dries. This is the detail most free plans omit entirely, and it is not something you can add convincingly afterwards.
Heater clearances and the electrical
Plans should show the heater model or at least the clearance envelope it needs to combustibles and to the bench, plus the guard rail. They should also show where the circuit lands and what rating it is, because the difference between a heater on a normal circuit and one needing a dedicated high current run changes the whole project cost.
Questions people ask about diy sauna plans
Are free sauna plans good enough?
Some are, but check them against the four items here. Most omit ventilation and the vapour barrier detail, which are the two that decide whether the room lasts.
What ceiling height should plans use?
Low by house standards, because the heat sits at the top. What matters is the gap above the upper bench rather than the height from the floor.
Do plans include the electrical design?
Rarely, and they should not pretend to. Use them to locate the heater and its clearances, then have a licensed electrician size and install the circuit.