A basement sauna is usually the right answer for an indoor cabin: the space is already cool and private, the floor is hard, the electrical panel is often a few feet away, and nobody upstairs has to look at it. Four things decide whether yours works, and the first one is not about saunas at all.
Damp comes first
Do not build a sauna into a basement with a moisture problem. A sauna adds humidity in bursts and needs to dry between sessions, and a space that is already struggling will not. Check for efflorescence, musty smell and a damp meter reading on the slab and walls, and fix the cause before you frame anything.
Headroom, which is an advantage
Basements are often lower than the rest of the house, and for a sauna that is good: heat sits at the top and a low ceiling over a high bench is what you want. What you need is enough height for a proper upper bench with a modest gap above it. Measure from the finished floor, allowing for any raised platform.
The panel and the circuit
The main advantage over a bedroom cabin. A dedicated circuit for a traditional heater is a short run in most basements, which puts real traditional saunas within reach of a normal budget. Confirm the panel has spare capacity at the rating the heater needs, not just a spare breaker slot.
Ventilation and the floor
A basement sauna needs an air path in and out like any other, and the room it vents into should itself be able to dry, which in a sealed basement may mean running the dehumidifier after sessions. A hard floor with a slight fall to a drain is ideal; a duckboard over sealed concrete is the common and perfectly good alternative.
Questions people ask about basement sauna
Is a basement a good place for a sauna?
Usually the best indoor site: cool, private, hard floored and close to the panel. The one disqualifier is an existing damp problem.
Is low basement headroom a problem?
Generally an advantage. Heat sits at the top, and a low ceiling over a high bench is what a sauna wants. Check you still fit a proper upper bench.
Do I need a floor drain?
Ideal but not required. A duckboard over sealed concrete works, provided the room can dry between sessions.