pre-pour - traditional sauna + cold plunge

Slightly better the a total newb. Planning a new house. They are pouring the foundation. I am think of adding a floor drain from the bathroom rough-in ("x" on the picture) to the sauna plunge space ("o" on the picture). What else? I know ventilation is a concern and am planning on getting the books around christmas. But the pour will be first. What else should I consider. The build will be after the unfinished basement (and the house) is built. Planning a traditional sauna, don't like infrared. Ever commercial space is without water so that's all I know. Probably a four person size, even though it will probably just be me. Not a barrel, in the basement.