tub/shower combo with no doors/curtains I just LOVE the idea of NO tracks or doors or anything to get all gunky! I hate the shower/tub situation usually, but I could possibly deal with it if it was a sunken bath.
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Space can be smaller - along with the spa. Keep floor single level and try to keep spa flush to the floor - as well as window. Try to keep edges and walls smooth. Let the view speak in more volume than you could ever utter.