I have a hot tub that measures 7x7 and weight is around 4400 pounds with 5 hefty people in it. I am in the process of building a new 16x40 deck and my wife wants the hot tub on it, the finished deck will be approximately 8 to 9 feet high. My thinking is to utilize this space instead of setting a post every 30 inches in a square which is what i found places on the internet have suggested. Would it be a better idea to build a 7x7 room. Dig down 4 feet which puts the footer 1 foot below frost line pour a footer lay enough 8 inch block to get back to ground level, then build a room with a 2x12 header over the door. Plywood it inside and out with 1/2 inch, and top it with 2x12s on 8 inch center and 3/4 marine plywood. Then build the rest of the deck. Will even put a rubber membrane over the top and even thought of running the 5/4 deck boards over the top of it tieing all the deck together.
Just I quick drawing the shades square is the hot tub placement area