I have a room behind mine. While it is nice it, is not large enough at about 10x6'. I would say the minimum sump/fish room would be around 10x10' when your dealing with a 6 to 8 foot long tanks, any less and you're cramped for space. The next house will have this planned out ahead of time.
. I just built a new house and getting a room built for a tank is not easy unless you go with a custom builder (which most are not, and those that are charge huge premiums). I did manage to get them to reinforce my floor under the tank, but a fish room/closet in a non-typical place is super hard to come by unless you are willing to sacrifice a normal room
