in a closet
From what I've read, they seem to be build into a closet, so that you have an acutal door someplace else that you can go into and have full access to the tank. Otherwise, you need to have cabinets below and access on the side for hoses to the caniter filter that must be lower than the tank. Also you'd need some access to the top of the tank, I've seen sliding doors that push aside to allow good access, but you'd need rather a lot of room.
I have a regular tank, 59 gallons, 45" long, that is in a corner, bad idea, it is amazing how limited I am in getting to the back of the tank from the front, very hard to unplug things and get lines across the back to the outlet. I live in fear that one day a hose will pop off and I won't be able to grab it.