I was in a house here locally that had a whole hidden room/attic area.
They had finished the upstairs, with a big set of french doors, balcony and the like. Looked great.
There was a large mirror on the back wall, that was actually a door to the attic, which was probably twice the size of the other part of the upstairs.
They used it for storage and an exercise room, but it had a LOT of potential.
The only drawback, is that someone rummaging through a house might break the mirror for spite, and that could reveal the room behind.
I am thinking of doing the same sort of thing with my basement. My basement entrance involves a door, which you enter for about 5 foot, then the stairs go down. I thought about putting a false door at the top of the stairs, and disguising it as a closet..
RipT