There’s a good overview in an October 5, 2006 New York Times article about ordinary homeowners who add secret rooms and hidden entrances to their homes.
Companies such as Niche Doors, the Hidden Door Company, Hide a Door, Secret Doorways and Decora Doors are all mentioned, with costs estimated at $800-$10,000 for hidden doors (not rooms).
One of the doors, a motorized bookcase, can be opened either by using a remote control or by knocking in a particular rhythm. “One time I accidentally left the remote on the other side of the door and forgot the knock code,” Mr. Sullivan said.
Turns out, Sullivan was about to get in by a “circuitious” route, a mistake, he says, he won’t “make for a long time.”
