A couple of years ago, I designed and built a home for a friend of mine in the San Francisco Bay area. The original plan had been to build a small office a few steps up from the second-floor hall, but during construction we raised the floor even more to take advantage of the view. Suddenly there was all this dead space below the room.

Not wanting the area to go to waste, my friend suggested turning it into a cool little playroom for his kids. Since he wanted it to be a surprise, we decided to access the room through a secret door. We considered using a fake bookcase or a hidden wall panel, but ultimately settled on something much more unusual — a hinged set of stairs like the...

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