In every episode I've seen, there is a flight of stairs in the kitchen on the opposite wall to the enterance to the sitting room. These stairs, if they were real, would come up in the center of Sam and Darrin's room, or popping out the side of the house.

These stairs also don't even look real. The part that turns parallel with the wall, Darrin once walked down this. But the rest is obviously just a painting on the wall and looks more like a ladder than a normal set of stairs.