In every show except the pilot, there is a room in the Dunphy house off to the left of the front door. In the pilot there is a solid wall there. Also, there is a set of double-doors at the top of the stairs in the pilot. In later episodes no room upstairs has a double door.
In the scene where Dylan visits Haley at home, when Claire grabs Dylan as he's going up the stairs you can see a member of the crew in the mirror in the background behind Claire.