I think the sets all occupied only one floor. But the must've utilized more for such things as storage, dressing rooms, etc. But then, maybe not. The actors/actresses and other personnel all mentioned what cramped conditions they worked in. The door under the staircase in the Collinwood foyer led to a dressing room, hence the famous blooper of Jonathan Frid passing through the door and cutting across the set, costume in hand, during the rolling of the credits.
Gerard