For a very small lot, this tall hexagonal house is only 36 feet in diameter.  A large number of rooms are stacked in spiral fashion around a triangular central stairwell.  (On my plans, an unlabeled arrow on stairs points in the direction you'd go to climb up.) Each little set of steps takes you three feet higher; by the time you've made the circuit of three sets of steps, you're one full story higher than you were on the previous lap.  When appropriate, you can also use a separate set of steps along the outside wall to go from one room to the next. Of course, such a house would be inaccessible to someone who couldn't climb stairs.  My next design takes care of that difficulty.