A 1.5 story house is where the height of any of the walls on the second floor are less than the height of the walls on the first floor. First floor walls that extend up to the second floor level are not included in the height comparison. Depending on your State, Provence or other local regulations there may be a specific height difference required. For example some areas required that the second floor walls be less than 70% the height of the first floor walls in order to be called a 1.5 story structure. Other areas are 50% and it is clear that this may vary substantially from region to region. Another common trait is angled ceilings that compensate for the short walls making all but the edges of the room high enough for normal use.