California Code, Health and Safety Code - HSC § 19955.3

(a) “Story” means that portion of a building included between the upper surface of any floor and the upper surface of the floor next above, except that the topmost story shall be that portion of a building included between the upper surface of the topmost floor and the ceiling or roof above. If the finished floor level directly above a basement or unused under-floor space is more than six feet above grade for more than 50 percent of the total perimeter or is more than 12 feet above grade at any point, the basement or unused under-floor space shall be considered as a story.

(b) “First story” means the lowest story in a building which qualifies as a story and which provides the basic services or functions for which the building is used. A floor level in a building having only one floor level shall be classified as a first story, if the floor level is not more than four feet below grade, for more than 50 percent of the total perimeter, or more than eight feet below grade at any point.

(c) “Mezzanine” means an intermediate floor placed in any story or room. When the total area of any “mezzanine floor” exceeds 33 1/3 percent of the total floor area in that room, it shall be considered as constituting an additional “story.” The clear height above and below a “mezzanine floor” construction shall be not less than seven feet.

(d) “Grade” means the lowest point of elevation of the finished surface of the ground, paving, or sidewalk within the area between the building and the property line or, when the property line is more than 5 feet from the building, between the building and a line 5 feet from the building.