City of San Mateo Law Library
City of San Mateo Municipal Code.

27.04.055 BASEMENT.

"Basement" is that portion of a building between floor and ceiling which is partly below and partly above grade. A basement is counted as a floor (story) of a building when more than one-half the area of outermost walls of the story is above finished or pre-existing grade (whichever is lower) and if the surface of the finished floor level above is more than four feet above average grade for more than 50% of the total perimeter or more than 12 feet above grade at any point.