Every building used or designed to be used for residence purposes shall be built upon permanent foundation walls and shall occupy a minimum building area of 900 square feet for a structure of one story in R, B and I Districts; and such buildings shall occupy a minimum building area of 900 square feet for a one-and-one-half-story building in R, B and I Districts; and such buildings shall occupy a minimum building area of 750 square feet for a two-or-more-story building in R, B and I Districts.