No principal building shall contain less than 800 square feet of usable floor space. Buildings used in whole or in part for residential purposes, exclusive of accessory buildings and exclusive of porches, entries, garages and terraces, shall contain no less than 900 square feet of usable living space if a one-story building used as a one-family dwelling, nor less than 600 square feet of usable first-floor living space if more than one story, provided that no such building shall contain a total of less than 1,000 square feet of usable living space if used as a one-family dwelling, and provided further that no such building shall contain a total of less than 600 square feet of usable living space for each one-bedroom family unit or apartment, 720 square feet of usable living space for each two-bedroom family unit or apartment and 1,000 square feet of usable living space for each three-bedroom family unit or apartment.