Lot Area Per Family. Every building or portion of building erected, moved or altered for residential purposes in the "LLO" District, zoned District "B," shall provide a lot area per family of not less than three thousand five hundred (3,500) square feet, in the case of one-family dwellings, two thousand five hundred (2,500) square feet per family in two-family dwellings, and not less than one thousand (1,000) square feet per family in apartment house as specified