Lot Area Per Family. Every building or portion of building hereafter erected or structurally altered for residence purposes in District "R-2" shall provide a lot area of not less than six thousand (6,000) square feet per family in single-family dwellings and five thousand (5,000) square feet per family in two-family dwellings; provided, that where a lot has less area than herein required in separate ownership, but not less than five thousand (5,000) square feet as of January 9, 1961, this regulation shall not prohibit the erection of a one-family dwelling. Where a public or community sewer is not available and in use for the disposal of all sanitary sewage, each lot shall provide not less than fifteen thousand (15,000) square feet per family.