No lot shall contain any building used in whole or in part for residence purposes unless such lot abuts for at least twenty feet on at least one public street, or unless it has an exclusive unobstructed private easement of access or right-of-way of at least twenty feet wide to a street; and there shall be not more than one single-family dwelling for such frontage or easement, except that a common easement of access at least fifty feet wide may be provided for two or more such single-family dwellings or for one or more two-family or multiple dwellings.