Except as herein provided, no lot shall contain any building used in whole or in part for residential purposes unless such lot abuts for at least 40 feet on at least one street. In situations of hardship, a special use permit may be issued for an exclusive unobstructed private easement of access or right-of-way of at least 20 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 50 feet wide may be provided for two or more such single-family dwellings or for one or more two-family or multiple dwellings.