Lots containing any building used in whole or in part for residence purposes shall abut for at least 40 feet on at least one street, unless it has an exclusive unobstructed private easement of access or right-of-way access at least 20 feet wide to a street; and there shall be not more than one single-family dwelling for such frontage or private access of easement, unless a common private easement of access at least 50 feet wide shall be provided for two or more such single-family or for one or more two-family or multiple dwellings.