Except as permitted below, any lot used in whole or part for residential purposes shall provide a minimum frontage of 20 feet along at least one public street, or shall possess an exclusive, uninterrupted private easement of access or right of way of at least 20 feet wide to a public street. There shall not be more than one single-family housing unit for such frontage or easement. A common easement of access at least 40 feet in width may be provided for two or more single-family units or for one or more duplex, two-family, townhouse, or multiple-family housing units.