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, except that lots containing single-family semidetached dwellings shall abut for at least 37½ feet, or have an exclusive unobstructed private easement of access or right-of-way at least 20 feet wide to a street; and there shall be only one single-family dwelling for such frontage or easement.