Two usable parking spaces for each dwelling unit shall be required except that garage space may be counted in meeting the required number. Where the paved widths of the streets serving the development are less than the standard 30 feet wide municipal street, common off-street parking areas shall be provided to the extent of one additional space for each dwelling unit. These common off-street facilities shall be conveniently located to all residences within the development and shall have a capacity of not less than six spaces nor more than 12 spaces at any single location.