Accessory uses and accessory structures on the same lot with and customarily incidental to any of the foregoing permitted uses, including the renting of not more than three rooms, without housekeeping privileges, in a main dwelling to not more than a total of four nontransient roomers or boarders, provided that off-street parking space is available exclusively for said roomers or boarders as follows: one space for each roomer or boarder having use of a motor vehicle, but in no case less than one space for each room so rented.