Any boardinghouse in existence in any R-1, R-2, OR-2, SR-3A, SR-3B or C-1A District as of the date of enactment of this chapter shall, at the expiration of five years from such date, or upon the death of the owner thereof, or upon the transfer of ownership thereof, whichever occurs first, become a prohibited and unlawful use and shall be discontinued, excepting, however, that this subsection shall not apply to any boardinghouse which is owner-occupied and which has received a special use permit from the Planning Board or Common Council prior to the enactment of this chapter.