us any rent increases subsequently made or granted pursuant to Sections 151.06, 151.07, or 151.08 of this chapter and less any rent reductions required by regulations promulgated by the Commission pursuant to Section 151.08 of this chapter or imposed pursuant to Section 162.00 et seq . of this Code; provided, however, as used in Section 151.06 of this chapter, this term shall not include: (1) any increase for capital improvement work or rehabilitation work, if the rent increase was approved by the Department on or after January 1, 1981, and the work was begun prior to June 1, 1982; or (2) any increase for capital improvement work where the application for a rent increase is filed with the Department on or after October 1, 1989; or, (3) any increase for smoke detectors installed on or after January 1, 1981; or (4) any increase for rehabilitation work where the application for a rent increase is filed with the Department on or after January 1, 1999. Maximum Rent. The highest legal monthly rate of rent which was in effect for the rental unit during any portion of the month of April, 1979. If a rental unit was not rented during said month, then it shall be the highest legal monthly rate of rent in effect between October 1, 1978 and March 31, 1979. If a rental unit was not rented during this period, then it shall be the rent legally in effect at the time the rental unit was or is first re-rented after the effective date of this chapter. Where a rental unit was exempt from the provisions of this chapter under Subdivision 5 of the definition of “ Rental Units ” in this section, the maximum rent shall be the amount of rent last charged for the rental unit while it was exempt. (Amended by Ord. No. 166,320, Eff. 11/22/90.) Mobilehome Owner. A person who has a tenancy in a mobilehome park under a rental agreement. (Added by Ord. No. 180,071, Eff. 8/30/08.) Preceding Tenant . The tenant who vacated the rental unit as the result of an eviction or termination of tenancy pursuant to Section 151.09 A.9. (Added by Ord. No. 165,251, Eff. 11/20/89.) Primary Renovation Work. (Added by Ord. No. 176,544, Eff. 5/2/05.) Work performed either on a rental unit or on the building containing the rental unit that improves the property by prolonging its useful life or adding value, and involves either or both of the following: 1.