(a) Where the amount of rent charged to and paid by the tenant is less than the legal regulated rent for the housing accommodation such rent shall be known as the "preferential rent." The amount of rent for such housing accommodation which may be charged upon renewal or vacancy thereof may, at the option of the owner, be based upon either such preferential rent or an amount not more than the previously established legal regulated rent, as adjusted by the most recent applicable guidelines increases and other increases authorized by law.

(b) Such legal regulated rent as well as preferential rent shall be set forth in the vacancy lease or renewal lease pursuant to which the preferential rent is charged.

(c) Where the amount of the legal regulated rent is set forth either in a vacancy lease or renewal lease where a preferential rent is charged, the owner shall be required to maintain, and submit where required to by DHCR, the rental history of the housing accommodation immediately preceding such preferential rent to the present which may be prior to the four-year period preceding the filing of a complaint.