24 C.F.R. § 891.640
(2) Annual and special adjustments. If the HAP contract provides that contract rents will be adjusted based on the application of an automatic adjustment factor and by special additional adjustments:
(b) Rent for unassisted units. The rent payable by families occupying units that are not assisted under the HAP contract shall be equal to the contract rent computed under paragraph (a) of this section.
(Approved by the Office of Management and Budget under control number 2502-0371)