D.C. Mun. Regs. r. 14-4202
For any rental unit previously excluded from coverage under the Act by § 205(e) of the Act and § 4105 of this title, the initial, lawful rent shall be determined in accordance with this section upon the occurrence of any event that causes the unit to lose its exclusion and come under the provisions of the Act; provided, that the unit is not otherwise exempt from the Rent Stabilization Program pursuant to § 205(a) of the Act and § 4106 of this title.
A housing provider of a rental unit previously excluded from coverage of the Act and not claiming an exemption from the Rent Stabilization Program shall file a Registration/Claim of Exemption Form in accordance with § 4101 within thirty (30) days of the event that causes the unit to lose its exclusion. If a tenant occupies the unit at the time the unit loses its exclusion, the Registration/Claim of Exemption Form shall state the amount of rent lawfully determined in accordance with § 4202.3. If the unit is vacant at the time it loses its exclusion, the Registration/Claim of Exemption Form shall state the anticipated rent in accordance with § 4202.3; provided, that if the rent charged upon occupancy is different, the housing provider shall file a Certificate of Rent Adjustment within thirty (30) days of the first date the actual rent charged is due.
The initial, lawful rent that shall be the basis for future rent adjustments for a rental unit described in § 4202.1 shall be the amount of rent charged to a tenant for the rental unit during the first month of the rental period in which the unit is occupied following the event that caused the rental unit to lose its exclusion.
D.C. Mun. Regs. r. 14-4202