D.C. Mun. Regs. tit. 17, r. 17-2522
Any notice required by § 2520 or § 2524.1 may be served either personally by the Director or by certified mail, return receipt requested, directed to the person concerned at his or her last known residence or business address as shown by the records of the Department.
If notice is served personally, it shall be deemed to have been served when delivery is made to the person concerned.
If notice is served by certified mail, it shall be deemed to have been served on the date borne upon the return receipt showing delivery of the notice to the person concerned or refusal of the person concerned to receive notice.
If the person concerned is no longer at the last known address as shown by the records of the Department, and no forwarding address is available, the notice shall be deemed to have been served on the date the Department received the return receipt bearing that notification.
D.C. Mun. Regs. tit. 17, r. 17-2522