D.C. Code § 42-3206
Every notice to the tenant to quit shall be served in English and Spanish upon him personally, if he can be found, and if he can not be found it shall be sufficient service of said notice to deliver the same to some person of proper age upon the premises, and in the absence of such tenant or person to post the same in some conspicuous place upon the leased premises. If the notice is posted on the premises, a copy of the notice shall be mailed first class U.S. mail, postage prepaid, to the premises sought to be recovered, in the name of the person known to be in possession of the premises, or if unknown, in the name of the person occupying the premises, within 3 calendar days of the date of posting.
D.C. Code § 42-3206
Alterations to units after notice to vacate, prohibition, see § 6-731.01 et seq. .