D.C. Mun. Regs. tit. 17, r. 17-3904
In any advertisement, the licensee shall include his or her license number.
All plumbing, gasfitting, electrical, or refrigeration and air conditioning work, or any combination of those services, to be performed under any contract between a property owner and a licensee, shall be performed in accordance with all of the requirements of the regulations applicable to that work, with particular reference to the use of qualified personnel (whenever required by the applicable regulations) in securing the permits and in the performance of the work.
A licensee shall maintain a list that includes information about all permits obtained and all contractors or subcontractors performing work on any project permitted or requiring a permit under this chapter. Such list shall include the contractor or subcontractor's name and address, and if applicable, their license number. If requested by the Director, the licensee shall produce this list within forty-eight (48) hours of the Director's request.
D.C. Mun. Regs. tit. 17, r. 17-3904