Wash. Admin. Code § 208-630-545
(1) You may add a trade or "DBA" name to your license if you first apply to the department, in a form prescribed by the department, and receive department approval. When the department has approved the trade name, you must conduct business under that trade name in at least one of the two following ways:
[Statutory Authority: Chapter 43.320 RCW, RCW 31.45.200. WSR 16-10-046, § 208-630-545, filed 4/29/16, effective 6/1/16. Statutory Authority: RCW 43.320.040, 31.45.200, and 2009 c 510. WSR 09-24-089, § 208-630-545, filed 12/1/09, effective 1/1/10.]