RCW 4.04.010 provides:
The common law, so far as it is not inconsistent with the Constitution and laws of the United States, or of the state of Washington nor incompatible with the institutions and condition of society in this state, shall be the rule of decision in all the courts of this state.

State v. Lutes, 230 P. 2d 786 - Wash: Supreme Court, 1st Dept. 1951 provides:
[A] person can change his name or adopt any name he may desire, provided the same is done for an honest purpose.

Washingtonlawhelp.org has this document regarding name changes, including common law

Affidavit