California Attorney General Opinion No. 00-205 provides:
But resort to the statutory procedure is not necessary either prior to commencing use of a new name, or afterward, for the purpose of rendering a prior name change valid. The statutory method for changing names does not repeal or displace the common law ability to change one’s name. (Code Civ. Proc., § 1279.5, subd. (a).) Accordingly, a person may change his or her name without legal proceedings simply by adopting another name and using it as his or her own. (In re Ross, supra, 8 Cal.2d at p. 609; Lee v. Superior Court (1992) 9 Cal.App.4th 510, 513-514; In re Ritchie, supra, 159 Cal.App.3d at pp. 1072-1074.)

California Civil Code section 22.2 provides:
“The common law of England, so far as it is not repugnant to or inconsistent with the Constitution of the United States, or the Constitution or laws of this State, is the rule of decision in all the courts of this State.”

California Code of Civil Procedure 1279.5 provides:
..nothing in this title shall be construed to abrogate the common law right of any person to change his or her name.