Wash. Rev. Code § 35.21.706
Every city and town first imposing a business and occupation tax or increasing the rate of the tax after April 22, 1983, shall provide for a referendum procedure to apply to an ordinance imposing the tax or increasing the rate of the tax. This referendum procedure shall specify that a referendum petition may be filed within seven days of passage of the ordinance with a filing officer, as identified in the ordinance. Within ten days, the filing officer shall confer with the petitioner concerning form and style of the petition, issue the petition an identification number, and secure an accurate, concise, and positive ballot title from the designated local official. The petitioner shall have thirty days in which to secure the signatures of not less than fifteen percent of the registered voters of the city, as of the last municipal general election, upon petition forms which contain the ballot title and the full text of the measure to be referred. The filing officer shall verify the sufficiency of the signatures on the petition and, if sufficient valid signatures are properly submitted, shall certify the referendum measure to the next election ballot within the city or at a special election ballot as provided pursuant to RCW 35.17.260(2).
This referendum procedure shall be exclusive in all instances for any city ordinance imposing a business and occupation tax or increasing the rate of the tax and shall supersede the procedures provided under chapters 35.17 and 35A.11 RCW and all other statutory or charter provisions for initiative or referendum which might otherwise apply.
RCW 35.21.706