The most frequently used method of annexing territory is by petition of property owners of at least 60 percent of the property value in the area (under RCW 35.13.125). This method is initiated by a property owner's written notice to city council announcing their intent to annex into the city.
The city council may initiate annexation proceedings by resolution (under RCW 35A.14.295) when there is an unincorporated area:
The city council may annex unincorporated territory, within the urban growth area (UGA), pursuant to a jointly approved interlocal agreement between the city and the county. Approval of such an agreement by special service districts such as fire protection districts, water-sewer districts and transportation benefit districts, within the proposed annexation area, is required.