RCW 35.58.030: Corporations authorized—Limitation on boundaries.

Any area of the state containing two or more cities, at least one of which is of ten thousand or more population, may organize as a metropolitan municipal corporation for the performance of certain functions, as provided in this chapter. The boundaries of a metropolitan municipal corporation may not be expanded to include territory located in a county other than a component county except as a result of the consolidation of two or more contiguous metropolitan municipal corporations.