Wash. Rev. Code § 35.55.010
If the city council of any city of the second class deems it necessary or expedient on account of the public health, sanitation, the general welfare, or other cause, to fill or raise the grade of any marshlands, swamplands, tidelands, shorelands, or lands commonly known as tideflats, or any other lowlands situated within the limits of the city, and to clear and prepare the lands for such filling, it may do so and assess the expense thereof, including the cost of making compensation for property taken or damaged, and all other costs and expense incidental to such improvement, to the property benefited, except such amount of such expense as the city council may direct to be paid out of the current or general expense fund.
If, in the judgment of the city council the special benefits for any such improvement shall extend beyond the boundaries of the filled area, the council may create an enlarged district which shall include, as near as may be, all the property, whether actually filled or not, which will be specially benefited by such improvement, and in such case the council shall specify and describe the boundaries of such enlarged district in the ordinance providing for such improvement and shall specify that such portion of the total cost and expense of such improvement as may not be borne by the current or general expense fund, shall be distributed and assessed against all the property of such enlarged district.
RCW 35.55.010