Wash. Rev. Code § 35.56.030
Such ordinance shall specify the boundaries of the proposed improvement district and shall describe the lands which it is proposed to assess for said improvement, and shall provide for the filling of such lowlands and shall outline the general scheme or plan of such fill. If any parcel of land within the boundaries of such proposed improvement district prior to the initiation of the improvement has been wholly filled to the proposed grade or elevation of the proposed fill, such parcel of land may be excluded from the lands to be assessed when in the opinion of the city council or commission justice and equity require its exclusion. The boundaries of any improvement district may be altered so as to exclude land therefrom at any time up to the levying of the assessment but such changing of the boundaries shall be by ordinance.
RCW 35.56.030