Wash. Rev. Code § 86.09.703
If funds are available the county legislative authority shall, at the expense of the county, refer the petition to the county engineer for a preliminary investigation as to the feasibility of the objects sought by the petition. If the investigation discloses that the matter petitioned for is feasible, conducive to the public welfare, consistent with a comprehensive plan of development and in the best interest of the district and will promote the purposes for which the district was organized, the county legislative authority shall so find, approve the petition, enter an order in his or her records declaring the establishment of the new boundaries as petitioned for, or as modified by him or her, and file a certified copy of the order with each county auditor, without filing fee, and with the board.
The board shall forthwith cause a review of the classifications and ratio of benefits, in the same manner and with the same effect as for the determination of such matters in the first instance.
The lands in the original district shall remain bound for the whole of the original unpaid assessment thereon for the payment of any outstanding warrants or bonds to be paid by such assessments. Until the assessments are collected and all indebtedness of the original district paid, separate funds shall be maintained for the original district and the revised district.
RCW 86.09.703