Wash. Rev. Code § 58.17.130
Local regulations shall provide that in lieu of the completion of the actual construction of any required improvements prior to the approval of a final plat, the city, town, or county legislative body may accept a bond, in an amount and with surety and conditions satisfactory to it, or other secure method, providing for and securing to the municipality the actual construction and installation of such improvements within a period specified by the city, town, or county legislative body and expressed in the bonds. In addition, local regulations may provide for methods of security, including the posting of a bond securing to the municipality the successful operation of improvements for an appropriate period of time up to two years after final approval. The municipality is hereby granted the power to enforce bonds authorized under this section by all appropriate legal and equitable remedies. Such local regulations may provide that the improvements such as structures, sewers, and water systems shall be designed and certified by or under the supervision of a registered civil engineer prior to the acceptance of such improvements.
RCW 58.17.130