Wash. Rev. Code § 85.06.500
Whenever it shall appear to the board of commissioners of any drainage district now organized or that may be hereafter organized under the laws of the state of Washington, that existing drainage systems or improvements are inadequate or insufficient to properly drain the lands within said district or any portion or portions thereof, such commissioners shall have the power and they are hereby authorized to construct such additional system or systems or to extend, add to, or enlarge any existing system as in their judgment is necessary. In such event the procedure for the establishment of such additional system or extension of existing system and the manner and method of the payment of the cost of construction and maintenance of the same by the assessment of the lands particularly benefited thereby, as well as the obtaining of necessary rights-of-way shall be the same as that provided by existing laws for the establishment of the original drainage system within said district. In the exercise of any of the powers herein granted it shall be immaterial whether the outlet of any of the ditches, drains, or other necessary structures or appliances are to be located within or without the boundaries of said district. This section is intended to grant supplemental and additional powers to such drainage districts and shall not be construed to limit or repeal any existing powers of such districts, nor to repeal any existing laws relating thereto.
RCW 85.06.500