RCW 35.51.020: Joint planning, construction, and operation of improvements.

A municipality may contract with any other municipality, with a public corporation, or with the state of Washington, for the following purposes:

(1) To have the acquisition or construction of the whole or any part of an improvement performed by another municipality, by a public corporation, or by the state of Washington;

(2) To pay, from assessments on property within a local improvement district or from the proceeds of local improvement district bonds, notes or warrants, the whole or any part of the expense of an improvement ordered, constructed, acquired, or owned by another municipality or a public corporation; or

(3) To integrate the planning, financing, construction, acquisition, management, or operation, or any combination thereof, of the improvements of one municipality or a public corporation with the planning, financing, construction, acquisition, management, or operation, or any combination thereof, of the improvements of another municipality or public corporation on such terms and conditions as may be mutually agreed upon including, but not limited to, the allocation of the costs of the improvements and the allocation of planning, financing, construction, management, operation, or other responsibilities.