Wash. Rev. Code § 35.97.080
A municipality may charge property owners seeking to connect to the heating system, as a condition to granting the right to connect and in addition to the cost of the connection, such reasonable connection charge as the legislative authority determines to be proper in order that the property owners bear their pro rata share of the cost of the system. Potential customers shall not be compelled to subscribe or connect to the heating system. The cost of connection to the system shall include the cost of acquisition and installation of heating facilities necessary or useful for the connection, including any heating facilities located or installed on the property being served. Connection charges may, in the discretion of the municipality, be made payable in installments over a period of not more than thirty years or the estimated life of the facilities installed, whichever is less. Installments, if any, shall bear interest and penalties at such rates and be payable at such times and in such manner as the legislative authority of the municipality may provide.
RCW 35.97.080