The authority granted by this chapter shall be considered an alternative and additional method for converting existing overhead electric and communication facilities to underground facilities, and for paying all or part of the cost thereof, and shall not be construed as a restriction or limitation upon any other authority for or method of converting any such facilities or placing such facilities underground or paying all or part of the cost thereof, including, but not limited to, existing authority or methods under chapter 35.43 RCW and chapter 35.44 RCW.