Wash. Rev. Code § 43.21C.034
Lead agencies are authorized to use in whole or in part existing environmental documents for new project or nonproject actions, if the documents adequately address environmental considerations set forth in RCW 43.21C.030. The prior proposal or action and the new proposal or action need not be identical, but must have similar elements that provide a basis for comparing their environmental consequences such as timing, types of impacts, alternatives, or geography. The lead agency shall independently review the content of the existing documents and determine that the information and analysis to be used is relevant and adequate. If necessary, the lead agency may require additional documentation to ensure that all environmental impacts have been adequately addressed.
RCW 43.21C.034