The policies and goals set forth in this chapter are supplementary to those set forth in existing authorizations of all branches of government of this state, including state agencies, municipal and public corporations, and counties. Any governmental action may be conditioned or denied pursuant to this chapter: PROVIDED, That such conditions or denials shall be based upon policies identified by the appropriate governmental authority and incorporated into regulations, plans, or codes which are formally designated by the agency (or appropriate legislative body, in the case of local government) as possible bases for the exercise of authority pursuant to this chapter. Such designation shall occur at the time specified by RCW 43.21C.120. Such action may be conditioned only to mitigate specific adverse environmental impacts which are identified in the environmental documents prepared under this chapter. These conditions shall be stated in writing by the decision maker. Mitigation measures shall be reasonable and capable of being accomplished. In order to deny a proposal under this chapter, an agency must find that: (1) The proposal would result in significant adverse impacts identified in a final or supplemental environmental impact statement prepared under this chapter; and (2) reasonable mitigation measures are insufficient to mitigate the identified impact. Except for permits and variances issued pursuant to chapter 90.58 RCW, when such a governmental action, not requiring a legislative decision, is conditioned or denied by a nonelected official of a local governmental agency, the decision shall be appealable to the legislative authority of the acting local governmental agency unless that legislative authority formally eliminates such appeals. Such appeals shall be in accordance with procedures established for such appeals by the legislative authority of the acting local governmental agency.