(1) This section contains criteria for determining whether an environmental document must be used unchanged and describes when existing documents may be used to meet all or part of an agency's responsibilities under SEPA.

(2) An agency may use environmental documents that have previously been prepared in order to evaluate proposed actions, alternatives, or environmental impacts. The proposals may be the same as, or different than, those analyzed in the existing documents.

(3) Any agency acting on the same proposal shall use an environmental document unchanged, except in the following cases:

(a) For DNSs, an agency with jurisdiction is dissatisfied with the DNS, in which case it may assume lead agency status (WAC 197-11-340 (2)(e) and 197-11-948).

(b) For DNSs and EISs, preparation of a new threshold determination or supplemental EIS is required if there are:

(i) Substantial changes to a proposal so that the proposal is likely to have significant adverse environmental impacts (or lack of significant adverse impacts, if a DS is being withdrawn); or

(ii) New information indicating a proposal's probable significant adverse environmental impacts. (This includes discovery of misrepresentation or lack of material disclosure.) A new threshold determination or SEIS is not required if probable significant adverse environmental impacts are covered by the range of alternatives and impacts analyzed in the existing environmental documents.

(c) For EISs, the agency concludes that its written comments on the DEIS warrant additional discussion for purposes of its action than that found in the lead agency's FEIS (in which case the agency may prepare a supplemental EIS at its own expense).

(4) Existing documents may be used for a proposal by employing one or more of the following methods:

(a) "Adoption," where an agency may use all or part of an existing environmental document to meet its responsibilities under SEPA. Agencies acting on the same proposal for which an environmental document was prepared are not required to adopt the document; or

(b) "Incorporation by reference," where an agency preparing an environmental document includes all or part of an existing document by reference.

(c) An addendum, that adds analyses or information about a proposal but does not substantially change the analysis of significant impacts and alternatives in the existing environmental document.

(d) Preparation of a SEIS if there are:

(i) Substantial changes so that the proposal is likely to have significant adverse environmental impacts; or

(ii) New information indicating a proposal's probable significant adverse environmental impacts.

(e) If a proposal is substantially similar to one covered in an existing EIS, that EIS may be adopted; additional information may be provided in an addendum or SEIS (see (c) and (d) of this subsection).