Agencies shall prepare environmental impact statements as follows:

(1) EISs need analyze only the reasonable alternatives and probable adverse environmental impacts that are significant. Beneficial environmental impacts or other impacts may be discussed.

(2) The level of detail shall be commensurate with the importance of the impact, with less important material summarized, consolidated, or referenced.

(3) Discussion of insignificant impacts is not required; if included, such discussion shall be brief and limited to summarizing impacts or noting why more study is not warranted.

(4) Description of the existing environment and the nature of environmental impacts shall be limited to the affected environment and shall be no longer than is necessary to understand the environmental consequences of the alternatives, including the proposal.

(5) EISs shall be no longer than necessary to comply with SEPA and these rules. Length should relate first to potential environmental problems and then to the size or complexity of the alternatives, including the proposal.

(6) The basic features and analysis of the proposal, alternatives, and impacts shall be discussed in the EIS and shall be generally understood without turning to other documents; however, an EIS is not required to include all information conceivably relevant to a proposal, and may be supplemented by appendices, reports, or other documents in the agency's record.

(7) Agencies shall reduce paperwork and the accumulation of background data by adopting or incorporating by reference, existing, publicly available environmental documents, wherever possible.

(8) Agencies shall prepare EISs concurrently with and coordinated with environmental studies and related surveys that may be required for the proposal under other laws, when feasible.

(9) The range of alternative courses of action discussed in EISs shall encompass those to be considered by the decision maker.

(10) EISs shall serve as the means of assessing the environmental impact of proposed agency action, rather than justifying decisions already made.