After a board has approved by motion and certified all or parts of a comprehensive plan for a county or for any part of a county, the planning agency shall use such plan as the basic source of reference and as a guide in reporting upon or recommending any proposed project, public or private, as to its purpose, location, form, alignment and timing. The report of the planning agency on any project shall indicate wherein the proposed project does or does not conform to the purpose of the comprehensive plan and may include proposals which, if effected, would make the project conform. If the planning agency finds that a proposed project reveals the justification or necessity for amending the comprehensive plan or any part of it, it may institute proceedings to accomplish such amendment, and in its report to the board on the project shall note that appropriate amendments to the comprehensive plan, or part thereof, are being initiated.