Before the construction of any project, and before the calling of any election for the issuance of bonds, the commission shall employ one or more engineers, or contract for engineering services with any city within the district or with the county, who shall make all necessary surveys, and prepare maps showing the location, cross-section and profile of the proposed project. The engineer shall also prepare specifications for the construction of the project and estimates of the cost of construction and of acquiring rights of way. These surveys, maps, specifications and estimates shall, upon approval by the commission, be promptly adopted by order entered upon the minutes and filed with its secretary, and when so adopted shall constitute the plan of the district for the proposed project.