The executive director or his designated representative may inspect a district construction project at any time. When individual contracts for construction are substantially complete, the engineer for the district will notify the agency's regional office of date and time of final inspection. The engineer will conduct, in company with the owner or his representative, a final inspection of the work for conformance with the design concept and compliance with the contract documents. The district shall not accept the project or release the statutory retainage on partial payments until work is determined to be in substantial compliance with plans and specifications as approved by the executive director.