The legislative body may adopt the plans and specifications furnished to it, but may not modify or change the same except with the consent of the public agency furnishing the plans and specifications. If no agreement can be reached the conduits, pipes and other appurtenances for supplying the water shall be omitted from the contemplated improvement. If the plans and specifications are adopted by the legislative body the engineer shall incorporate the plans and specifications in the plans and specifications furnished by him to the legislative body pursuant to Section 5130, and the plans and specifications shall thereupon be deemed incorporated in the plans and specifications for doing the work.