Before submitting a proposal to incur any bonded indebtedness, the district shall employ such engineers, economists, fiscal experts and others as is necessary to develop general plans, estimates and general specifications pertaining to the projects for which the bond issue is proposed, sufficient in the opinion of the board to enable the board to determine the feasibility of such projects, provided that no such reports or the approval thereof specified in Section 29156 of this code shall be applicable to any proposal for the issuance of additional bonds to complete the project referred to in the original reports.