Whenever a district shall have adopted a general comprehensive plan and bonds to defray the cost thereof shall have been authorized by resolution of the board of commissioners, and before the completion of the improvements the board of commissioners shall find by resolution that the authorized bonds are not sufficient to defray the cost of such improvements due to the increase of costs of construction subsequent to the adoption of the plan, the board of commissioners may by resolution authorize the issuance and sale of additional revenue bonds for such purpose in excess of those previously issued.