If an election is held to vote upon the question of issuing bonds to be paid by general taxation and if the issuance is authorized by the taxpayers, there shall be included in the amount of bonds issued all costs thereof including legal, engineering and other special costs of the improvement. Whenever a drainage district authorizes improvements to be paid for by the levy of special assessments, necessary charges for engineering, legal services and assessors may be included as a part of the necessary construction costs or bond issue therefor.