Said board shall, upon the entry of the condemnation judgment, file in the same proceeding a supplementary petition, praying the court that an assessment be made upon the lands in the district for the purpose of raising an amount necessary to pay the compensation and damages awarded for the property taken or damaged, with costs of the proceedings, and for the estimated cost of the proposed improvement; and the court shall thereupon appoint three competent disinterested persons as commissioners to make such assessment. Said commissioners shall include in such assessment the compensation and damages awarded for the property taken or damaged, with legal interest from the date of entry of the judgment, and with all costs and expenses of the proceedings incurred to the time of their appointment, or to the time when said proceedings was referred to them, together with the probable further costs and expenses of the proceeding, including therein the estimated cost of making and collecting such assessment. The petitioners for the improvement shall be entitled to have included in the costs of the proceeding, and repaid to them, such reasonable sums as they may have expended in preparing the maps and plans of the improvement and procuring the names of landowners for filing with the petition. Such expenditures to be approved and allowed by the court.