The department may make a study of the works necessary to drain the district and prepare maps, plans, estimates and reports showing a complete system of drainage therefor, the estimated cost of the various ditches and other works and parts thereof and the boundaries of the land which will be served by and therefor should contribute to the cost of each structure. These estimates of costs shall be made on the assumption that no drainage work has ever been done in the district. In addition the department may show on the plans any drainage work which has heretofore been done in the district, may estimate the cost of those works on the same basis used for the estimate of the cost of the proposed system and in similar manner may determine the boundaries of the land served by the existing ditches.