Notwithstanding the provisions of section 43-701, Idaho Code, whenever the cost of making and collecting assessments against residential tracts, one (1) acre or less in size, is determined by the board of directors to be burdensome on the district and on the owners of such lots, the board may assess all such tracts at a flat rate, to be determined by dividing the total amount assessable against all such tracts by the total number of such tracts, and the flat rate assessments thus determined may be certified to the county officers as specified in section 43-701C, Idaho Code, for collection or may be collected by the district in substantially the same manner that other assessments are collected pursuant to this chapter. The board, in the reasonable exercise of its discretion, may establish the maximum lot size for flat rate assessments at any size within the range between one-fourth (1/4) acre and one (1) acre. Tracts included or retained in an irrigation district for drainage purposes only shall be assessed only for drainage costs and may be certified to the county officers on a separate list. The board may, in its discretion, exclude from the flat rate assessment procedure any tracts upon which liens for construction costs exist under the provisions of sections 43-328 through and including 43-330, Idaho Code.