(a) When assessments of benefits are made in drainage and other improvement districts, the landowners shall have the privilege of paying the assessment in full within thirty (30) days after the assessment becomes final. However, the assessments shall be made payable in installments so that not more than twenty-five percent (25%) shall be collectible in any one (1) year against the wishes of the landowner. In the event that any landowner avails himself of this indulgence, the deferred installments of the assessed benefits shall bear interest at the rate of six percent (6%) per annum and shall be payable only in installments as levied.
(b) The levy of the assessment may be made by way of proportional amounts of the total assessed benefits. Interest need not be calculated until it is necessary to do so to avoid exceeding the total amount of benefits and interest.
(c) The amount of interest which will accrue on bonds issued by districts and subdistricts shall be included and added to the tax, but the interest to accrue on account of the issuing of the bonds shall not be construed as a part of the cost of construction in determining whether or not the expenses and costs of making the improvements are, or are not, equal to or in excess of the benefits assessed.