When the board proposes to make any special assessment under this chapter, the board or its agent, before the hearing or the mailing of written notice required under section 61-24.8-13,     shall inspect any and all lots and parcels of land that may be subject to assessment and shall determine from the inspection the particular lots and parcels of lands which, in the opinion of the board, will be directly benefited by the construction of the work for which the assessment is made and shall assess the proportion of the total cost of acquiring right of way and constructing and maintaining such improvement in accordance with direct benefits received but not exceeding such benefits against any lot, piece, or parcel of land that is directly benefited by the improvement. Property belonging to the United States is exempt from assessment unless the United States has provided for the payment of any assessment that may be levied against its property for benefits received. There must be attached to the list of assessments a certificate signed by the chairman and certified by the secretary that it is a true and correct assessment of the benefit described to the best of their judgment and stating the several items of expense included in the assessment.