Tennessee Code Title 54. Highways, Bridges and Ferries § 54-12-402

(a) If the county legislative body determines that the estimated cost of drainage of the district or road improvement district is greater than should be levied in a single year upon the lands benefited, the county legislative body may fix the amount that should be levied and collected each year, and may issue road improvement bonds of the county pursuant to title 9, chapter 21, and may devote the bonds at par, with accrued interest, to the payment of the expenses and work as it progresses or may sell the bonds at not less than par, with accrued interest, and devote the proceeds to the payment. If in the sale of the bonds a premium is received, the premium shall be credited to the improvement fund.

(b) Should the cost of the work exceed the estimate, a new apportionment of the assessment may be made and levied and other bonds issued and sold in like manner, but in no case shall the bonds run longer than twenty (20) years.