Mont. Code Ann. § 17-5-103

All special assessments levied by a political subdivision must bear interest at a rate or rates determined by the political subdivision's governing body, except that the rate may not exceed the greater of 7% per annum or, in the event that the special assessments are appropriated for the payment of principal and interest on bonds issued by the political subdivision, the rate of interest on the bonds plus an additional rate of interest, if any, on assessments authorized by statute to pay the principal and interest on those bonds.