Mont. Code Ann. § 7-12-4191

Any special assessment made and levied to defray the cost and expense of any of the work enumerated in this part, together with any percentages imposed for delinquency and for cost of collection, shall constitute a lien upon and against the property upon which such assessment is made and levied from and after the date of the passage of the resolution levying such assessment. The lien can only be extinguished by payment of such assessment with all penalties, costs, and interest.