S.D. Codified Laws § 31-13-41

After twenty days from the adoption and publication of the resolution referred to in § 31-13-40, unless the referendum be invoked, pursuant to § 31-3-14, or unless a written protest has been filed with the township clerk and signed by the owners of more than fifty-five percent of the frontage of property liable to assessment, the board of supervisors may cause the improvement to be made, may contract therefor, and may levy and collect special assessments therefor as provided in this chapter. Any proceedings taken prior to the adoption of the resolution shall be deemed ratified.