Wis. Stat. § 893.72: Actions contesting special assessment.

An action to avoid any special assessment, or taxes levied pursuant to the special assessment, or to restrain the levy of the taxes or the sale of lands for the nonpayment of the taxes, shall be brought within one year from the notice thereof, and not thereafter. This limitation shall cure all defects in the proceedings, and defects of power on the part of the officers making the assessment, except in cases where the lands are not liable to the assessment, or the city, village or town has no power to make any such assessment, or the amount of the assessment has been paid or a redemption made.