Wis. Stat. § 72.23: Acceleration and interest.

(1) If the tax imposed by this chapter is not paid when it is due under s. 72.22, interest is due and payable at the rate of 12 percent per year from date of death. In computing time under this section, the day of death is excluded. If any payment of tax or interest under s. 72.225 is not paid when due, the tax and interest due shall be paid upon notice by the department.

(2) The department or circuit court may waive interest on any additional tax arising from the discovery of property omitted in the inventory of total assets or in the original tax determination, if due diligence was exercised in determining the assets.