S.D. Codified Laws § 21-52-12
As to any redemptioner the right of redemption shall exist for one year after the sale of the property. The period of redemption herein limited may be extended in the manner described in §§ 21-52-13, 21-52-22 and 21-52-23 and may also be extended by a written agreement between the purchaser of the property at a judicial sale of the class described in § 21-52-1, and any redemptioner, provided such agreement is in writing, signed and acknowledged by the purchaser at such sale and recorded in the office of the register of deeds where the certificate of sale was recorded, within one year from the date of sale, or within such further period as may be provided by §§ 21-52-13, 21-52-22 and 21-52-23. When the time for redemption is extended by written contract as herein provided, it shall be extended in favor of all redemptioners whether they are parties to such agreement or not.
SDCL 21-52-12