South Korean first lady Kim Yoon-ok will be questioned over allegations of irregularities in a land deal that has implicated the president's brother and son, an investigator said Monday.

"We have decided to question first lady Kim, but we are still discussing the method and the timing of the questioning with the presidential office," assistant special counsel Lee Chang-Hoon said.

The ongoing probe by a team of independent prosecutors centers around President Lee Myung-bak's now-scrapped project to build a post-retirement home in an affluent Seoul neighborhood.

Lee's son Si-hyung and the presidential security service jointly bought 462 square meters of land last year in the junior Lee's name, a violation of South Korea's real estate law that prompted allegations of real estate speculation and tax evasion.

The 34-year-old is also suspected of purchasing the land at a below-market price by letting the security service pay more.

The scandal drew public criticism, prodding Lee to scrap his plan, though prosecutors who initially probed the case acquitted everyone involved in the deal.

First lady Kim will be accompanying Lee on a five-day visit to Indonesia and Thailand starting Wednesday. Lee's five-year stint ends early next year.