Somsak, a key figure in the party and a former education minister, said he accepted the court’s verdict Thursday.
Public prosecutors last year filed the case, asking the court to seize the ex-party list MP’s property, which he had failed to declare when he was a minister.
The house was built in 1998 in his hometown in Angthong province.
The National Anti-Corruption Commission ruled against him with a majority vote in response to his assets declaration as a political-office holder last year, before public prosecutors proceeded with the case.