Yanyong Kangkan, director of the 8th Forest Management Office (Nakhon Ratchasima), said the buildings, which are connected to land legally owned by former DSI chief Tarit Pengdith, encroach on the foothills of Pa Khao Sied-Ar national forest reserve.
Staff will pull the buildings down tomorrow, he said.
Mr Tarit and his wife earlier presented a document identifying themselves as owners of the land next to the buildings.
They denied owning the attached buildings even though they are connected to the land, said Mr Yanyong.
A notice was placed on the buildings in April, seeking to identify the owners, but no one has come forward.
"It's a common practice for the department to seek the owners first. If no one shows up to declare ownership, we can demolish the buildings, according to section 25 of the National Forest Reserves Act," he said.
About 40 forest officials will start to bulldoze the illegal buildings in Pak Chong's Ban Nong Nam Daeng tomorrow.
Activist Tawan Srikanin said his alliance and former senator Prasarn Maruekapitak will witness the operation.
He said he would also file a charge of forest land encroachment against Mr Tarit's wife, forcing her to prove the buildings don't belong to the family.
Previously, the Royal Forest Department had denied that Mr Tarit's family was involved in any land encroachment, insisting all buildings on his property were legal.
However, the department failed to specify whether the two attached buildings were legal or not.