About fifty firefighters tackled the blaze at the three-storey building at Mount George Road in Penelewey, Feock, at about 10:30 GMT.
The roof of the building was badly damaged as a result of the fire. No one was injured during the blaze, Cornwall Fire Service said.
Six appliances were sent to the scene. An investigation is to be carried out into the cause of the fire.
A fire has destroyed a detached house near Truro.