In the early hours of Sunday morning, smoke and the sound of fire meant that Cornwall Fire and Rescue Services were called to Harrowbarrow, near Callington.
A unit from from Callington was sent to the incident where firefighters found an outbuilding well alight.
The Incident Commander called for a further crew from Callington and one from Saltash to be sent to the incident.
Crews wore breathing apparatus and used a hose reel to extinguish the fire.
The cause of the fire is thought to have been accidental.