Fifteen fire appliances are attending a severe fire at Sydenham House at Lewdown in Okehampton.
The historic building's roof is involved in fire and 50 per cent of the first floor.
Crews were called to the scene at 4:02pm and are using six jets, one hose reel jet, and water from a river and lake adjacent to the four-story househouse, and carrying out salvage.
Crews at the scene include Tavistock, Okehampton, Greenbank, Plympton, Yelverton, Bere Alston, Ashburton, Hatherleigh, Crownhill, Bovey Tracey, Chagford, Holsworthy, and several from Cornwall.
A spokesperson for the fire service said part of the building is at risk of collapse.
The building has been evacuated and everyone has been accounted for.
Road closures are in place in the area.