Fire at Cholderton House in Wiltshire tackled
Some 30 firefighters are still at the site of a Grade II listed building in Cholderton, Wiltshire
The "severe fire" at Cholderton House started at 07:22 GMT in the sitting room and spread to the top two floors, Wiltshire Fire and Rescue Service said.
Two people escaped from the house uninjured and 22 horses which were stabled at a neighbouring equestrian centre were led to safety.
The nearby A338 is expected to remain closed overnight.
A Wiltshire Fire and Rescue Service spokeswoman said at the height of the blaze there were 12 fire engines from Wiltshire and Hampshire, but this has now been reduced to six.
"Salvage from the house has been completed and firefighters are now damping down and checking for unseen fire damage.
"The fire has caused substantial damage to the entire building and there has been a significant collapse of timbers within the property.
She added: "An investigation into what started the fire will start once it is safe to do so but crews are expected to remain there for some time."
Cholderton House is a privately owned 17th Century manor house.
The nearby equestrian centre is leased by dressage rider Anna Ross Davies, who has competed for Great Britain.
She said: "To see such devastation is simply heartbreaking."