A fire broke out at about 6.45am in one section of the Tudor building, with 10 crews from around Berkshire called to the scene.
A family of four including two young children were evacuated and were unharmed, along with a maid.
Crew leader Paul Jacques, from Wokingham, said about 30% of the building has been severely damaged.
He said: "The fire broke out on the ground floor and spread vertically to the first floor and into the roof.
"Initial crews quickly called for six pumps and subsequently 10.
"The main operations have involved stopping the fire spreading to the main residence.
"Firefighters have been using breathing apparatus and hose reels for the inside of the building.
"Conditions were extremely hot and dangerous."
An aerial appliance was also used to tackle the roof fire from the outside.
Three ambulances were called to the scene as a precaution.
A fire investigation officer is on scene to determine the cause of the blaze.
It is understood the family has returned to London following the incident.
A representative of the family refused to give any further details.