A fire in the grounds of Castle Ashby House was started accidentally by workers, a fire service investigation has concluded.
The estate office building, which is very close to Castle Ashby House itself, caught fire on Wednesday, July 9.
Between 50 and 60 firefighters eventually brought the fire under control and the incident commander later said they had to battle hard to prevent the flames spreading to other buildings.
A Northamptonshire Fire and Rescue Investigation now shows that the cause was accidental.
A spokesman said: “The cause was determined as accidental. It was found that friction and sparks caused while contractors where renovating windows in the estate office building led to it catching fire.”
Insurers acting for the Marquess of Northampton, Spencer Compton, who owns Castle Ashby, as well as firefighters inspected the site over the course of three days before arriving at their conclusion.
The building had been safely evacuated following the discovery of the blaze and no-one was injured.