Floor tiles and piping work were discovered whilst excavation work was taking place on the grounds of the National Trust site, Clumber Park, near Worksop.
The original mansion was once home to the Dukes of Newcastle and the area uncovered is thought to be located between the main kitchen and the Duke’s study.
The mansion was completely demolished in 1938.
Rachael Hall, National Trust archaeologist for the East Midlands, said: “It was an unexpected find.
“We have been undertaking a programme of work to update and enhance the sewage system for the last three months.
“It had been particularly uneventful until last Thursday.
She added: “The floor is in such a brilliant condition.
“We expected that area just to be demolished rubble.”
The floor will be covered up and protected but may be open to the public to view at some point in the future.
Trevor Pressley, visitor experience manager at Clumber Park: “When we started to plan the installation of the new drainage systems it was always a possibility that we would unearth some of the archaeology left behind after the demolition of Clumber House in 1938.
“It’s been an exciting discovery for us to see such a relatively large a section of the flooring that’s remained covered up for over 70 years.
“We are currently thinking about how we might be able to display the floor to visitors and excavating the mansion site further in the future to see what other parts of house may still be waiting to be discovered.”