Clandon Park House in Surrey is to be restored to its “original glory” after being reduced to a shell by a fire last April.
The National Trust said the ground floor rooms in the stately home are to be restored, while the upper floors are to be used for exhibitions and events.
The Palladian mansion near Guildford was bequeathed to the NT in 1956 by the Onslow family.
About 95% of the Grade I-listed house was damaged during the fire.
The trust said it was confident a number of principal rooms on the ground floor, including the Marble Hall, Speakers’ Parlour and Saloon, could be restored.
A spokesman said: “The fact that so many features survived and items from the rooms are being recovered from the ashes made the case for restoration compelling.
“The enduring significance of the architect Leoni’s original designs means it will go back instead to look at the 18th Century decorative schemes and layout of the house.”
The trust said the upper floors were “less architecturally significant” and had already been altered since the house was first built.
Helen Ghosh, director general of the National Trust, said: “The fire at Clandon was shocking, but gives us the opportunity not only to show our respect for the heritage of the past, but also to create new heritage for the future.
“The loss of so many of the contents of the house means that we cannot return it to how it looked the day before the fire.
“However, we now know more about the original layout and recognise that the enduring significance of the house is its architecture.”
A spokesman said the cost of the work was expected to be met largely through the trust’s insurance policy, although it would also be asking supporters for donations.