Wentworth Woodhouse is to become the first stately home in South Yorkshire to be supported by the National Trust.
The Grade I listed property claimed to be the largest privately-owned house in Europe before it was sold this year.
The Wentworth Woodhouse Preservation Trust (WWPT) is planning a multi-million pound renovation before opening the mansion and gardens to the public.
The National Trust has pledged financial support and advice on how to turn it into a visitor attraction.
The WWPT bought the property for £7m, funded by grants.
It had initially thought restoration work would cost £42m but that figure has now more than doubled.
Sarah McLeod, chief executive of WWPT, said: "The original estimates were based around £42m but I think realistically... we will be looking at somewhere in the region of about £100m for the entire site.
"It's going to be a very long-term project over a number of decades I would have thought, but the really key thing over the next five to 10 years is to get a large majority of the work under way."
The government gave £7.6m towards the renovation, with the trust planning to raise the rest through grants and a public appeal. The National Heritage Memorial Fund has already pledged to contribute £3.5m.
Harry Bowell, National Trust director in the north, said: "The Wentworth Woodhouse Preservation Trust have saved an incredibly important house.
"We're pleased to be able to work in partnership with their team to open the house and grounds to the public so that everyone can enjoy it."
Built by the Marquesses of Rockingham between 1725 and 1750, the front of Wentworth Woodhouse is more than 600ft (180m) long. The main house has more than 200 rooms.
It was bought in 1999 by architect Clifford Newbold, whose family sold it after he died in April 2015.
The staterooms and gardens are to be opened to the public, with the ancillary parts of the building and grounds converted into a business centre and events space.