The final bill to save a Grade I listed mansion is expected to reach £200m, its owners have revealed.
Wentworth Woodhouse is the first stately home in South Yorkshire to be supported by the National Trust.
Wentworth Woodhouse Preservation Trust (WWPT) bought it for £7m last year and is restoring the property and its gardens before opening to the public.
Chief Executive Sarah McLeod said she believed the total bill would be between £150m and £200m.
WWPT initially thought restoration work would cost £42m.
Ms McLeod said the renovation work could take more than two decades, but she is expecting Wentworth Woodhouse to become a national landmark in a few years.
"I don't think it will take anything like 20 years for that to happen, I think that will happen much sooner," she said.
"There's a lot of groundwork going on behind the scenes. But I think, certainly over the next three to five years, we will see a huge change in both the trust itself and how the site is seen by the public."
A £7.6m grant from the government is being used to make the roof, which is nearly four acres in size, watertight.
Ms McLeod said a public consultation would go out to decide on the mansion's future use.
A current mixed-use plan would see Wentworth Woodhouse become a visitor attraction, business centre and events space, with the possibility of housing.
Before its sale the property, hidden in countryside just outside Rotherham, claimed to be the largest privately-owned house in Europe.
The trust said it would look at raising the money needed to restore the mansion via public grants and private donations.
"We've made huge progress this year, I'm really, really proud of what we've achieved," Ms McLeod said.
"How many people get to come to work in a Grade I listed mansion every day? It's fantastic."
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.