The bill to save the Wentworth Woodhouse stately home in Rotherham could reach £200 million, according to the group restoring the property.
But the Wentworth Woodhouse Preservation Trust, which bought the Georgian mansion for £7 million last year, could be as well-known as Chatsworth or Blenheim Palace in a few years time.
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Trust chief executive Sarah McLeod believes that between £150 and £200 million needs to be spent to reverse years of decay.
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Wentworth Woodhouse's 606ft facade is wider than Buckingham Palace and its floor area is about the same as the Grand Kremlin Palace in Moscow.
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It is reputed to have 365 rooms.
Ms McLeod said: "Occasionally, I find a new room I've not found before."
The current priority is to make the nearly four acres of roof watertight - a job which is being funded by £7.6 million earmarked by chancellor Philip Hammond in his 2016 budget.
Ms McLeod said the complete renovation could take more than 20 years but she is confident Wentworth Woodhouse will become a national landmark much sooner than that.
She said: "I don't think it will take anything like 20 years for that to happen, I think that will happen much sooner."
Ms McLeod said the trust was taking care not to blow Wentworth Woodhouse's trumpet too early - especially as she currently has a staff of just 15, compared with the 1,000 people who ran the house and estate in its heyday.
She said: "We need to ensure, before hoards of people come and visit us, that we're in a position to really look after them when they get here.
"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."
Ms McLeod said the masterplan for the house's future was close to completion and will inevitably involve a mix of uses for Wentworth Woodhouse - almost certainly a visitor experience, definitely an event location and probably some housing.
She said: "A big part of this process is consultation - talking to the community around us, talking to the people of South Yorkshire and finding out what they want for this site.
"After all, this is not a privately owned house any more. This is now owned by a charitable trust which, by definition, means it's owned by the nation for the benefit of the nation."
Ms McLeod added: "We've made huge progress this year, I'm really, really proud of what we've achieved."