A cash windfall of £750, 000 will give visitors the chance to see areas of South Yorkshire's historic Wentworth Woodhouse that were previously off limits to the public.
Wentworth Woodhouse Preservation Trust, which runs the Grade-I listed property just outside Rotherham, received the money from the National Trust.
It will be paid in £250, 000 installments over the next three years and the trust has also pledged a team of extra staff over the next six years.
Guided tours are currently held in certain parts of the mansion but both trusts hope to use the money to open up more of the 365 rooms and allow visitors to have freer access to the property.
The windfall will see dilapidated areas spruced up ahead of public tours.
The National Trust said the close partnership was the first of its kind for the organisation in the north of England.
Harry Bowell, director in the north for the National Trust, added: "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.”
Sarah McLeod, chief executive of WWPT, said: "We’re excited to invite the general public into this incredible property to enjoy the many exciting features.
"Support, advice and expertise from the National Trust is invaluable and will ensure we are equipped to make Wentworth Woodhouse a fantastic experience for everyone.”
The house was built by the Marquesses of Rockingham between 1725 and 1750 and contains five miles of corridors.
At 180 metres wide, its eastern front is believed to be the longest of any English country house, and the buildings is also said to have been the inspiration for Pemberley in Jane Austen’s Pride and Prejudice.
WWPT bought the property, along with its outstanding collection of classical statues and and the surrounding grounds of 83 acres, for £7 million in March. It was partly funded by a £3.5 million grant from the National Heritage Memorial Fund, with the rest of the cash coming from a range of institutional and private donors.
In recent years the Government has also pledged £7.6 million for urgent repairs to crumbling parts of the property.