When Reg Nash was growing up near Rotherham in South Yorkshire, he would walk along the public right of way through Wentworth Park, stop by a fence and gaze at the enormous country house across the fields.

Wentworth Woodhouse, the historic home of the Fitzwilliam family, was the biggest private residence in the UK, sitting on 23,000 sq metres (250,000 sq ft) of land and boasting the longest facade of any house in Europe – at 185 metres, twice the length of Buckingham Palace. The main part of the house dates back to the mid-18th century and it once employed up to 1,000 staff, requiring an infirmary and a dentist on site.

Nash, now 64, first saw inside the house on a rare guided tour in 2012. “It was just … wow,” he says. “The splendour and the scale of the place is just beyond anything that you can imagine. Even the stable block is huge. People see it and think it’s the house.”

From a distance, Wentworth Woodhouse looks like something from the most extravagant of period dramas, but as you get nearer you start to notice crumbling stonework and boarded-up windows. After years of neglect, the building is in a critical condition, dry rot has set in, the roof is leaking and drains have collapsed.

A derelict part of the property. Photograph: Christopher Thomond/The Guardian

Nash retired after a career in Sheffield’s steelworks five years ago and is now one of 165 local volunteers helping with an ambitious project to renovate and repurpose the house. In 2016 the Wentworth Woodhouse Preservation Trust – established after a campaign by a local business owner, Julie Kenny – bought the building from its last private owners, the Newbold family, for £7m, which was £1m under the asking price. (The architect Clifford Newbold, looking for a restoration project, had bought it for £1.5m in 1999.)

When the trust moved in to the house, the building had only a single phone line, an intermittent internet connection and a single vacuum cleaner. The government gave the project £7.6m in the 2016 autumn statement, and the chancellor, Philip Hammond, told MPs the building was said to be the inspiration for Pemberley in Jane Austen’s Pride and Prejudice – a claim that has since been disputed.

The £7.6m sum was immediately swallowed up in fixing 20% of the roof. The final bill for the restoration is expected to be about £130m, the majority of which has not yet been raised.

So far, about 100 structural surveys have been carried out, more than 100 drains have been inspected, 110 tonnes of slate has been ordered for the roof, 200 tonnes of asphalt has been put on the new driveway, and 350 tonnes of rubbish has been removed from the site. It will take an estimated 15 to 20 years to fully restore the property.

The Marble Saloon, the principal room in the house, is regarded as one of the largest perfectly proportioned Palladian-style rooms in the country. Photograph: Christopher Thomond/The Guardian

For Nash, it’s about being part of something. “Because something’s got to happen to it, because otherwise it’ll fall down,” he says.

When work on Wentworth Woodhouse is finished, the buildings will host events such as weddings (the space is big enough for multiple weddings to be held at once without the parties being aware of each other), and will offer residential accommodation, holiday flats and office space.

In the 20th century, the Fitzwilliam family’s wealth took a hit when they had to pay death duties twice in quick succession (the 8th Earl Fitzwilliam died in a plane crash in 1948 along with his lover Kathleen Cavendish, the sister of the future US president John F Kennedy) and the coal industry was nationalised in 1947.

The house sits on the Barnsley seam coalfield, and the postwar Labour government ordered that coal should be mined from opencast mines within 100 metres of the back of the building, making it an unattractive place for the family to live. The house was leased to West Riding county council in 1947 and it was used as a training college for female PE teachers until 1974 when it was taken over by Sheffield City Polytechnic, which later became Sheffield Hallam University.

Repair work in progress at Wentworth Woodhouse. Photograph: Christopher Thomond/The Guardian

For Sarah McLeod, the trust’s chief executive, the restoration of the house is not just about heritage but also about regenerating the local area, much of which is socially deprived. “We’re regenerating a site and we’re helping to regenerate a community as part of that, so it’s about creating jobs and providing training and work opportunities and work experience,” she says.

Oliver White, 23, first started volunteering at the house under its previous owners in 2012. He is now a paid “house assistant”, doing everything from helping to organise events to working with building contractors. He boasts that he has been in every one of the house’s 365 rooms.

“This place draws you in – a lot of the other members of staff say the same. It’s got a strange feeling about it,” he says. Does he think he will see the project to completion in 20 years’ time? “I hope so. I love this place.”

McLeod says: “It’s got a very patriarchal quality to it, this house. It always has and it always will have. Even though it was owned by the aristocracy and the money came from – in some ways – the blood, sweat and tears of the miners, those mining communities are incredibly proud of it. People feel like it’s theirs because it is.”