We approach the famous ruins through a curtain of rain. Through native woodland, rich and sluiced with cold water from heavy grey skies. Then, over a barricaded bridge, like something from a war, and through a muddy track, past more gates, and then, out of the watery haze, the ruin looms: Scotland’s finest modernist building, St Peter’s Seminary, which is on the threshold of transformation.

The story of this remarkable building, near Cardross, Dunbartonshire, is known: St Peter’s, designed and built by late Scottish architects Andy MacMillan and Isi Metzstein, was completed and consecrated in 1966 after five years of building. But the seminary - leaky, draughty, otherworldly, wildly ambitious - was only used for 13 years. It closed in 1980 and was used for a few years as a drug rehabilitation centre.

After, the buildings - massive piles of intricate concrete, huge volumes of weight that, even today in the abandoned central chapel space, seem to float on air - fell into dereliction. The older Kilmahew House, at the centre of the site, was ablaze twice before being demolished in 1995. The seminary, called the greatest 20th century building in Scotland, had been given Category A status in 1993. The World Monument Fund scheduled it as one of the world’s most endangered cultural landmarks.

Over the many years of its disuse, the site had lain in gradually worsening ruin. It was covered in graffiti and the subject of arson and vandalism. Like an archaic ruin, its features became obscured by nature, by mud and detritus. In that time, various plans for the building and its surrounding 40 hectares of woods were raised and came to nothing. Before the financial crash of 2008 it seemed as if the Church, which still owned the building, would sell it to developers who wanted to make it into a plush hotel.

However, in 2011, the public arts organisations NVA, led by Angus Farquhar, took over the entire site on a conditional basis from the Catholic Church. It is now, essentially, theirs. Huge work, costing around £10m, is being done on the ruined building so that it becomes one of the most extraordinary venues for public arts, walks, education, performance and learning.

In 2018, the reborn St Peter’s will officially be re-opened to the world. In the meantime, next year will see NVA staging Hinterland, the first public event at the site.

Farquhar, who leads us around the building on this sodden day, is clearly enthralled by the building and its surroundings.

"I have brought many artists here from all manner of disciplines, some of the artists producing some of the best work in Scotland, and for every artist, every theatre director, opera director, composer, artist - it is an incredibly inspiring place," Farquhar says.

"Imagine giving this to the National Theatre of Scotland for a month. Imagine the productions they could stage here, it is a place that will inspire new work. What's brilliant is that the building will force you to make work in new ways. It will force you to respond to it, it takes no prisoners."

He adds: "You will have to use what is here - the sightlines, the different levels, its openness. You cannot stage things in a conventional way. So for me it is a place for new public art. And by public art, I mean not the kind of conceptual cul de sac I see a lot of gallery work stuck in - it's work that useful for society, useful for the evolution of the site."

Farquhar has many angles on the site and it potential. One is political. "I want to do work that reflects the new politics of Scotland. That is borne out of a progressive mind set.

"I could see this as the site for an alternative parliament, as they have in Iceland - where you can really debate the issues with wider society. Here would could have debates about what society we really want to live in, how can we work together to make a better world."

When we visit, the rain is constant. Paths are drowned in places, and to cross the moat, we teeter on the bridge’s parapet. The main building is windowless and much of its wood has gone. The gaping eyes of its many windows and its many shadows and dark graffiti give the building a sense of both physical and existential danger. But the graceful climb from the crypt to the chapel is still a beautiful piece of architecture, and the central chapel is a theatrical space ready for upgrade and performance.

In the dark rain, the smashed altar of the ruined chapel posits a post-apocalyptic scene amid the trees. The altar was not smashed by vandals: before the seminary was handed over to the care of NVA, the Church sent a man with a jackhammer to break it up, in case it was used for unholy purposes. Now, oddly, its table top fallen away in several pieces, its crucifix-shaped base has been exposed and stands as a unviolated Christian symbol amid the disarray.

The saving of the building is a constantly evolving, challenging business. When we visited, Farquhar had just sanctioned £160,000 of emergency work to save some of its architectural details from falling through.

“It’s weird the transitions it has been through,” he says, “it has lived twice as long as a ruin as it had as a working building. It is famous for being a ruin. Now, that someone like us can get our hands on it, that’s fantastic: buildings can be reborn.”

Farquhar's strategy- who sees this project as his group’s biggest and perhaps defining project - is not to rebuild or refashion the seminary. Instead they are conserving much of it as a ruin: cleaning it up, stabilising it, revealing its decades-hidden architectural details - lines of metal in the paving here, a slope and carefully fashioned concrete steps here, the moat - making it safe and slowly but surely breathing new life into its frame.

NVA’s plan for a “consolidated ruin” is being developed in partnership with Avanti Architects, ERZ Landscape Architects and Nord. The chapel will become a 600-capacity performance and events space. There will be teaching spaces and restored woodland paths. Eventually, Farquhar says, people will be able to stay and work there.

So only a part of NVA’s plan will be introduced to the world in next March’s Hinterland project, as part of Scotland’s Festival of Architecture. It will heavily involve the surrounding land - the semi-ancient woodland of Kilmahew, ‘the chapel of St Mahew’.

Hinterland will, Farquhar hopes, play to 10,000 people - between 800 and 900 people a night.

The buildings will be animated by monochromatic light installations and a choral work by composer Rory Boyle, performed by St Salvator’s Chapel Choir, from St Andrew’s University. Phil Supple will be lighting director, and installations will be created by the 85A art collective. It will be launched almost 50 years to the day that the seminary opened, from March 18.

What is Hinterland? Apart from a "very, very beautiful piece of music".

"I see it as a kind of open manifesto. Firstly: it is about the building in the landscape - do not obsess about the building in isolation,” Farquhar says.

"It is the tension and the juxtaposition of this bucolic land - if you come in spring when the gorges are running with water, the bluebells are out, it is a very beautiful place - with a late brutalist building dropped into the middle of that, not apologising for itself on any level.

"Our start is to introduce people to that. And for the building itself? Just to reveal what is there. We’ve said right along this is not about a finished work, but it is just as important as what will happen in 2018."

Farquhar is clear that, whatever NVA bring to the seminary, it will not be a complete resurrection. Instead it will find new life in its jagged but still beautiful ruin.

He says: "We are not attempting to turn the clock back. Its the transition through ruination. It's an understanding also that not all the ideas did not work. The Church has receded. Institutional certainty has gone from many areas of life: we live in uncertain times, and that is reflected here. And often it is artists who poke about in the ashes - and rebuild things."

Hinterland

Near Helensburgh, Argyll & Bute, Scotland

March 18 to 27, 2016

Tickets on sale now at www.hinterland.org