Manchester's gallery in the park is re-made by Scots

Tomorrow the assignation will be fulfilled when the parkland-set Whitworth Art Gallery re-opens after £15m worth of re-development and 18 months work. It is the latest phase of a story that began in 1889, when this precursor of Glasgow's Burrell, with its vast collection of textiles, watercolours, prints and sculpture, was the first English gallery in a park. The new extension on the back of the building has doubled the amount of public space without the same increase in the building's footprint on the environment around it, and the journey from its impressive Victorian facade through a typically 1960s earlier addition to the new "back door" entrance from the park is through some of the most impressive gallery space anywhere in the UK. It is the work of a trio of architects who trained at the Mackintosh School at Glasgow School of Art in the 1980s.

Scots, or artists who live and work in Scotland, are very much to the fore even before you enter the building. Among the artworks outside is a new version of Christine Borland's Hippocractic Tree, which had its origins in her work with the medical faculty at Glasgow University. Nearby sits the aluminium tubing of Jacqueline Donachie's Crawfurd Heights, named after a Glasgow towerblock. Above the rear entrance ,Nathan Coley's illuminated sign Gathering of Strangers has found a permanent home and welcomes newcomers in. The Whitworth's dynamic catalyst of a director Maria Balshaw, has not only chosen many Scottish artists to showcase in her new building, she has also echoed Borland's practice in partnering Cornelia Parker with Nobel Prize-winning and Manchester University-based inventor of graphene Kostya Novosolev for elements of the major exhibition for the reopening.

But the chief transformation at the Whitworth is in the building itself, the design for which was the subject of a competition that attracted 139 entries, and where the longlist of ten included names like Zaha Hadid, Amanda Levete, and Edward Cullinan. It was won by MUMA (McInnes Usher McKnight Architects), whose best known work previously had been for the V&A in London, with a proposal that defied the instruction to extend on to a greenfield plot next to the existing building.

Stuart McKnight, who is from Callendar, met his Glaswegian partner Gillian McInnes at Glasgow School of Art in 1983 and Simon Usher when he came to the Mackintosh School to complete his Diploma in 1987. All three moved to London for their first jobs and then joined forces in their own practice in 2000.

"All our work has come from winning competitions," said McKnight on the eve of the Whitworth opening. "There are only ever nine to 12 of us in the practice, working on just three or four projects at one time."

Of these, the acclaimed new Medieval and Renaissance Galleries at the V&A, a £30m undertaking which opened in December 2009, has been the largest and most high profile completion yet, but McKnight thinks MUMA has achieved more with half the budget in Manchester.

"The cultural and social consequences here are beyond what we did at the V&A," he says. "The V&A was not finished when we were selected, so it was a leap of faith to choose us. And we didn't use the site specified but went for a different strategy that built on servicing the three floors of the existing building, using the existing framework."

The plan reclaimed the Victorian Great Hall from poor use, subdivided as offices and storage, and removed a low ceiling and back wall to create a vast light central gallery. A new glazed promenade, finished in the finest materials but using all of the red brick of the old building, was wrapped around the back of the structure, with extension arms to accommodate a new cafe and study spaces. McKnight says the plan was cost-effective, although as the project developed further funding had to be raised. The major contributors to the work were the Heritage Lottery Fund (£8.5m), the Arts Council of England (£2m) and the University of Manchester, of which the Whitworth is part (£3m). The transformation looks like a remarkable bargain by contemporary standards and the four months delay in completion a serendipitous trifle. When the gallery went dark for its makeover, it "Closed with Love" and a special weekend of events. Reopening on St Valentine's Day is an event that Cornelia Parker's major show has carefully bought into. At the end of that central gallery, and clearly visible from the park outside, is her work The Distance (A Kiss With String Attached), which uses Rodin's sculpture The Kiss, on loan from the Tate, and winds a mile of string around the couple. It dates from 2003, fifty years after public subscription helped buy the Rodin for the London gallery, and thus some time before any recent novel or movie that no-one was mentioning in Manchester.

One parallel room houses Parker's breakthrough work, Cold Dark Matter: An Exploded View, the 1991 piece which exploded a shed and its contents with the help of the British Army. Opposite, War Room is a new work that responds to it by draping the whole space in the red paper left when the shapes are cut out for Remembrance Poppies by the British Legion, lit by four bare bulbs. That parallelism is present in other new Parker works of black-painted bronze casts of pavement cracks in the city of Jerusalem, and the fissures between the slabs at William Blake's burial ground in Bunhill Fields. It is also there in her re-mounting of the backing canvases of Turner's paintings in the same room as the artist's watercolours, donated to the Whitworth by John Edward Taylor, once owner of the (Manchester) Guardian.

Dr Balshaw and her curatorial team have been extraordinarily thoughtful in the way they have combined the Whitworth's vast collection of 55,000 objects with new work in this way, particularly in a huge display of portraiture that leads up the stairs to a 1960s gallery, "Boom" where some fine Eduardo Paolozzi prints and an Ian Hamilton Finlay Poster Poem hang next to Richard Hamilton, Colin Self and David Hockney, leading into a Sarah Lucas room, startlingly wallpapered with her "Tits in Space". The corridor to the portraiture has a collage of the photography of Johnnie Shand Kydd, documenting the Young British Artists partying on the Greek island of Hydra at the invitation of collector Pauline Karpidas. She is a patron of the Whitworth, whose donation of nearly 100 works is the gallery's most recent. The artists include Hayley Tompkins and that Nathan Coley work that is the building's illuminated invitation for tomorrow's opening, when Gathering of Strangers will surely witness the Whitworth make many new friends.

www.manchester.ac.uk/whitworth.