At first you think you might be the victim of a practical joke. You come through the glass wall to a mid-air point in the huge, empty atrium, and descend the open, curving staircase among the towering white pillars to the panelled floor.

In the Merchant City, it looks and feels like the headquarters of some megabucks corporation. But at the foot of the stairs, there is the personal assistant to reassure you that this is indeed the new home of the Scottish Youth Theatre.

The theatre staff, who have had the keys to the building for barely a month, still have to pinch themselves as a reminder. Even Mary McCluskey, the artistic director, admits that she stopped short in the middle of a drama session in the new theatre to remember where she was.

As in some folk tales, they are the paupers who have been given the keys to the palace, and for anyone who cares about youth theatre, not before time.

The company has made a long journey from the rented school rooms of 20 years ago. It began when the College of Drama moved up the hill from the Old Athenaeum - a venerable old theatre, but with a spiral staircase to the box office and an auditorium that rumbled with the Glasgow Underground. It was a building that nobody wanted, except the Scottish Youth Theatre, who at last had a roof they could call their own, and at a peppercorn rent.

What no one foresaw was that the tide of retail prosperity would flow through Buchanan Street and make the old theatre a prime site. “It was the money we got for the Old Athenaeum that made all this possible,” says Ms McCluskey. “And then when someone had the bright idea that the Old Sheriff Court should be redeveloped with cultural and retail as well as residential use, Persimmon Homes asked us if we would be the cultural partners.”

It was almost three years in the planning, the Scottish Youth Theatre talking with the architects, with dance leaders, theatre arts and technical specialists, and all the time the building organically evolving. In the end, Persimmon was generous; at first, the SYT was to have half the basement; in the end, they had it all.

Not that there is any sense of being in a basement. The glass walls open into the centre of the complex, a long, paved courtyard open to the sky, with pebbled path leading from the atrium to the theatre. Not the largest of auditoriums, it is nevertheless fully flexible for audience and players, and has a more than useful gallery on three sides.

“Access for everyone” is the building’s mantra; the motorised lighting rig can be lowered 20 metres to ground level, so lantern work can be accessible to anyone. Provision for special needs users is apparent everywhere, with ramps and lifts, one even to the theatre control box, and state-of-the-art dressing rooms with wheelchair access to showers and toilets.

On each side of the courtyard are the six rehearsal rooms, the dance studio and the sensory room for those with sight or hearing impairment.

Everywhere is airy and light, white paint and glass, with as yet no doors or furniture to challenge the spaciousness. With one exception - the claustrophobic, almost windowless prisoners’ cells, almost as an unconscious mark of the creative artist’s enthusiasm for committee work, are now the boardroom.

After the furniture will come the young people in the autumn term to make the building live, though already the summer school is active in one of the rehearsal rooms, with the young people on the musical theatre course in full cry as they learn the songs for Geordie, which some may remember as a novel and film of the 1950s. If you remember neither, Ms McCluskey, who directs the piece, adds that this story of the Highland lad who becomes an Olympic champion inspired the picture on the porridge oats packet.

Elsewhere in the city the “performance and production” courses are hooking their shows to the bicentenary of Hans Christian Andersen’s birth, rehearsing a new version of The Ugly Duckling and Stuart Paterson’s ever-welcome The Snow Queen at the Citizens, and at the Tramway, Into the Light, a modern version of The Little Match Girl. They go into performance in early August in Glasgow, Edinburgh and Stirling.

It will be another summer of memories for the 100 young people at work, but Mary McCluskey thinks she may have her “magic moment” already: “There is a girl who has been with us for 10 years - she joined when she was six. When she came into the building for the first time - the look on her face. It made everything worth it.”

Scottish Youth Theatre, tel: 0141 221 5127