Pitlochry Festival Theatre, the famous "theatre in the hills", has always been an exceptional place.

Ever since its inauguration as a tented summer festival venue in 1951, the Highland Perthshire playhouse has done things very much its own way.

Currently attracting over 100,000 visitors to its six-play summer festival from across the UK and beyond, PFT has, in recent years, become Scotland's leading producer of professional stage musicals. Its summer and winter musical productions have carried all of the hallmarks of professional performance, design and technical stagecraft which have made the theatre such a success.

Now, 11 years into the artistic directorship of John Durnin, PFT has announced a seven-year, £25-million expansion plan. Entitled Vision 2021, it is not only the most ambitious project in the theatre's history, but also one of the most remarkable plans for artistic and infrastructure development that Scottish theatre has ever seen.

The plans include a major expansion of the theatre itself, including the reconfiguration of the building to provide it with two auditoria, rather than the one it has at present. The main house (which will seat 500 patrons, as opposed to the current 540) will have a full-size fly tower, thereby enhancing PFT's already considerable reputation for technical accomplishment.

The studio theatre (which will seat an audience of 300) will allow the theatre to further broaden its repertoire, thereby enhancing a process which has taken place during Durnin's reign of widening out from the traditional well-made plays of the likes of JM Barrie and Sir Arthur Wing Pinero to include more contemporary dramas, such as Stephen Greenhorn's Passing Places and Liz Lochhead's Perfect Days (both of which appear in the 2014 summer programme). This artistic expansion will be pushed further by the beginning of a touring programme, which will take PFT's work to audiences across Scotland and the UK.

The PFT building itself will be transformed. Lovers of the Perthshire playhouse will be relieved to learn that its famous glass wall, which provides glorious views of the River Tummel, will remain. Indeed, if initial artist's impressions of the 2021 PFT are any indication, the new building will be something of a modernist palace in glass and concrete.

Should funding for the ambitious expansion plan be secured, PFT patrons will have two years (2019 and 2020) in which they will return to the summer festival's roots in a tented theatre. By 2021, the season should be back in the expanded new theatre.

Fascinatingly, another aspect of Vision 2021 aims to tackle a problem which, according to Durnin, has become something of an elephant in the room for Scottish theatre and the performing arts more generally - namely, the looming shortfall in trained stage technicians. "The average age of a theatre technician in Scotland is 50," Durnin explains. "There's nothing wrong with being 50, but if the average age of those working in the technical disciplines is increasing, that's actually quite terrifying. It suggests that we're not training enough young people, and we're not explaining to them how they can forge a career in the industry. Nor are we explaining the transferable skills which a performing arts training equips young people with."

The loss of Queen Margaret University as a performing arts training school has been disastrous, says Durnin, as it leaves the Royal Conservatoire of Scotland in Glasgow as the sole educational institution in the country which trains technical staff to the required level. Under the Vision 2021 plan, PFT hopes to assist in a process whereby organisations which are not ostensibly training organisations, such as producing theatres, begin to supplement the work being done at the Conservatoire in training stage technicians.

The overall vision for the expansion dates back, Durnin remembers, to 2003, the year in which he took up the directorship of the theatre. He was part of research trip to Canada which looked at programmes, such as the Shaw Festival at Niagara-on-the-Lake and the Stratford Festival, which are comparable to Pitlochry's summer season, but have expanded beyond the scale of their Scottish cousin.

Over the last six or seven years Durnin and his team have, in consultation with partners, applied their observations of the Canadian festivals to create the new development model for PFT. In 2012, the theatre had a set of specific proposals and a feasibility study was then commissioned by arts funding quango Creative Scotland, Perth and Kinross Council and Scottish Enterprise.

The result of all of this work is, the artistic director says, a set of proposals which represent "very much a melding of the recommendations from the feasibility study consultants with our particular thoughts about how the organisation needs to transform itself in order to, not just survive, but thrive in the 21st century".

It is appropriate, perhaps, that Vision 2021 should be announced in the second year of Scottish Homecoming. PFT's 2014 summer season - which ranges from JM Barrie's The Admirable Crichton to the new musical Whisky Kisses - is, says Durnin, "a celebration of Scottish theatrical voices". Such is the scale and ambition of his theatre's development plans, all of us who care about the performing arts in Scotland, wherever we are based, may well be celebrating the positive impact of the changes taking place at the Perthshire playhouse.

Details of Vision 2021 and this year's PFT summer season can be found at pitlochry.org.uk