The redevelopment of the theatre, described as one of the "jewels in the crown of Glasgow's cultural life", will involve the complete redevelopment of its front of house, a significant improvement of its facilities for the public, and a new wing for education, children, rehearsals and performances.
Glasgow City Council owns the building, long in need of major repair, and is funding a feasibility study which will lead to bids for capital funding from Creative Scotland and the Heritage Lottery Fund this summer.
The Edinburgh-based architects Bennetts Associates has won the competition for the revamp, which will also radically overhaul the theatre's offices, backstage areas and utilities, in a design which will again reveal the original stone walls of the Victorian theatre.
The same firm has recently won awards and plaudits for its work on the transformation of the Royal Shakespeare Theatre at Stratford-upon-Avon.
The Bennetts design opens up the 4740-square metre footprint of the theatre without greatly expanding it, but makes improved use of its space, re-utilises many redundant or cramped internal spaces, will enable disabled access to all floors, and includes one completely new building on the south side of the site.
The much-loved auditorium, which is Grade 2 listed, will not be dramatically changed, but the steeply angled stage, which is raked on a scale of 1:17, one of the steepest in the UK, will be lowered to 1:24, allowing for more productions with movement and dance.
Over the years, the theatre has produced a stellar cast of theatrical alumni, with actors such as Rupert Everett, Tim Roth, Gary Oldman, Pierce Brosnan, Alan Rickman and Glenda Jackson treading its boards, as well as Robbie Coltrane, Stanley Baxter, Leonard Rossiter and Moira Shearer, among many others.
However, the fabric of the building has long needed upgrading, its offices and backstage areas need modernising, and its central bar/foyer and revolving door entrance are dated and restrict movement for the public.
If grants are secured, and a £2m public appeal is successful, work on the revamp will begin in 2016 and last two years, with, the theatre hopes, no full closure of the venue or dark period.
Construction costs will be around £6.5m but with fees, tax and other costs will in total rise to £10m, the theatre's administration believes.
Dominic Hill, artistic director, said: "I think this is about giving our audience the best overall experience we can, from the front of house, to the cafe, to what we offer for children, to what we offer for the community as a whole. We want it to be the at the centre of the Gorbals community and hopefully a better experience for visiting companies and artists."
He added: "I'm looking forward to working with Bennetts Associates to realise our vision for this beautiful and historic theatre, and to provide audiences, artists and staff with quality spaces in which to make and enjoy theatre.
"My ambition is to have a building that reflects the Citizens' reputation for contemporary interpretations of classic texts, while respecting the theatre's important heritage."
The original Victorian building on the Gorbals site has been home to the Citizens Theatre Company since 1945.
Adrienne Scullion, chairwoman of the Citizens, said: "This is an important capital project for both the company and the city. The Citizens has always valued its place in the heart of the Gorbals and now, with the Laurieston regeneration project under way, it's the right time for us to invest in a more efficient, more sustainable building for future generations."
Alasdair Gordon, associate director at Bennetts Associates' Edinburgh office, said: "From breathing new life into the wonderful Victorian auditorium to providing new rehearsal, education and studio spaces, this ambitious project will transform the experiences of visitors, staff and performers and will secure the future of one of Scotland's leading theatres."
Bailie Liz Cameron, executive member of development and regeneration services at Glasgow City Council said: "The Citizens is a jewel in the crown of Glasgow's cultural life, and the plans for this capital project will mirror the regeneration of the neighbouring Laurieston area.
"The council has been a supporter of the theatre for many years, and will continue to do so through our funding of the feasibility study. This is just the start of the process."