The brief sounds straightforward: fix what was broken. The reality was open-heart surgery on one of the world's busiest theatres and the most watched building in Australia – with every architect in the country looking over your shoulder, none closer than the eminent members of the Opera House's own Design Advisory Panel.

By 2012, the Concert Hall was facing end-of-life theatre machinery, a poor reputation for acoustics, and mobility issues that left some areas impossible for wheelchair users. Acoustic problems had plagued the building since it opened in 1973. And the entire thing sat inside a World Heritage shell that couldn't be touched in any way that showed. Our job was to fix all of it and make it look like nothing had happened.

What did happen was enormous. Threaded into an already jam-packed heritage structure, the works amounted to a complete reinvention of what the hall could do – while matching the architectural brilliance of Jorn Utzon and Peter Hall. The result is a venue that turns from orchestral to amplified mode in a matter of hours, stages more shows with more diverse programming, and is finally, fully accessible. 

140 tonnes of new steel installed into a ceiling never designed to carry it. A complete stage rebuild with automated risers (the Sydney Symphony's horseshoe layout deploys at the touch of a button). An invisible but operationally critical flying system to safely handle the demands of contemporary production. And 18 acoustic reflectors above the stage – clarinettist Chris Tingay says it's the biggest change in 24 years playing here with the SSO.

Richly modelled timber panel box fronts provide acoustic diffusion and a warmth the interior previously lacked. Automated drapes flip the hall from acoustic to amplified mode – the Sydney Symphony one afternoon, a contemporary performance the next – with a speed and flexibility the building never had before. A new passageway and lift give wheelchair users independent access to the Northern Foyer and its harbour views for the first time.

We came to this project from Hamer Hall and the Shrine of Remembrance – two buildings that demanded the same discipline: understand the original authors, work in conversation with them, leave no scars. People walk in and ask what was actually done. That's the point.