Due for completion by summer 2012, the council-owned A-listed building is set to undergo work to provide more seating, improve acoustics and new front-of-house facilities.
Internally decorative finishes will be restored. The ground floor will be returned to its original use with three new shops. A fine dining restaurant is to be located in the Supper Room.
The Assembly Rooms was originally opened in 1787 and has been a principle performing arts venue vital to social and artistic life within the city.