The gallery will house part of the collection of Charles Saatchi, former advertising mogul, including Tracy Emin's unmade bed and Damien Hirst's shark (top picture) and slices-of-cow-in-formaldehyde (bottom picture).
The gallery was designed by RHWL, the architect that has spent the past 10 years turning the home of the former Greater London Council into a cultural centre.
Since RHWL was selected by Shirayama Shokusan, the Japanese property developer that own County Hall, it has played a large part in finding tenants for the building. These include two hotels, a gallery space dedicated to the Spanish surrealist Salvador Dalí, the London Aquarium, ticketing and lounge facilities for the London Eye, and several restaurants.
This latest addition occupies 4000 m2 of County Hall's first floor, and the architect's work has focused on renovating and enhancing Ralph Knott's beaux-arts-inspired interiors. The original fixtures have been retained throughout the grade II-listed interior.
Project architect Jelena Tomic said that the emphasis has been on exploiting the existing characteristics of the building. She said: "The intention was to deploy techniques and create space that looked as if nothing had been done at all."
To bring large art works into the building without damaging the structure, the entrance to the bridge that connects the former conference hall to the lobby can be dismantled, and a full-height door installed.
Saatchi chose County Hall for the gallery because of its prominence on the South Bank, and some in the art world view it as a direct challenge to Bankside's Tate Modern.
He said: "New British art is the most exciting in the world and needs a showcase. The extraordinary rooms in County Hall will make an interesting setting for works like Hirst's shark and Emin's bed."