Sisk has been confirmed to build the £50m redevelopment of London’s Royal Academy.

The David Chipperfield-designed revamp of the Royal Academy (RA) will start on site this year – seven years after Chipperfield won the job. He is the third architect to work on it.

The project will link the RA’s main Burlington House gallery with Burlington Gardens for the first time in their history.

All but £6m of the cost has been secured – along with agreement from all the buildings’ users – so work can finally begin. It is due to be completed in time for the RA’s 250^th anniversary in 2018, chief executive Charles Saumarez Smith said yesterday.

The two buildings back on to each other but are at different levels and the area between them is filled with service yards and the studios of the RA’s art schools.

This has made what sounds a simple job highly complex, said Chipperfield, himself a Royal Academician.

“You would think it would be quite easy to connect two buildings separated by a 15m gap but the bricks and mortar are quite simple compared to the complexity of what goes on in here,” he said.

“Finding a route between the two front doors was a significant problem. We have to go into the underbelly of the RA and connect through the schools.”

His major architectural intervention is a bridge that will lead from behind the stairs in the foyer of the main Burlington House building, down through the art schools’ cast corridor and studios and up into Burlington Gardens, emerging from behind the main staircase into the foyer of that building. By luck both entrances are within a metre of being on a direct axis.

The bridge will not only improve circulation in the expanded Royal Academy and provide access to newly accessible undercroft galleries, it will also provide a free public route between Piccadilly and Mayfair, said Chipperfield. The RA’s two entrances will have equal prominence.

Other aspects of the project include a 260-seat lecture theatre and a dedicated learning centre in Burlington Gardens as well as new galleries to display academicians’ and students’ work plus the RA’s rarely seen permanent collection. Visitor facilities will be improved and two extensions on the roof will contain offices and plant.

The Burlington Gardens entrance will be revamped to improve access and the public realm. And the Burlington House foyer will be enlarged, with ticket desks recessed in existing spaces.

“The scope of what we are doing varies between the substantial, with major construction pieces… and minor interventions and the renovations of galleries,” said Chipperfield.

The project was forcing the RA to re-think its purpose, as previously hidden aspects of its work, such as the schools and permanent collection, prepare for public scrutiny, he said.

“The physical interventions will hopefully be in the background. It’s what those interventions will allow us to do that’s interesting,” he added.

The Royal Academy bought Burlington Gardens in 2001. It was built for London University in 1870 and was for years home to the Museum of Mankind. It will retain a slightly more academic feel, said Saumarez Smith.

Chipperfield is the third architect to work on the connection project. Michael Hopkins won a previous competition in 1998 which was scrapped because of lack of funds. Colin St John Wilson was next in 2006 but when he died the following year a new competition was held which was won by Chipperfield. He is working with Julian Harrap, reprising their Stirling-shortlisted collaboration on the Neues Museum in Berlin.