Tate Modern, the world's favourite modern art museum, will increase its size by half under new plans announced today.

Half of the space currently used by an electricity substation attached to the building will be turned into display halls, visitor facilities and a learning centre.

Herzog and de Meuron, the acclaimed architects that turned an old power station into the existing museum, have been chosen to design the new section.

The move follows a decision by EDF Energy to update its substation housed in the huge wing south of Turbine Hall. This will halve the space it occupies, making the rest available for Tate.

"This opportunity would not have recurred for another 50 years," a Tate spokesman said.

"We also want to improve the facilities for the huge number of people who visit, especially over weekends."

Since Tate Modern opened its doors in May 2000, more than 20 million people have passed through. Last year it saw 4 million visitors, including nearly 6,000 in one hour on a July Saturday.

Only the Centre Pompidou in Paris comes close with 3.5 million visitors. The Guggenheim New York and Guggenheim Bilbao falls far behind at 0.9 million. No figures are available for the relocating Museum of Modern Art in New York.

Tate Modern has regenerated Southwark, with a knock-on effect of helping to create over 20,000 cultural jobs in Bankside, South Westminster and Bermondsey.

Now the architects will also develop its surrounding open areas into pedestrian routes and gardens. This will complete the vision of creating an "international centre for visual culture" on the South Bank.

New galleries will include warehouse spaces designed to show performance, film and new media.

Details will only become clear when the plans are submitted this summer. Work is due to start in 2008, when the substation will be largely relocated, and should finish by 2012.

The cost is still to be assessed. "Tate has a good record of raising money from both public and private funds," the spokesman said.