LONDON — Tate Modern disclosed designs for a bold new extension, planned for 2012, that will feature a glass pyramid and cost $305 million — as much, in today’s money, as the museum itself in 2000.

The 11-story structure, a jagged pile of cast-glass boxes forming an irregular pyramid, will occupy the eastern wing of the building. It will displace an EDF Energy Networks electricity substation that will shrink by about 50% in size, giving Tate more space that the museum hopes to fill by the 2012 London Olympics.

The plans were drawn up by the Pritzker Prize-winning Swiss architecture firm Herzog & de Meuron, which originally converted the bulky power plant that became Tate Modern. Its project will be submitted for planning approval in autumn 2006, with a decision scheduled in the spring of 2007.

Most of the funds have yet to be raised. “We don’t start with a huge war chest,” the Tate director, Nicholas Serota, said at a press conference. He added that he was “confident we can raise that money. We’ve done it before.”

The project got a helpful push from the mayor of London. Ken Livingstone, whose Greater London Authority gave it $12.88 million. Tate will apply for U.K. National Lottery funds, Mr. Serota said, though he added there were increasing demands on them.

Mr. Serota said he expected most of the funding to come from individuals, a number of whom had “expressed interest.” He would neither name them nor say how much had been raised so far.

The new building will stand 77 yards tall, hovering over the converted power plant that is the museum yet dwarfed by the chimney that dominates the riverside facade.

It will be built over three giant disused oil tanks that once held fuel to power London and sent smoke spouting out of the chimney. Reporters in hard hats and fluorescent vests toured the circular tanks, the size of skating rinks, and still stained with leftover tar. These will be turned into performance and storage spaces.

Commenting on the building’s odd look, architect Jacques Herzog said, “This is clearly a shape that is ambiguous. It’s a pyramid, but it’s also a simple stack.”

Mr. Serota, who was asked about potential criticism of the design, said London would be a “sad city” if “the only new buildings representing this age were commercial buildings.”

The Tate Modern director, Vicente Todoli, said curators have been constrained by the linear and rectangular shape of the display rooms. “Now, acquisitions won’t be driven by space,” he added.

The building will exhibit oversize works that Tate currently lacks space for, such as Louise Bourgeois’s monumental spider sculpture or Bill Viola’s video installations. It will also house new art from Latin America, Asia, and Africa, as well as educational rooms.

Tate Modern also is overcrowded. Designed to take in 1.8 million visitors a year, it admits more than 4 million. Commenting on a slide that showed dozens of people crammed into one place, Mr. Todoli joked, “This is not a Sunday in Benidorm: It’s a weekend at Tate Modern.”