by Lucy Bullivant

A catalytic expansion, a new building by its architects Herzog & de Meuron, ambiguous in appearance and opening up the south west side of Tate Modern, will give the most visited contemporary gallery in the world the opportunity to play an active role in the regeneration of its context in time for London's Olympics in 2012.

The proposed new 11 storey building adjoins the south side of Tate Modern, using the footprints of the power station's disused subterranean oil tanks, and has already come in for predictable criticism from skittish members of the UK architectural world for its looming scale (70m; Tate Modern's chimney is 99m) and visibility from the north side of the Thames. More than 25 million visitors since it opened in 2000, 4.1m per year (MOMA in New York gets 2.67m) have prompted Nicholas Serota, Tate's Director, to create a new complex that will be a key hub in a rapidly emerging cultural quarter in Southwark.

Until ten years ago this was a depressed, unexploited part of the riverside rather than an attractive magnet for locals and tourists. A cultural quarter would suggest the need for, rather than the prohibition of, vibrant architecture, especially as there has long been a lack of good new buildings on this stretch of river bank. Foster's GLA is one; the London Eye is hardly a building; Foster's SwissRe in the City of London stems from its traditional largesse for new architecture so a rebalancing act seems entirely fair.

The situation is due to change dramatically in the next few years, with Alsop's Palestra, Zaha Hadid's Architecture Foundation, Allies & Morrison's Bankside One, Two, Three, all set to transform this 'neck of the woods', Renzo Piano's Shard at London Bridge growing up soon after and the prospect of four new buildings by Richard Rogers, and there is an agreement with Tate Modern for the Design Museum to take space on the site to build its own new home in the next six years. Herzog & de Meuron's design, a series of stacked boxes in a pyramidal form housing variegated types of spaces connected via a dramatic 'vertical boulevard' (long escalator) creates improved facilities for visitors.

The new centre of gravity with the existing and new buildings becoming one increases Tate Modern's capacity by 60%, with 23,000m2 of new space the team hope will offer an intimate, informal atmosphere. A new north-south pedestrian route will bring urban permeability, enabling you to walk from the city of London via the Millennium Bridge, through the building and south into Southwark. A new park, a local scheme, will connect landscaped areas around the gallery, terraces down to the river and open spaces behind Tate Modern.

The identity of art and role of curating have changed, yet no new buildings in London were realised at the time of the Millennium reflecting this, and Tate Modern has been itching to move forward. 14,000m2 of new exhibition space (currently 9,000m2 excluding the Turbine Hall), an increase of over 60%, widen the scope of activities with 10 new galleries (5,063m2), some double height, for photography, film, video and performance and other long-term installations. The two currently rat-ridden oil tanks will be turned into a flexible auditorium for 400 people and an informal performance space. Eating sandwiches sitting in the corridor, just one sign of overcrowding at Tate Modern, should become a thing of the past with six new cafes, bars and restaurants including a second top floor restaurant and a public roof terrace.

A mini-city sequence of variegated hubs - educational spaces for formal and informal learning - will add 1,567m2 of facilities to the existing 1,000m2, including Young Tate, a space curated by young people, 'incubators' organisations can use with display space, Tate Lab, a video art, performances and sound archive, film, web-casting and video conference suite, and Tate Forum, for the growing adult education programme. Herzog & de Meuron, who won the international competition for the building back in 1995, were invited back in Jan 2005 to develop a further scheme for the south west side of the building.

The synergistic, community-oriented nature of Tate's ambitions prompted a free flowing design, light and crystalline, sited behind the power station's sober neo-classical structure. Structurally orientated on an orthogonal layer, the complex spatial organisation is contained in a rough cast, fragmented formal envelope of delicate, cast 'vision glass' with darker areas of opaque insulated glass panels. The architects offer the interpretation is that this is both 'the erosion of the pyramid.. and a pyramid in the process of emerging'. The two will form 'a single, though disparate organism', Whatever dualistic game the new architecture will play, Tate Modern always intended to 'complete' itself by reactivating the derelict areas of the building for use in the future.

EDF, the electricity supplier that owns the substation currently located in the switchhouse of Tate Modern, needed to modernise the station's equipment, so Tate has taken advantage of that rare opportunity to move the plant south into a smaller area of the building, freeing up space for expansion on the south side of the site. Herzog & de Meuron's designs are to be submitted for planning approval this autumn. Turning the Tate Modern around to open up to its impoverished southern hinterland should win favour. Fortunately the new plans are not about 'looking through a glass darkly' as an organic futuristic experience Tate Modern's audience might well buy.