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Nearly 30 years on, the Sainsbury Centre for Visual Arts at the University of East Anglia has established itself as a modern classic. Instead of the traditional black box approach to gallery design, Foster and Partners came up with a lightweight shed into which daylight percolates through silvery and diaphanous screens of glazing and venetian blinds.

The original gallery has been extended down through a slot cut in the ground floor

So beautifully integrated is the building that even the original architect has been loath to tamper with it. Foster and Partners' first extension, completed in 1991, discreetly burrowed underground at one end to create archive and conservation areas. The practice's second extension, newly completed at a cost £10m, performs a smaller and more delicate operation by joining up the two earlier phases.

"Suddenly everything has been tied together," says Spencer de Grey, Foster's deputy chairman. This is achieved physically by cutting out a large rectangular area of the ground floor, which transforms the basement below into a daylit lower extension of the main gallery space. From here a long but wide passageway leads to the 1991 subterranean wing. It too admits borrowed daylight at either end.

More than that, the facilities in the two earlier phases have been blended. "The basement areas were private spaces, and some of these are now turning into public facilities," says de Grey. Not only does the open basement area serve as a public gallery, but so too does the link passageway. Its walls display paintings, and a series of niches on either side exhibit furniture and other 3D objects. In addition, the public is given free access to the reserve collections housed alongside the link passageway.

Slimline canopies over the two entrances are the only changes visible from the outside

The 12-month refurbishment contract has also enabled all 40 original air-handling units to be replaced, as they had come to the end of their natural life, and to install controls that are more sensitive to external conditions. Cooling can now be laid on in summer through a connection to a new chilled water mains.

From the outside, only one change is visible. Slimline, artificially lit glass canopies have been added to the two main entrances. They manage to be welcoming without marring the perfection of the original.