Great Court, British Museum

The idea may have been simple – it could have been conceived by the Victorians – but its successful realisation depended on advanced engineering and constructional techniques. The aim was to create a well-tempered but not heavily-serviced environment, neither over-hot in summer nor cold in winter. Buro Happold's responsibility for both structure and services allowed a close integration of the engineering (see the detailed account in AJ 23.09.99). On an intermittently sunny autumn day, the 'balancing act' between providing a calm and generous natural light and excluding direct sunlight appeared to have succeeded.

More sensitive areas – the Reading Room, the education centre sunk below the Great Court, the Great Court Gallery housed in the 'ellipse' which hugs the north side of the Reading Room drum, and the ethnography galleries located north of the Great Court (a separate Foster project, due for completion in 2003) – will, inevitably, be more heavily serviced. The form of the Great Court's high-performance roof, supported on the walls of the quadrangle and on columns conveniently sunk within a cavity behind the new 60mm stone cladding of the Reading Room, has obvious roots in other Foster projects – the Cambridge law faculty, the 'Fosteritos' of Bilbao's metro and Canary Wharf Station, for example.

Its lightweight, ethereal look harks back to Lord Foster's past inspirations, not least the work of Buckminster Fuller. By making this roof float above the space, the architect achieves a potent contrast with the cleaned and repaired Smirke facades and the refaced drum, encircled by monumental staircases. Spencer de Grey, Foster's partner-in-charge, describes the recast Great Court as 'a symphony in European limestones'. The quadrangle facades had to be extensively patched and even partly rebuilt to remedy the damage done when they were unseen and unloved – the bookstacks had been partly rebuilt after the ravages of wartime bombing – while the much-hacked-about brick skin of the Reading Room, complete with ugly new windows, could not be exposed to view. 'Our strategy was to keep the detailing of the new stonework very “hard” and simple, as a contrast to Smirke,' says de Grey.

Only now, with all the scaffolding down, does the variegated character of the original fabric become obvious. Pressed to comment on the contentious issue of the south portico, de Grey is unapologetic: 'It's a new structure, incorporating new elements – lifts, for example – and it looks new. That seems to me to be in accord with the Society for the Protection of Ancient Buildings' principles.' Such arguments have apparently not cut much ice with English Heritage, whose commissioners issued a pompous and inappropriate statement demanding that Lottery funding for the offending portico be withheld, further encouraging the posturings of Camden council's planning committee – not a body noted for its encouragement of either conservation or good new design.

When the public is admitted to the Great Court, it can safely be predicted that the debate over the south portico will soon fade. The sheer quality and consistency of this project is likely to quell all but the more vexatious critics. In tune with the status of the institution, Foster and Partners has built on its previous 'old and new' schemes – the Royal Academy's Sackler Galleries, for instance, and the Reichstag – to create a fusion which has both dignity and a sense of place. The limited range of materials – natural stone, matt stainless steel, opaque glass – and the rigorous eschewal of extraneous detail is part of a formula for good looks and hard wear. The completed scheme has an apparent inevitability which conceals the problems and risks which it involved – the excavation of the basement, for example, had to be 'incredibly accurate', according to de Grey. 'The engineers would allow no more than 9mm movement in the frame of the Reading Room'.

Back in 1912, when Burnet's extension was nearing completion, W R Lethaby drew up proposals for a great avenue, linking the museum with the River Thames and connecting with another ceremonial route leading north towards Euston. The development of London University's central precinct blocked the route to the north, while the casual network of streets to the south survived to defy the best efforts of Leslie Martin and Colin St John Wilson in the 1960s. Between the two, the British Museum formed a massive and impermeable obstacle. With the opening of the Great Court, open late into the evening, the obstacle has been removed – the new space forms part of a pedestrian route across central London.

The Great Court has equally transformed the way in which one perceives the museum itself. It is possible to use it to move quickly from Greek marbles to African tribal masks or medieval antiquities, or merely as an oasis of calm and relative emptiness amid so many riches. You can sample the collection, drift easily from one culture to another, and avoid a wearisome march through apparently endless galleries. For the first time, the British Museum has added an element of enjoyment to the visitor experience. It starts when you step into the Front Hall and find that a richly polychromatic decorative scheme of the 1840s has been restored, replacing the psa grey imposed by a former director (on the grounds that aesthetics had no place in a scholarly institution). Such attitudes are now, hopefully, dead: the sleeping giant of Bloomsbury has emerged triumphant from its millennium makeover.