Getting it right at Tate Britain

For a few heady moments last year, around the time that Madonna and Tracey Emin came to the party that reopened the old Tate Gallery as Tate Britain, we were led to believe that British art was in some way cool. Until then you may have thought it was about horses, and trees, and more horses, and portraits of 18th century Sloane Rangers standing in windy fields, and yet more horses. But, fleetingly, it was suggested that the art of these islands was actually rather radical.

It turns out that there was a terrible mistake, and it is not radical at all. The Sloanes have seen off Madge and Tracey, and Tate Britain has established itself as the Radio 4, not the MTV, of art galleries. The evidence is the £32.3 million redevelopment and new galleries, creating a third more space than before, which opens to the public on 1 November. John Miller and Partners, the architects of the development, have opted for discretion, for caution, for not frightening the gallery's awesome equine constituency.

The Centenary Development, as the project is called, consists of a new entrance on the flank of the original Tate building, new temporary exhibition galleries at a lower level, both new and refurbished galleries above for the permanent collection, a grand staircase connecting the two levels, and the usual shop, cafe, cloakroom and toilets.

The rhetoric is not of a "world-class" or "innovative" or "spectacular" piece of architecture, which is what was said about Tate Modern and the British Museum's Great Court. It is about getting the lighting right, and matching the existing 1926 galleries, and not closing the whole institution during building works, and making sure that "the return-air grilles were not too assertive", as the avuncular John Miller says.

The publicity material proudly proclaims "new hard and soft landscaping" by Allies and Morrison, who are British architecture's maestros of the inoffensive. Herzog and de Meuron, the hard nuts of the global celebrity architecture circuit who designed Tate Modern, were not called for here.

The new galleries are barrel-vaulted, classically proportioned and oak-floored, and the mildly daring decision not to varnish the boards at Tate Modern has not been followed. Here they shine like the haunches of a Stubbs.

It is very clear that no one wants the new work to be James Stirling's Clore Galleries, the last big addition to the Tate's Millbank site, where Turner's giant canvases do battle with the architect's postmodern stunts.

Just to confirm that this is not a place of the 21st century, it will open with an exhibition in the basement of The Victorian Nude. If we have arrived in a time when it's possible to be upfront and open-minded about displays of the body, this show will go back to the days of furtive lusting at pornography disguised as art. A Tony Cragg sculpture, showing Britain made of coloured rubbish and lying on her side, may decorate the new staircase with suggestions of alternative Britishness, but don't be fooled.

The thing is, notwithstanding the critic's visceral urge to cry "boring!", John Miller and Partners did the right thing - almost. There may be some subtle freakiness in the Elizabethan portraiture, but much of the art is genuinely unchallenging, and, as John Miller says: "The architecture shouldn't send up the contents." We can be grateful there is no inane bubble by some high-tech egomaniac, and this isn't the moment for a poetic explosion by Frank Gehry or Daniel Libeskind. You don't want a Gainsborough of some Lady Twittering presented as if her spiritual home was a Shoreditch loft, because it wasn't.

It's a pleasure to see an art gallery where the decent display of pictures comes first. The new staircase is serene, well made, a cool, churchlike shaft of stone and light that only goes awry when the architects get uncharacteristically expressive and put in clunky air vents and an industrial crane in the ceiling.