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Britain's great national museums and galleries were built in the 19th century as elevated affairs with grand stone staircases leading up to the main entrance. But in the current spate of museum refurbishments and conversions, many public entrances have been shunted below ground level.

London's Tate Britain (formerly the Tate Gallery) was built in 1897 with the grandest flight of stone steps on its frontage to the Thames. It now sports a basement-level side entrance, as does the Tate Modern, which opened last year. Does this represent a new, egalitarian approach to art presentation? Perhaps, but the real reasons are more practical: to comply with the Disability Discrimination Act by providing a ramped public entrance, and to channel visitors directly into gallery space. In the case of Tate Britain, there was also a need to provide an entrance on the main visitor route from Pimlico Tube station.

This new low-level entrance forms part of a £32m reordering of the Tate Britain, which opens to the public on 1 November and is claimed to be the gallery's most significant development since it was built. The project conforms perfectly with the current generation of gallery and museum refurbishments. As well as adding a third entrance, it provides 35% more viewing space and public amenities on two floors, including the mandatory shop and toilets, and stitches in state-of-the-art air-conditioning, lighting and power. It provides all this within the listed external envelope of the building, and it has been mainly funded by the National Lottery.

Designed by Allies and Morrison Architects, the new entrance has requisitioned an old arched window in the stone flank wall. In the internal reordering, designed by John Miller & Partners, a large hall of hefty rectangular columns leads into a grand marble staircase below a glass roof. Four new galleries have been created within an old service courtyard, and 10 existing galleries have been rebuilt with stronger floors, new servicing and, in the basement, more headroom.

The architectural styles of both practices follow current approaches to historic building refurbishment – respectful yet contemporary in style. Allies and Morrison's ramp cuts into a new stone plinth and is screened by a 70 m ribbon of grit-blasted frameless glazing. Miller's interventions are in a solid modern style with painstakingly discreet detailing that sits well with the ornate classical building. In the upper galleries, internal and external sun-screen louvres follow the contours of the vaulted glass roofs, and uplighters and tracks for spotlights are tucked above the cornices, dispensing with conspicuous pendant booms for lighting or sunshades.

John Miller, a former professor of architecture at the Royal College of Art, argues that "the design of art galleries is an art form in itself." The new Tate Britain conforms to that principle.