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Rick Mather, architect for the refurbished Dulwich Picture Gallery, has performed a similar service for the Wallace Collection near London's Oxford Street. The building, refurbished at a cost of £10.6m, is due to be officially re-opened on 22 June by Prince Charles, one century to the day after it was opened by Edward VII.

In 1995, Mather won an architectural competition to address a familiar accommodation problem of our times. The building housing the collection, a listed Georgian mansion house overlooking Manchester Square, was bursting at the seams.

Mather's solution was no less familiar: he requisitioned an inner courtyard by slinging a glass roof over it. A similar solution has been used at the Louvre in Paris, the British Museum in central London and the National Maritime Museum in Greenwich, the last also by Mather. In addition, he has burrowed out four new galleries and a 150-seat lecture theatre in the basement.

Perhaps the most novel aspect of the project has been the fact that it was carried out while the ground and upper floors remained open to the public. According to project manager Malcolm Reading: "I don't think anybody believed the collection wouldn't move out, but the gallery director, Rosalind Savill, said it would be an even greater risk to move it." And that was even though excavations turned out to be much more extensive than had been anticipated from pre-contract surveys. "We had to do an awful lot of monitoring of dust levels, humidity and vibration. We had sealed ducts to extract the dust and temporary dehumidifiers, and we took great care to minimise vibration," says Reading.

By adding the glass roof, the 20 × 20 m courtyard has been transformed into an atrium and indoor circulation hub. This now operates on two floors, as existing lightwells down to the basement have been expanded and two sweeping stone staircases added.

The courtyard also functions as a spacious all-weather sculpture garden that includes a historic bronze fountain and five palm trees. And it serves as an ideal venue for that other facility that has become de rigueur in museum refurbishments – a fashionable restaurant and café – in this case run by cult chef Stephen Bull.

In detailed design, the glass roof is hipped, with four tubular steel beams transferring the load to nine concrete padstones built into the existing enclosing walls. Unfortunately, the delicacy of the steel and glass structure is marred by large plastic rainwater pipes left exposed in all four corners of the courtyard.

As for the unforeseen problems encountered during construction, Ian Blake , director of construction manager Exterior International, explains: "Originally we only expected to underpin one wall. In the end we had to dig up to 5 m by hand and remove 10 000 m3 of spoil. Everything had to be lifted up through the courtyard by tower crane and over the roof. It was quite an ordeal."