Oxford's first rooftop cafe will be a new attraction at the Ashmolean Museum following a major redevelopment.

The first stage of construction work for the £61m extension of the Beaumont Street museum was completed on Monday.

In a ceremony to mark the latest stage in the building project, Dr John Hood, Vice-Chancellor of Oxford University, and Richard Gregory, of the construction group HBG UK Ltd, poured concrete into the highest section of the new building.

The extension will double the museum's display space, allowing thousands of objects previously in storage to be shown for the first time.

The development features 39 new galleries, a new education centre, state-of-the-art conservation studios, and a walk-through between the museum and the cast gallery. It is due to open next year.

James Purnell, former Secretary of State for Culture, Media and Sport, said: "When the development is complete, and the expanded museum opens in autumn 2009, visitors will be provided with a world-class museum to match the Ashmolean's world-class collections."

Throughout this year, the new galleries will be fitted out in preparation for the installation of custom-made cases and exhibition displays.

A grant of £15m came from the Heritage Lottery Fund, with other major donations from the Linbury Trust, foundations and individuals, towards the total project cost.

Founded in 1683, the Ashmolean is the UK's most important museum of art and archaeology outside London.

Among treasures it contains are the best collection of Raphael drawings in the world, the most important collection of pre-Dynastic Egyptian material outside Cairo, the greatest Anglo-Saxon collection outside the British Museum, and Gunpowder Plotter Guy Fawkes' lantern.