When it was rebuilt in the late 18th century, Somerset House became Britain’s first purpose-built office bock. Ever since, these handsome neoclassical buildings have been stuffed with paperwork and staffed by penpushers, the place of work of civil servants down the ages from The Navy Board to the General Register Office and the Inland Revenue. It used to be that amateur genealogists were the only other people to set foot in the place.

The General Register Office moved out in 1970 but it lives on in the national consciousness as the first port of call for people in search of their ancestors. A few people still call every week.

Little do they know that these days Somerset House is an entirely different place. Its vast central courtyard - which, with a bureaucrat’s regard for usefulness over beauty, had been a car park - is a continental-style piazza, and the site of the largest public fountains to be commissioned in London since Trafalgar Square in 1845. In the summer months, 55 jets of water dance above the granite cobblestones in choreographed displays, while winter visitors can be found attempting manoeuvres on the ice rink installed every Christmas.

And this weekend, a giant pirate ship will drop anchor in the square, the centrepiece of Free Time, a four-day festival of performances, art, music and storytelling that typifies the open-house policy of the place.

Families, and not just people trying to trace theirs, are welcome.

It wasn’t always this way. At the top of the stairs leading down to the new learning centre is a reminder of the bad old days. One late 19th-century writer’s description reads like a Victorian version of the TV comedy series The Office. “In these damp, bleak and comfortless recesses the clerks of the nation grope about like moles and stamp, sign, examine, indite, doze and swearI unconscious of the revolving sun.”

The Inland Revenue still occupies two sides of the square, but not for much longer. The idea is that eventually the east and west wings will be opened up to the public just as the north and south wings have been.

Impressive though its exterior may be, the most extraordinary parts of Somerset House are within its walls, which hold three quite different but equally fascinating collections of art - the Courtauld Gallery, the Hermitage Rooms and the Gilbert collection.

The Courtauld Gallery is housed in the northern wing, above the grand arch that separates the calm of the square from the bustle of the Strand outside. The semi-circular stairs that sweep upwards from the entrance have the feeling of a fairly grand country house rather than a world-renowned gallery. And inside, there are not the vast rooms - or the crowds - found in other museums. But the understated atmosphere just serves to make the art on display seem even more outstanding.

There’s a household name on every wall. Works by Renoir, Manet and Gauguin are instantly recognisable, and there’s the peculiar thrill of seeing for real paintings such as Degas’s “Dancers”, Cezanne’s “Card Players” or Van Gogh’s “Self-Portrait with Bandaged Ear” viewed so often in books or reproductions.

Besides this all-star line up of Impressionists, the Courtauld boasts dazzling displays of silverware (Samuel Courtauld who founded the gallery in 1932, was descended from a long line of Huguenot silversmiths), a roomful of Rubens, a large display of Bloomsbury ceramics and local curiosities such as Pissarro’s landscape of Lordship Lane station in Dulwich, south London, and Derain’s bright rendition of Tower Bridge.

There’s also an array of Degas’s bronzes - figures of dancers, horses and bathers for which he is well known but which nearly never saw the light of day. These experimental models cobbled together with wax and clay were rescued from his studio and cast into bronze only after his death. This gallery alone is enough to satisfy the most ravenous culture vulture.

But across the courtyard, even older and, in some ways, more remarkable riches await. In the audio guide to his collection, the late Sir Arthur Gilbert explains: “I never intended becoming a collector. It just evolved.”

Born in north London’s Golders Green in 1913, Sir Arthur set up an evening gown business with his wife, the designer Rosalinde Gilbert, and by 1950 they had made enough money to retire to California, where he spent the next half-century amassing one of the greatest collections of gold and silverware, furniture and objets d’arts in the world. “I decided very early it belonged not to me but to the world,” he said. And so it was bequeathed to his home country and, after his death in 2001, put on display at Somerset House.

Breathtaking in its detail and craftsmanship, and literally dazzling to the eye, this collection is a showcase for the decorative arts. Many of the 17 galleries are in near darkness save for a spotlight trained on the bejewelled items, adding to the exoticism. There are grand silver howdahs (elephant saddles), tiny micromosaics and a golden ewer dating back 4,000 years, one of the oldest in the world.

Upstairs in the Hermitage Rooms are items of similarly astonishing pedigree. This collection - loaned from the museum in St Petersburg - is like a lavishly illustrated history lesson in Russia’s relations with its neighbours. And the Heaven on Earth: art from Islamic lands exhibition, which runs until August 22, is a reminder that 1,000 years ago some of the Middle Eastern countries currently in turmoil were the leading civilisations on earth.

Things in the West had advanced a bit by 1775, when Sir William Chambers was commissioned to rebuild Somerset House on the site of a ruined Tudor palace. It became the Navy Office, headquarters of the British fleet and all who sailed in her. The Nelson stair, a soaring flight of steps at one end of the south building, is a reminder that Horatio was here.

The great arch that allowed ships to dock directly under Somerset House was off from the Thames by the building of the Embankment and now houses the King’s barge, once used to ferry VIPs up and downriver. But the location remains one of Somerset House’s great attractions. The spacious promenade overlooking the river is the venue for one of several cafes at Somerset House. It’s the perfect place to reflect on the fact that, as architects strive to outshine each other in the City behind you, they just don’t build offices blocks like this any more.

www.somerset-house.org.uk

In the house

Educational visits

Free Time runs from July 23 to 26 and includes performances from Oily Cart theatre company, Akademi dancers and the London African Gospel Choir. There will be workshops where visitors can make musical instruments, flags, model boats and watercolours and a Treasure Seekers game to find the works of art that inspired the activities.

Somerset House’s well-equipped learning centre offers a range of courses and workshops for key stage 2 and above, including the archaeology-themed “Overground Underground”, sculpture in “2D and 3D” and “Myths and Legends” based on the artefacts in its three galleries. Painting Quickly and the History of Dress courses are suitable for all key stages. For details, tel: 020 7420 9406.

A series of family days and practical workshops based on the building and its collections runs every Thursday (for under-6s) and Saturday (for six to 12-year-olds) until August 28. They are free and run on a first-come, first-served basis. For full details: www.somerset-house.org.ukwhatson familyeventsindex.html.

Anywhere else like it?

* County Hall, London - once home of the GLC, now houses the Saatchi art gallery, the London Aquarium and booking office for the London Eye.

* Beatrix Potter Gallery - National Trust collection of sketches, watercolours and manuscripts displayed in the former offices of Potter’s husband, solicitor William Heelis, in Hawkshead, Cumbria.

* Tower of London - until the mid 19th century, the Mint, Record Office, War Office and Office of Ordnance were all based here.

* The Scotsman - new hotel on Edinburgh’s North Bridge, converted from the baronial former offices of the newspaper of the same name.