In recent weeks, several gleaming new galleries have opened to a predictable fanfare of stories about their budgets, the attitude of the local population, their size and appearance: Turner Contemporary in Margate, the Hepworth in Wakefield and the extension to the Holburne in Bath. One might reasonably have expected that our luck, not to mention our cash, would by now have run out. And yet the most thrilling opening of the year is still to come. Only this one involves no astonishing new construction, no (possibly) thin-skinned architect and no whiff of overspend. Nor, as the doors swing open, will any hot young artists be available for interview on the subject of their unfathomable installations; the painter whose work is gathered inside died a century ago. As for the locals, they could not be happier. It is the gallery's doting neighbours who have planted its flower beds with lavender and verbena, and they, too, who will bake the cakes for its tea room.  The Watts Gallery in the village of Compton, Surrey, is the loveliest of places. Lots of people, of course, thought it was lovely in the days when it was still on English Heritage's "At Risk" register, before it closed and began its £10m restoration. But it is even lovelier now. The original galleries, which first opened to the public in 1904, have been returned to their former glory, their walls ruby red and emerald green, their tiled dado rails, previously hidden beneath thick paint, shiny as new. In the sculpture studio, a judicious use of glass – the architects are Adam Zombory-Moldovan and Lucy Clark of ZMMA – has brought in light and drama.  Meanwhile, fresh spaces have been subtly created, carved from back rooms and corridors. There is a fine new learning studio, complete with working kiln, and a study room for truffling scholars, book-lined and soothing sage. Never before has the Watts collection, which comprises some 6,000 objects including 250 oil paintings, 800 drawings and watercolours and 200 sculptures, had a finer home, or a better chance of capturing the imagination of a new generation for whom the great GF Watts, and his wife, Mary, are still an unknown quantity.  In the Weston room, where some of Watts's most famous paintings hang (Found Drowned, which depicts the body of a suicide by the Thames; Under a Dry Arch, a study of a destitute), the gallery's director, Perdita Hunt, surveys the result of seven years' slog. Is she thrilled? "Oh, yes," she says. "We've created a centre of excellence, and I think it will mark the beginning of a reassessment of Watts's reputation."  In his day, Watts was extremely famous, as popular and acclaimed as his near contemporary Dickens (both used the distressing inequalities of Victorian life in their work, to stunning effect). His reputation was, moreover, international; Watts was the first living artist to be given a one-man show at the Metropolitan Museum of Art, New York. Born in London in 1817, his artistic ambitions were encouraged by his father, a piano-maker, and in 1837 he exhibited at the Royal Academy Summer Exhibition for the first time (his entry, A Wounded Heron, hangs in the room that was formerly known as the Green Gallery). Thereafter, it seemed there was nothing that Watts could not do: narrative painting, landscapes, portraits (his Hall of Fame, featuring the most famous figures of his age, among them Lillie Langtry and John Stuart Mill, is a Watts Gallery highlight), even sculpture (stand before his giant Gesso grosso model of his Monument to Lord Tennyson and feel your knees buckle).  His most famous painting is still Barack Obama's favourite: the allegorical Hope, that curled, blindfolded woman, perched atop a globe, caressing a broken lyre – and at the Watts, the finest version of it, on loan from a private collection, comprises the heart of a small exhibition about its amazing life (it has even appeared on a Jordanian stamp). But there is more to Watts than his adventures in symbolism. He is a Victorian who feels oddly modern, an enigma who also happens, sometimes, to be an open book.  And the life! In 1864 Watts married the actress, Ellen Terry, then just 16. But the relationship did not prosper, and they separated a year later. It was not until 1886 that Watts married again. For his new wife, Mary Seton Watts, who was 32 years his junior and his former pupil, this was the climax of a long campaign. She had admired him ever since their first meeting in 1870, and by 1880 her feelings were so clear to her, if not to him – he discouraged her affections – that she told him: "Signor, I think I have been looking for you my whole life."  Together they left London for Compton, where they built their house, Limnerslease, and commissioned Christopher Hatton Turner to create a nearby gallery in Arts & Crafts style, "a simple & rural type of building", to be constructed of rendered concrete and Surrey tiles. It opened in 1904, just three months before Watts died. Thereafter, it was Mary, his devoted amanuensis, who kept the flame alive, and it is her that we have to thank for the richness of the gallery's collection – something the refurbished Watts sweetly acknowledges with its display of her remarkable bronze triptych in memory of her nephew, Death Crowning Innocence.  Mary was, of course, a supremely talented artist in her right; the Compton Potters' Arts Guild she founded survived until the 1950s. And so, after you have wandered the galleries – after you have gazed on Watts's charmingly innocent self-portrait from 1834, on his exquisite busts of Clytie and Daphne, and on his almost Bloomsbury-esque double-headed study of his favourite model, Long Mary – stroll along the lane to see her masterpiece, the Watts Chapel. It astounds, every time. First, walk around the building – still a working mortuary chapel – examining the terracotta tiles that adorn it like some embroidered veil, each one made by the villagers, mostly unemployed agricultural workers, whom Mary trained. Then go inside and gaze on its painted blue-green-bronze panels. You will feel, perhaps, that you have walked into the Morte d'Arthur. Finally, step outside again and follow the path up to the loggia. Beside it is the grave where George and Mary, who died in 1938, lie together. The fact that down the hill their other legacy lives on, transcendent and amazing, seems only to add to the deep sense of peace.  The Watts Gallery reopens on Saturday (18 June)
The legacy of Barack Obama's favourite painter is alive and well in a Surrey village, thanks to a £10m restoration, writes Rachel Cooke