The new Serpentine Sackler Gallery: A modern classic takes shape

Zaha Hadid's new creation is unveiled tomorrow. It shows how to update but not upstage a revered building, says Jay Merrick

We may be in the grip of austerity, but the new Zaha Hadid Serpentine Sackler Gallery in London's Hyde Park, which opens to the public on Saturday, offers a vivid, architectural antidote. Hadid's £14.5m transformation of an early-19th- century gunpowder magazine at the kink of West Carriage Drive redefines the Serpentine Gallery's image and allows its co-directors, Julia Peyton-Jones and Hans-Ulrich Obrist, to roll out a much wider arts and culture offering – as if exhibiting the work of 1,600 artists in the past 43 years were not enough. Their plans for the Sackler include new kinds of art, cinema, and literary events.

Hadid's first building in London was the Serpentine Gallery's inaugural temporary summer pavilion in 2000, and the new gallery – four minutes' walk away – is her first completed permanent structure in central London, despite the fact that she has studied, lived, and worked in the city for more than 40 years. Her association with dramatic architectural bigness may have something to do with that. But now we glimpse something more finely crafted in her work.

The headline feature of Hadid's renovation and extension of the 215-year-old building is the languid lily-white roof of the new gallery's café and social space. The canopy ripples outwards from the old brick facade, and melts down over the glass walls like a subsiding Modernist soufflé to touch the ground at three points.

Some may declare it an outrage that a Grade II* listed building, designed by Decimus Burton, should be carbuncled with a kind of Mr Whippy splodge minus the Flake bar. And not a few architectural aficionados will wince and murmur: "Designer chic." It's true that the curves of the Sackler's glass walls seem almost too perfect. And true that the silicone-coated cloth on the underside of the roof is stitched and tailored as beautifully as a lady's shirt from Anne Fontaine in Sloane Street and that the five sinuous columns supporting the roof look a bit like hollowed-out Prada high heels.

But the default connection between Zaha Hadid and all things fashionable is not entirely relevant here. Indeed, the extension is the latest of two pieces of new London architecture in the past year that have added something absolutely modern to historically important buildings, without upstaging them. The vast wave-form canopy over the new concourse at King's Cross station, designed by John McAslan + Partners, is the other example. Both stop safely short of being wow-factor eye-cons, and interventions of this design quality show how historic buildings, regardless of scale, can retain their historic character, yet reach forward ingeniously into the 21st century.

The changes at the gunpowder magazine have been quite simple. Gallery spaces have been created by roofing the gap between the original storage building and the additional single- storey protective facade around it, and Hadid and her historic buildings consultant Liam O'Connor have added a deliberately plain office segment along its northern side. The magazine, an encyclopaedia of 18 different brick types, remains much as it was in the early 19th century. According to the historian, Leo Hollis, a certain Mr Walter, armed with a sparkless copper shovel, tended the gunpowder packed into the church-like chambers. These are now two highly atmospheric central gallery spaces, with a rectangle of four longer spaces around them.

The creation of the new gallery was not simply about the Serpentine's ability to raise money, which in this case, depended primarily on the generosity of the Sackler Foundation, with significant further funding from Bloomberg and other donors. Originally encouraged by the Serpentine's chairman, Lord Palumbo, Peyton-Jones and Obrist were off the blocks quickly when, in 2009, the Royal Parks advertised the availability of a 25-year lease on the old gunpowder magazine. And those competing for it had to submit detailed proposals, plus confirmed funding, within four months.

"It was like a thriller," recalls Peyton-Jones. "We hadn't really investigated the building. The site was completely overgrown. You saw only the main facade. There was a pretty little meadow in front of it, and one Italian urn which the Royal Parks didn't seem to want. It was a fascinating process, because we knew the licence was a once in a lifetime opportunity.

"And it was by no means a foregone conclusion that we would get it. Actually, we were at a disadvantage: we already had one building in the park, and some people couldn't see why we should get another. But what we have here now is absolutely Zaha's concept from day one. And it isn't just about galleries, it was about creating social space, and supporting the parkland setting."

By Hadid's standards, the new extension is remarkably deferential to its context. As Obrist points out, the new structure has very little sense of weight; the roof is linked to the historic west facade in a very refined way; and the quality of internal light, and the inside-outside ambience, is calm and refreshing.

And so, Hadid's original concept sketch – as languid and serpentine as a squiggle of Golden Syrup – has come to life in a setting where Queen Caroline once hoped to create an ultra-grand landscape to rival the gardens of Versailles in the early 19th century. There is certainly nothing grand or blingtastic about the transformation and extension of the old magazine. The ghost of Mr Walter, shuffling around cautiously in his gunpowder-caked stockings, would still recognise his old haunt – but he might be stunned by the first exhibit in the Sackler's opening show, by Adrián Villar Rojas.

It's an almost life-size concrete elephant kneeling painfully under the weight of a massive cast of the historic architrave across the building's main façade. A few steps away, Zaha Hadid's billowing, finessed canopy makes much lighter work of history.

Today We Reboot the Planet, by Adrian Villar Rojas, Serpentine Sackler Gallery, London W2 (020 7402 6075) 28 September to 10 November