Stairway to heaven: Thomas Heatherwick stuns Manhattan with his new store for Longchamp.

To paraphrase Oscar Wilde, it all began with a handbag. In 2003 the perpetually inventive Thomas Heatherwick was invited by French conglomerate Longchamp, patrician purveyor of luxury accessories, to design a handbag. He duly obliged with the Zip Bag, a typically audacious piece of lateral fashion thinking in which a super-long zip was wound in horizontal concentric rings around the entire bag, allowing it to expand and contract like a small but extremely stylish accordion. From this initial coup de foudre, the Heatherwick/Longchamp relationship has now consolidated into a serious affair, with the recent much hyped completion of a new Longchamp store in New York, the firm's first contemporary flagship.

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Occupying the first floor of an existing corner block in Soho, Longchamp NY is, from the outside at least, more tugboat than flagship, with surprisingly little pavement presence. Originally built in the mid '30s, the building suffered from a bad '80s makeover. Enticing fretful bag ladies beyond the unprepossessing frontage up to a luminous haven of luxury leather goods was Heatherwick's principal design challenge, and he has risen to the occasion with characteristic aplomb. A gorgeously theatrical staircase oozes and cascades languidly around a toplit circulation core, so that legions of happy shoppers barely register the fag of actually having to get upstairs. More sculpture than staircase, the construction might best be described as a series of ribbonlike forms, made from very unribbon-like one and half inch thick (38mm) hot-rolled steel. Together the polished metal strips gently swell and heave to create a vague yet intensely seductive semblance of treads and risers.

This stairway to bag heaven is enclosed by an impossibly ethereal glass balustrade, that billows and shimmers like transparent fabric. All too familiar with the industry standard of flat sheets of glass and their associated rigidity and lack of reflectivity, Heatherwick wanted something more tantalisingly fluid and sparkling. A year's development yielded a final design that exploits aerospace windscreen technology to create the requisite effect, each panel like a warped and buckled gossamer wing and each one unique.

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The main retail floor continues the theme of disarming undulation. The laminated timber ceiling is sliced open with forensic precision and the slashed strips peeled downwards to provide instant display units, while also exposing the building's disembowelled guts. By a similar slice 'n' peel logic, the floor plane is also thus modified, forming a handy series of gracefully curved surfaces on which to array Longchamp's exquisitely hand tooled produce.

A new second floor contains the business end of things, with offices, showroom and a wonderfully sybaritic roof garden. It's all very New York, as confirmed by the Bacchanalian scenes at the opening party (broadcast as part of a recent television profile on Heatherwick's oeuvre), but beneath the frivolous froth of fashion, there are some serious points to be made about the importance of craft, invention and construction, and how boundaries can, and should, be pushed. It is, perhaps, an architectural version of the familiar fashion editor argument for the innate superiority of haute couture over pret-a-porter. Longchamp could have easily opted for the familiar comforts of luxury blandness, but chose instead to take a more scenic route in the enquiring and engaging company of Heatherwick. Happily, their faith has been rewarded. Toujours l'amour.