After Zaha Hadid, Daniel Libeskind, Toyo Ito, Oscar Niemeyer, MVRDV and Alvaro Siza/Eduardo Souto de Moura, it was high time Koolhaas had a go at the annual “pavilion” architecture commission for London’s Serpentime Gallery in Kensington Gardens. It is certainly well different to the ground-hugging beast given us by the Portuguese pair last year. The architectural heavyweight has gone lightweight.

How light? Has Rem managed a zero-gravity building, a Bucky Fuller dream? Yes, if you do the calculations on the moving top part, which includes an aluminium-framed rectangular frieze by Thomas Demand let into the transparent underside of the balloon. The weight of this top half is three tonnes, and the lift generated by the balloon is six tonnes. Result: a building with a weight of minus three tonnes. But only if you don’t count the bottom section, which looks very light – it is walled in translucent polycarbonate – but has a lot of steel in it. Perhaps we should regard this bottom half as the airship’s mooring mast.

These commissions are done lighting-fast, without a contract to slow things down, funded entirely by sponsors and the sale of the building at the end of the season. The only brief is that they must act as a lecture space and café. Despite this speed, there is time for the occasional exquisite detail: here it is the vertical steel tension cables bracing those polycarbonate walls. The cables run fully encased in moulded grooves in the transparent stiffening fins. Just lovely.

Rem mumbled on at the launch as he does, munching grapes constantly. He sees it as an extension to the existing Edwardian Serpentine Gallery rather than as a separate entity. He sees it as being about the activities that will take place there, rather than being a building in itself. With the Thomas Demand frieze, it incorporates art but also – since the frieze is one of Demand’s “wallpapers” – it “avoids an encounter with art”. And so on.

It was left to Arup’s Chris Carroll to be a bit boosterish about it. “This is the first buoyant structure we’ve realized,” said Carroll, buoyantly. “It is a structural form without precedent.”

So: this year we’ve got a big squishy misshapen balloon on a rigid drum base. It doesn’t sound like much, put that way. But it does edge just a bit nearer that old idea of the flying building, the ultra-minimal structure, the featherlight footprint. I’m glad the Serpentine continues with this series of short-life modern follies. They serve to advance the art and science of architecture.

Oh, and oddly this is Rem’s first building in London, a city he has lived in intermittently since 1968. But he has a big HQ coming along for the N.M. Rothschild merchant bank in the financial district.

Links Serpentine Gallery: http://www.serpentinegallery.org/
Rem Koolhaas’s Office for Metropolitan Architecture: http://www.oma.nl/