In most years you can spot the Serpentine Gallery’s Pavilion a mile off. Every summer its director, Julia Peyton-Jones, and the curator, Hans Ulrich Obrist, invite a contemporary architect untested in Britain to create a “temporary wing” to the gallery, open to all for outdoor events and impromptu picnics. And each year architects from Daniel Libeskind to Oscar Niemeyer respond by sparing not a rivet to plonk a monumental opus in Kensington Gardens. This year’s pavilion (the ninth), by the Japanese architect SANAA, does the opposite. Approaching the gallery, you can hardly make out the thing.

Made from a single, slim, horizontal plane of aluminium perched on skinny columns, and polished to a mirror, it so reflects its surroundings, the park’s trees and lawns, as to be almost absorbed by them. It disappears. The camouflaged pavilion is a stealth bomber. Is it there? Is it not?

This, say SANAA’s Kazuyo Sejima and Ryue Nishizawa, is not an icon. This is no monument. Instead, they call it a “collection of atmospheres” or moods, like a 3-D Mark Rothko. Last year’s, by Frank Gehry, took architecture’s iconic bent of recent years perhaps as far as it can go, creating a massive monument of hulking great timber almost as large as the neighbouring permanent gallery itself. SANAA’s, though, they say, is a delicate “pool of water”. Under the roof your eye is forever distracted by reflections caught on the ceiling. A tree. A person. A bus on the road next door. A blond comet streaks across it — a labrador chasing a ball in the park outside.

When it rains, Obrist says, “beads of drops cascade from the roof”, which, from inside, reflect on the ceiling as if falling upside down, turning a summer downpour into installation art.

The pavilion may be ethereal, but it’s not unreal like a computer graphic. It’s chunkily riveted together like an aircraft or a 1950s Airstream caravan, so the mirrored “pool of water” shimmers with dimples as if someone has thrown in a stone. It’s mesmerising, and fun. Undulating from waist to tree height, it might tempt the naughty to transform it into London’s biggest slide. The building is only really finished when the public take it over, popping in for a cuppa or a lecture, jogging through on the morning run. The intent always is to make contemporary architecture unintimidating, but not dumbed down. You can touch it. It won’t bite.

And in this, once again, the Serpentine succeeds, big time. The pavilion series has had its ups and downs. This is one of the ups. Architects just never get the chance to create projects of such conceptual richness in Britain. Most of the spaces we are surrounded by are like watery slop. This, though, is a shot of the hard stuff.

From Sunday until October 18

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