A cloud appears to have dropped out of the sky and landed among the trees in Kensington Gardens, in west London. This hazy lattice of spindly white rods, which hovers above the ground like a digital apparition, is the 13th annual Serpentine Gallery pavilion, designed by the Japanese architect Sou Fujimoto. At 41, Fujimoto is the youngest architect to be selected for the commission, and he brings a fresh, experimental energy to the project.
"I wanted to create a structure that was somewhere between architecture and nature," he says, "something like the primitive beginnings of a building."
His pavilion is not composed of walls or roofs, but rather is seemingly grown from a steel matrix that extends upwards and outwards in all directions, like a garden trellis on steroids. It forms a shape-shifting mass with no discernible edges. Here and there, it rises into pert peaks and swells outwards in dramatic overhangs; from other angles, it appears to slump like a deflated meringue.
As you walk around, over and through the structure, the layered grids play games with your eyes, creating ever-changing patterns in a three-dimensional tartan weave. In some places it seems solid, forming dense, rocky crags; in others, it is barely there at all.
Stepping inside, surrounded by this all-pervasive grid, feels like entering a computer mainframe, as imagined in the 1980s. It is a dazzling, Tron-like landscape of infinite white lines, the modular, cubic units of which form terraces of seating and steps, side tables and a coffee bar, as if Fujimoto has revealed an invisible geometric order of which the whole world is made. The cubes are not closed, but rather extend in projecting rods, like the frayed edges of fabric, suggesting the network might continue to grow. On an overcast day, it's hard to tell quite where it ends.
Relating his design to a distinctly Japanese conception of nature, Fujimoto describes how the process of crafting the form was "like clipping a bonsai tree". Working from hand drawings and large-scale models, he "pruned" sections of the grid to provide nooks and crannies, creating a multi-levelled terrain of places to perch and explore.
"Architecture," he says, "should provide a background structure that allows people to behave in a different way. I want visitors to find cosy spaces here." It is an idea that runs through all of his buildings, which he compares to caves and nests, trees and forests: natural landscapes to be inhabited.
His transparent House NA, in Tokyo, is as close as you could get to a treehouse of steel and glass, with clusters of rooms suspended within an impossibly slender frame. It is, like the pavilion, a beguiling nest of what he calls in-between spaces that elegantly dissolve the boundaries between the private interior and the busy street on a cramped urban site.
But Kensington is a long way from the dense plots of inner-city Tokyo. Here, his utopian frame looks like it could do with something to rub up against. It is a shame that, where it meets the realities of safety and access, his continuous landscape is fettered with balustrades and safety rails to stop you hitting your head – as well as layers of acrylic discs to keep out the rain.
Like the dreamy megastructure projects of the 1960s before it, the relentless aerial grids of Yona Friedman and the tree-like towers of the Japanese Metabolists, with their plug-in living pods, Fujimoto's cloud is somewhat deflated when it meets the real world. But as a powerful distillation of a young architect's ideas and one of the most radical pavilions to date, it sets a promising direction for the Serpentine programme.