IT CROUCHES. It definitely crouches. In a slightly malevolent way, too. Like a medieval Stealth bomber armoured in dark, stained chainmail, or like a beast, rangey and muscular, with hooves and spikey scales, “poised,” says the press release, “with an arched back and taut skin, ready to pounce”, to gobble Kensington Gardens’ innocent Frisbee throwers and picnic eaters and the Serpentine Gallery itself in one terrifying bite. Jeepers.

Malevolence and beastly crouching aren’t qualities hitherto much in demand for tea pavilions. Elegance, Madeira-cake lightness, Pavlova-ish frilliness, with a slight nostalgic whiff of faded grandeur, yes. Dark, brooding and gothic, no. But this, of course, is no ordinary tea pavilion. This is the Serpentine Gallery’s annual summer pavilion, where you come not so much to sip tea as admire architecture at its most adventurous and experimental. It’s where risqué architects previously banished from the cautious UK are let loose untrammelled by planners and interfering clients, with naught but a budget to hold their imaginations in check.

Last year, though, the merry Dutch pranksters MVRDV railroaded through the budget with their fantastical plan to cover the entire gallery in an artificial hill to be clambered over by innocent Frisbee throwers and picnic eaters. Funds were not forthcoming enough, and the gallery, slightly backpedalling now, has largely consigned it to the archives as a “virtual” pavilion.

This year the Serpentine’s fifth (realised) pavilion is the work of the Portuguese architects Alvaro Siza and Eduardo Souto de Moura: not household names here like some of their predecessors in Kensington Gardens — Zaha Hadid, Daniel Libeskind — but just as talented. The pavilion, on first sight, is not characteristic of their work. Neither really does malevolence, beastly crouching and dark, brooding gothic. They ’re more your grace and poise kind of guys. Siza in particular, revered the world over by those who like their architecture subtle — minimalist to the uninformed, is weighty, tectonic and modern, with nods to the vernacular. Buildings such as the Serralves art institute in his home town of Porto, or the glorious Expo pavilion in Lisbon, with its column-less, bed-sheet canopy in hefty concrete, have earned Siza the Pritzker Prize for their structural daring and unerring ability to create hushed, monastic awe, their ever-so-carefully placed architecture, packed with subtle, deft twists, turns and proportional tricks that you or I might not instantly “get”, but which somehow innately feel right.

The pair are earnest, cultured and intellectual. They smoke fags. They come over like a couple of kindly, wise, if cloakless, Yodas, uttering what could be soundbites in lesser beings but which, on closer inspection, are actually rather profound. When they say this is a crouching beast, that’s what it is. And not for the PR. For the emotion.

The pavilion might not automatically look Siza-ish, but it’s all about evoking mood and feeling, and there’s nothing more Siza-ish than that. The Serpentine’s pavilion is in the great British tradition of the fantastical, picturesque folly of our country house estates, there to evoke emotions and allusions in the mind of the beholder. And surprise, too. Yes, the outside looks foreboding, ugly even. But step inside and the building is transformed into a light, airy net, the beast’s polycarbonate scales, which outside look so opaque, throwing a soft, gentle light onto the network of arching beams and the floor of cosy, cool brick. It feels warm and welcoming. The hi-tech, curving, zoomorphic shape, whizzed up on computers, in reality looks attractively bodged and cobbled, not sleek and mechanical. The struts don’t even meet up, for goodness’ sake, and the loose grid looks as if it could collapse at any moment.

It looks like something knocked up in the back garden by your uncle last bank holiday weekend. Again, pure Siza. The man has a feeling for both the humbly vernacular and practicalities. Charged with whacking up a formally daring pavilion from design to completion in three months — the Serpentine’s challenge — the man has made a virtue of bodge, turning it into an aesthetic perfectly targeted at our nation of garden shed DIYers.

Those low-tech “mistakes” are actually high-tech miracles. All that bodge — perfectly caught between loose and baggy and taut and energetic — is devilishly hard to do. The pavilion, like Libeskind’s and Toyo Ito’s, has been engineered by that engineer to the stars Cecil Balmond, deputy chairman of Arups, for whom the project is another experiment in his Advanced Geometry Unit, charged with making aesthetic sense and harmony out of the chaotic patterns suggested in nature. He calls it a “shuffling, shambling grid” in which each little strut leans on its neighbour for support: “Let’s stretch the modernist box. Here’s humble homely plywood, old-fashioned timber, let’s see it dance in space.”

Actually it’s not-so-humble Kerto, a new super-plywood stronger than conventional timber, because all the knots and inconsistencies have been removed, but warmer and more tactile than steel. The pieces are cut by computer-order, “then it went together like a jigsaw puzzle”, says Balmond.

Siza and Souto de Moura’s pavilion is perhaps the quietest, least immediately appealing of the series so far. It isn’t as eyecatching as Libeskind’s gyrating spirals or as elegant and crowd-pleasing as Ito’s high-tech jewel box or Oscar Niemeyer’s effortless curves. But it has presence, it demands exploration. It’s even, perhaps, the cleverest, with its fusion of boldness and subtlety, casualness and precision — pure Siza, in other words. Your cup of tea and a nice sit-down won’t be restful. But it’ll feed the mind and leave just enough room for cake.

• The Serpentine (020-7402 6075) summer pavilion is open July 2-October 2