The garden of minimalist delights

A summerhouse made of wood and plastic has beaten big-time projects to a national architectural prize. Jay Merrick views a small but perfectly formed structure

In a narrow back garden in north London, there is a shed which proves that the art of architecture can be applied to even the most modest arrangement of walls, floor and roof. The award-winning summerhouse designed by Ullmayer and Sylvester Architects is a great little essay in lightweight design - a trick of the light composed of plywood, pine, milkily translucent polycarbonate and mirror-foil.

In a narrow back garden in north London, there is a shed which proves that the art of architecture can be applied to even the most modest arrangement of walls, floor and roof. The award-winning summerhouse designed by Ullmayer and Sylvester Architects is a great little essay in lightweight design - a trick of the light composed of plywood, pine, milkily translucent polycarbonate and mirror-foil.

The summerhouse is the practice's first completed solo project, and has just won second prize in Architects' Journal's annual Small Projects competition. The architectural quality of the summerhouse transcends its size: this £35,000 fidget of a building beat off competition from projects costing up to £250,000 and designed by established practices such as Studio Baad, Glas Architects, Bauman Lyons Architects and Sarah Wigglesworth Architects.

Just off Stoke Newington Church Street is an unremarkable avenue terraced on one side by late Victorian houses and lined, on the other, with modernist council blocks. The garden of the house in question is unusually long and, from the mezzanine-level terrace, the summerhouse is barely visible through an elegant scrim of trees, shrubs and climbing plants.

There is something of the stealth object about the structure, though not in a glibly ironic manner. Sylvia Ullmayer and Allan Sylvester have achieved one of the neatest of architectural tricks because the summerhouse, an obvious demonstration of minimalism, offers a form and demeanour that displays precisely arranged materiality, yet gives a glimpse of the charmingly dematerialised.

This might seem unsurprising if we consider the architects' key shared antecedent: they completed their degrees at the Bartlett School of Architecture at University College London, a hotbed of techno-trippy architectural phantasmagoria. And Sylvester was tutored by Neil Spiller, whose polemic is as likely to invoke the post-Dadaist lyrics of Captain Beefheart in his riffs on binary poetics, as the jellyish semiotics of computer-generated architectural space.

The summerhouse seems to mock such New Age science fiction for the virtualised chattering classes. Reality pervades every high-res detail: the knots and grain of the timber; the screw-heads in the plywood; the crisply lucent edges of the polycarbonate. Here is a very directly conveyed structure whose artifice - the extra ingredient that makes it notable architecture - is conveyed via the exposed grid of the timber frame and joists of the interior, and their relationship with a couple of design googlies that spin the summerhouse's overall form away from shed-ness and into an engaging realm of subtly skewed visions.

The key to the summerhouse's finessed presence, in a garden planted with some deftness by Sybil Caines, is its gently dog-legged north-facing façade, and the similarly kinked "butterfly" roof profile. Put another way, the building is bent in both plan and elevation - but only slightly. There is no obvious drama here, no graphic punch. It takes a moment or two for the eye's natural assumption of the building's orthogonal form to be pleasantly derailed. And when it is, the quietly engrossing beauty of the summerhouse crystallises.

Its vibe owes something to the Scandinavian tradition of sauna and weekend cabins: ultra-lightweight construction, complete clarity of structure, minimal interference with nature. The ethos and aesthetic that produced the summerhouse might be compared with, say, the small Finnish timber buildings of Georg Grotenfelt or Aarno Ruusuvuori. Sylvester also mentions the work of the remarkable US architectural collective Rural Studio as an influence - though a necessarily faint one here, given the site and required function of what is, ultimately, a graphic designer's workspace-cum-chill-room.

The only stark digression from Scandinavian architectural imperatives is that the summerhouse is anchored to a rectangular foundation made of steel joists, set on small concrete pads just below a layer of shingle.

The decision to use exposed steel rather than a bolted frame of robust, but much less noticeable, Douglas fir or jarrah beams seems suspiciously like an act of heightened contrast - the softly, creakingly organic apparently floating above something brutish, hard and essentially inert.

Other, more finely contrived architectural contrasts are the keys to the design. Ullmayer speaks of an attempt to screen out the summerhouse's urban setting by tapping "baroque ideas of illusion, distortion, camouflage and capture of nature to create a contemporary, rigorous and surprising building".

The decisive move is the 8m-long swathe of mirror-foil that completely covers the exterior plywood sheathing of the dog-legged, northern side of the summerhouse. Looking along its length towards the house - which can barely be seen - the viewer is confronted with two floral and herbaceous realities, one orderly and actual, the other warped in both the horizontal and vertical planes. The effect is brilliant, and rendered surreal by the very slim projecting edge of the polycarbonate roof - something absolutely definite set against what might as well be a giant screenful of Photoshop trickery. Baroque? Virtually baroque, at a pinch, in the same way as Softroom's Kielder Belvedere, whose luscious polished-steel surfaces caused such a stir four years ago.

But the most interesting thing about the summerhouse, seen from this corner of the garden, is that it frames the nominally "spare" space between the summerhouse and the garden fence in such a way that it becomes an active rather than a passive presence: the space seems as clearly defined as the outline of the building; one almost expects to see ghostly dotted lines tracing its containing outline.

A similarly teasing relationship between form and detail is experienced if one looks, head-on, at the summerhouse's open end. Ullmayer and Sylvester have used a subtle device to play with the perspective of the building's interior. By angling the glazed facade one way, and the edge of the cantilevered roof the other, the exposed ceiling joists running straight back through the summerhouse look decidedly kinked. They're not. It's an illusion, an entertainment.

Several of the Architects' Journal Small Projects shortlisted projects - including Nord Architects' winning house in Scotland - reveal strong design, yet radiate a self-conscious facility whose engaging details seem to dilute raw architectural presence. Imago's glass staircase in a house - an avowedly private house, no doubt - at Holland Park, London, is quite wondrous. But it is, alas, nothing else. We must assume that the architects are capable of more than precious flights of Elle Decoration and Wallpaper* fancy.

A number of the other finalists provide comparable entertainment, some of it as taughtly conveyed as the shed in NW16, some veering less interestingly into des-res territory. I wrote about Glas Architects' block of jet-black modernist semis in Ealing, London, some weeks ago in these pages and, in the absence of the summerhouse, they would have been worthy winners - as would two or three other buildings in the AJ's distillate.

At the sharply modernist end of the scale, we find Studio Baad's primary school at Gorse, highly graphic in its treatment of key façades. Bauman Lyons Architects' garden-room extension to a house in Leeds is even more striking - a self-consciously heroic demonstration of refined structure that filters Bauhausian health-and-efficiency through the practice's hi-tech filter. If you're familiar with Bridlington, then you'll have had the pleasure of walking along Bauman Lyons' crisply revamped promenade.

The antidote is the coherent architectural relativity achieved by John Pardey Architects in a context that is ostensibly ideal, yet turns out to be oddly fraught. Their task was to create a new wing for a post-war Modernist house near Great Missenden, Buckinghamshire, the kind that would originally have been advertised as "architect-designed", a worrying tautology at the best of times. The setting - open countryside, sloping site - is absolutely delightful.

But the existing property is, in my opinion, something of a minger, with few distinguishing marks, let alone any general "architect-designed" distinction. Pardey have delivered a handsome, calmly composed extension that is almost striking, but manages to stops decently short of being an implicit put-down of the original property. This is skilled and thoughtfully improving architecture.

Thankfully, such heated considerations evaporate at the mere thought of that summerhouse just off Stoke Newington Church Street, which shimmers playfully in the mind's eye like a cold, crisply cleansing slab of architectural sorbet.

Architects' Journal Small Projects, Royal Institute of British Architects, 66 Portland Place, London W1 (020-7580 5533) to 28 May