Architecture: Will it get a welcome in the hillside?

The design panel that selected the new Welsh National Assembly building thinks so. Richard Rogers has come up with a clear winner.

WHEN SPITTING Image made a puppet of Richard Rogers, he wore his heart on his sleeve, an obvious reference to the architect's predilection for putting essential services such as lifts and escalators on the outside of his buildings. But seeing his latest masterpiece, the new National Assembly for Wales, Spitting Image will have to make a skeletally thin, utterly transparent ghost of his former self. There is nothing obvious about the chastely beautiful glass and slate building that will rise out of Cardiff Bay on a windswept site. The selection of the pounds 13.5m project from a shortlist of six, by a panel chaired by Lord Callaghan, is expected to be endorsed next week by Ron Davies, Secretary of State for Wales.

A slate plinth rising out of the sea will accommodate the assembly. High above the debating-chamber, the public will be able to walk the length and breadth of the site on a glazed ceiling. They will be sheltered by a great billowing lightweight roof structure made of steel net. Different kinds of glass - opaque, clear and wired - create effects of translucency in a pavilion that is Miesian in its simplicity. "God is in the details," Mies van der Rohe memorably said, and it is the lack of obvious details that usually pockmark buildings in the digital age - grilles and power outlets, ventilators, extractors and visible lights - that makes this building so obviously the winner. This is Richard Rogers at his finest.

Lord Callaghan described the design as "simple, straightforward, elegant and economical. It allows for light and transparency and will provide generous public spaces. In symbolic form, it represents the open modern democracy which the National Assembly for Wales must be." Adding that the panel's task had not been easy, because of six very different approaches using Welsh materials, he confirmed that the decision had been unanimous. The fact that Lord Rogers and Lord Callaghan are both Labour peers did attract some waspish speculation as to why he won. "Secretly, we thought that might count against us. I'm delighted that it hasn't influenced the panel's choice of our design," says Ivan Harbour, the project director from Richard Rogers Partnership.

Of the six, the Rogers Partnership design is the most transparent, all the more unexpected on a windswept seafront site that Ivan Harbour describes as "terrible, an Eighties attempt at regeneration". It is surrounded by fenced-off commercial properties, and two big brick blocks, Pierhead House and Crickhowell House, have been leased to the Welsh National Assembly for administrative offices and the Cabinet office. In 2001, the neighbouring Millennium Centre for Wales, nicknamed "Sellafield" for its white sterility, opens adjacent to the site.

"Rather than make another building as an object, we accept that those buildings that surround us create the space. In that space we simply keep out the rain, inspired by the simplicity of Mies van der Rohe's museum in Berlin," Harbour explains. There is accommodation in the slate plinth, which steps up in tiers, and public space from the waterfront as far back as Crickhowell's offices. Glazing from eye-height means that people wandering around the public spaces can glance into the debating-chamber at all times rather than be intimidated by having to gain access to the public gallery. For the debating-chamber, the architects have just drawn a circle, the ultimate democratic form. The concept will be extended to the seating.

This will be the first parliament in the UK to recognise sustainability. In the brief for the competition, organised under the rules of the Royal Institute of British Architects, there was a requirement for energy conservation and ease of maintenance. The environmental engineers BDSP, working with Richard Rogers Partnership, aim to use water from the docks to achieve 50 per cent higher efficiency than is normally achieved from heating and cooling systems.

Wales has more daylight hours than London but the frequency of sunlight and its duration are less, which affected calculations. In the brief, architects were also asked to use Welsh materials, for a building with a 100-year design life.

Rival architects on the shortlist resolved these issues differently. A secretive copper-clad building (already called the Welsh Dragon for its sinuous silhouette) by Benson & Forsyth reflects the fact that Phoenicians quarried copper in Wales, even as it reflects the sunlight and sea that would play upon its surface. Shafts of light penetrate the conical red assembly tower behind granite slabs that hold the glazing in a transparent, layered design by Eric Parry. Beached in a diagonal slab of glass, the timbered vessel of the chamber, designed by MacCormac Jamieson Prichard, is wrapped around at the back with a slate wall, while Itsuko Hasegawa Atelier with Kajima Design Europe chose to wrap the assembly hall in a sushi-shaped glasshouse. The functionalist Neils Torp from Norway, with Stride Treglown Davies, expresses with humour a timber building that is anything but wooden. He describes it as "a grand piano". Layers of wood sitting on stilts in a landscaped garden have two rationalist wings in sandstone and slate on either side to create a plaza. An exhibition of all six shortlisted designs will be on public view for a fortnight at four locations: St David's Hall in Cardiff, The National Library of Wales in Aberystwyth, the Shopping Centre in Colwyn Bay and the County Library in Carmarthen.