What's in store?

Not a museum, but not quite a warehouse either.

"It was a nice challenge to be faced with," says Harry Gugger, partner at Swiss architect Herzog & de Meuron. "We had to invent a new typology." This must be one reason why the Schaulager, on the southern outskirts of Basel, in Switzerland, has a unique form.

Translated literally, Schaulager just means "show-storage". The word sums up the paradox presented by the new home of the Emanuel Hoffmann Foundation art collection. Works will be stored, looked after and lent out to exhibitions, but they will also be on show. Herzog & de Meuron's original idea was to hang everything on one giant wall, but the architects quickly moved on to the more practicable warehouse format.

The next imaginative leap was to build the world's largest adobe structure, as adobe is perfect for regulating temperature and humidity. "The brief was really strict," says Gugger. "The building had to have an average of 20ºC with a tolerance of plus or minus one degree. This is hard to achieve in our climate, where we have hot summers and very cold winters."

But the adobe, too, fell by the wayside, as it can only be applied in the dry summer months and there weren't enough of those in Switzerland to accommodate the build schedule.

Finally the architects decided that the aggregate excavated from the site would insulate the building perfectly. But they also wanted the building to look as though it had risen out of the ground – and the pursuit of this aesthetic entailed going to extraordinary lengths. To expose the aggregate they had to scratch away the smooth surface of the concrete walls, but this meant using a retarder in the concrete, which left the walls "like pudding".

To convince the client that the walls would last 100 years and that the surface deterioration had not damaged the structure, Herzog & de Meuron built testing machines. "Because the machines included a heater, a refrigerator and a water pump, we were able within a month to simulate the life-cycle of 50 years, and we had to do that several times to come to a result that was acceptable," says Gugger.

Gugger reveals that one reason why the exposed walls will last is that they include an alpine mica that becomes soapy when wet: "The wall is like a self-healing body; it has millions of tiny cracks in it but if rain washes it, the super-fine mica particles are washed into the cracks and these are filled over time."

This technical battle finds a metaphorical reference in the windows, which from the outside read as giant cracks tearing through the side of the building. "We had the idea of creating a huge crack, which is an easy idea but controlling it and defining it architecturally is a different issue," says Gugger. In fact, the window was moulded from a battered copper tube one-20th of its actual size and translated into a digital 3D model.

The same tube was used to model the ceiling of the cafeteria, which undulates like the roof of a cave. Gugger's analogy for the ceiling is dough rolled out by the misshapen tube. Again it was digitalised and this time rendered in a dense alabaster-like plaster in which Jasper Morrison's globe lamps nestle "like fruit in a fruit cake". This is Herzog & de Meuron thinking at its most cohesive: from tiny crack to giant crack, from the inside surface of the crack to the inner surfaces of the building. "We don't like to invent forms," says Gugger. "All the forms you find in the Schaulager are a homologue of the surface of the concrete wall."

One form that does stand out from the overall scheme is the quaint little gatehouse, which provides a human sense of scale before visitors enter the four-storey high reception area. This makes clear reference to the surrounding neighbourhood, which shifts suddenly from small-scale housing to massive industrial sheds.

Notwithstanding the difficulties the architects had in reaching their aesthetic goals, the toughest challenge was installing the building services, which are entirely hidden within the walls. "The whole building is activated, every wall is like a radiator," says Gugger. "So if the systems fail, you have to get a hammer and open up the structure, like you would have to cut into your body to get to your liver. But we made sure they won't go wrong."

The most difficult thing was installing the services during construction, when the site was a mess. It was worth it: the Schaulager has such low energy use that planners originally thought the architects had miscalculated. "The boiler is the size of one in an ordinary house. When we submitted the building application they sent it back saying: 'No-one can heat such a huge structure with that boiler'," says Gugger.

He credits much of the building's success to the client, which speedily responded to each of the architect's decisions as they were made. "People think the best clients are the ones that let the architect do what they want. Which is seriously wrong," says Gugger, "because the best clients are the ones who are able to challenge the architect on a similar level."

That relationship was clearly a fertile one, as the Schaulager has just won the Swiss Architectural Award for best building of 2003.