The headquarters of the Norddeutsche Landesbank in Hanover is a startling building in a boring city. But, says Marcus Fairs, the one thing it isn't shouting about – its green technology – is its most impressive feature.
It's not often you visit a foreign city and decide you'd rather spend the afternoon in front of the TV in your hotel room. But then, Hanover is a uniquely dull place: the streetscapes are unmemorable, the architecture banal, the shops uninspiring and the restaurants dreary. As for the bars and cafes – well, it's difficult to find any. The capital of Germany's Lower Saxony is prosperous and friendly all right, but it doesn't let its hair down.

A couple of years ago, the city had a rush of blood and held an Expo – but fellow Germans couldn't be bothered to travel to Hanover, so it flopped. "It's one of the most provincial towns in Germany," says Martin Haas, project leader at Stuttgart architect Behnisch, Behnisch & Partner.

Surprisingly, then, Haas has been allowed to build an exhilaratingly modern office block right next to the flamboyant Rathaus (town hall), challenging that building's status as the most interesting edifice in town. And it is even more surprising that the client is a bastion of conservatism: the state-owned regional bank.

In fact, the jaunty new headquarters of the Norddeutsche Landesbank – the North German Clearing Bank – came about almost by accident. In 1996 the bank held an invited competition for a new overflow building, to be sited a couple of blocks from its main offices. Behnisch, Behnisch & Partner won with a restrained, low-rise block.

The next year, in a transparent display of his power, the bank's chief executive decided that he would decamp the whole company to the new building. This required a much larger headquarters of 70,000 m2 to house 1500 people. The only way to fit all this on the l-shaped site was to build high, so the architect had to completely rethink the design.

"There was a big debate in Hanover because people said this is not a good part of the city to have towers," Haas recalls. "It was not easy to get the city to agree. The heights of the street elevations were restricted by zoning laws so the only possibility was to have a higher area in the middle of the building. We said to the bank, to do that, you have to give something back to the city."

Hence the schizophrenic nature of the finished £300m building, which opened in July. A narrow, six-storey block runs around the perimeter of the site, presenting somewhat austere glass facades to the surrounding streets – apart from on one side, where a listed facade has been incorporated.

The ground level is given over to shops and restaurants and the centre of the block is scooped out to provide a rather harsh public piazza with a a cascade of angular water pools. From the centre of this square rises a glassy, constructivist 17-storey tower, the floors of which progressively shift direction and cantilever outwards to resemble precariously stacked Jenga bricks.

At 86 m, it is just three metres short of the Rathaus' cupola (no building in the city centre is allowed to be taller). Most undemocratically, much of the tower is the private lair of the chief executive, and the boardroom occupies the uppermost floor. Not even Hanover's drab skyline can ruin the spectacular panorama from this lofty eyrie.

The base of the tower is given over to a triple-height atrium containing reception areas and a staff restaurant, all enclosed by a sloping slab of glass that sweeps down to touch the water pools. The glass is held up by leaning columns and punctured by two tubular glass walkways that provide aerial shortcuts across the piazza to the offices in the perimeter blocks.

The building's narrow floorplates are a result of Germany's strict laws governing office environments. "You have to give 11-15 m2 of workspace to each worker," explains Haas. "Every desk must have natural light, and be no more than 50 cm from the facade. Every worker has to have an external view from their seated position. It's very difficult as an architect in Germany to be creative because the laws are so strict."

British employees accustomed to battery-farm conditions would be amazed at the exceptional quality of these offices. Every workspace has fully controllable lighting – tasklights and uplighters but no dazzling ceiling-mounted strips – as well as sunshades controlled by a central computer. The external blinds are configured to direct sunlight onto the reflective ceilings but not onto desks, and workers can override the computer via touch-sensitive panels. (The computer takes over again if bad weather is forecast. During Building's visit, all the blinds suddenly lifted out of sight, as strong winds were on the way).

Behind each desk is a large storage cupboard designed by the architect, which doubles as a partition. The corridors, stairs and public areas are lavishly decorated with artworks and every floor has an open terrace for use on sunny days.

Internal walls in the buildings are also glass – a novelty in Germany, where enclosed cellular offices are still the norm. "The idea was to have light, light, light, light, light in the building," says Haas. "One of the main problems with glass walls is people can see who's working and who isn't. But they're getting used to it now."

But what is most remarkable about this building is its environmental performance: it is expected to consume half the energy of a comparable office. It will achieve energy savings of 15% on heating and an incredible 90% on cooling thanks to a combination of natural ventilation, groundwater cooling, heat exchanges and solar panels. On the upper floors, where the cost of pumping water for the natural cooling system would have been prohibitive, there is a conventional air-handling system. But the first seven floors have no air-conditioning at all – almost unheard of in a prestige office block, even in eco-sensitive Germany. Instead, workspaces have openable windows plus computerised ventilation panels, with solar chimneys inside the building creating a convection current to draw stale air out.

Temperatures are further modified by a geothermal system. In summer, cool groundwater drawn from 20 m beneath the building is pumped through the concrete floor slabs. The heat absorbed by the water is then removed by a subterranean heat exchange, and stored in the subsoil. In winter, the process is reversed to draw on the stored heat, and warm water is pumped through the building.

The external elevation facing the inner ring road has a double glass facade. The outer layer shields the offices from noise, and the inner one has the same openable windows as the other offices. The void between is fed with fresh air drawn in from the piazza, so workers do not have to breathe in traffic fumes.

The system appears to work, although temperatures do fluctuate more than in an air-conditioned building. "In the summer, if it's hot outside for long periods it gets hot inside, but only like in your own house," says Haas. Staff have already noticed that the elevated glass walkways can get very chilly, and have dubbed them "coldways". But Haas says workers simply put on their coats before crossing.

Haas' colleague, architect Kerstin Burmester, adds that the bank was initially sceptical about the energy concept, but was won over by the design team's enthusiasm: "It was a kind of education for the bank to realise you didn't need this constant temperature; you didn't need to have rooms that are cold all year round."

Other green features include solar panels to provide hot water for the bathrooms and kitchens, and banks of roof-mounted "heliostats" – computer-controlled mirrors that track the sun and reflect its rays into dark areas such as the atrium and piazza. These silvery devices look sensational, but Haas admits: "This is a bit of a gag. It's not really necessary for anything."

Likewise, the flat roofs are planted with sedum and wild flowers, partly to reduce solar gain but also because they look pretty. However, the designers stopped short of sticking banks of electricity-generating photovoltaics on the building, judging them uneconomic.

Even the water pools in the piazza are eco-friendly. The water contains no chemicals; instead, a carefully thought-out series of waterfalls aerate the water to keep it healthy. "We are still working on it," Haas admits, pointing to a green patch in the corner of one of the pools. "It has a little algae in it."

The architects concede these features are hardly revolutionary but believe it is the first time so many environmental techniques have been incorporated into a normal-looking, glass-clad office block. "The concept isn't new, but green buildings have always tended to look like green buildings. In this one, you can't tell."

Probably just as well – otherwise it would have been just too interesting for Hanover to cope with.