"I'm really pleased at the way it sits in its context," says Grimshaw. "For us, it's an achievement. I think we've brought something extra." And indeed, the new spa building is something new: not just for Bath, but for Grimshaw as well. If you were expecting something like the glistening bubbles of Eden, or one of the huge exhibition halls, stations and airport buildings his firm is famous for, be prepared for a shock. Close though it is to the centre - just a hundred yards or so from the Roman Baths - the Thermae Bath Spa, to give it its full name, sits very happily cheek by jowl with its listed Georgian neighbours. A crisp stone-clad cube and linked staircase rotunda of golden Bath stone, pierced with tiny portholes, hover within a taut glass skin that wraps right round the site. The big cube - which contains the spa's changing, massage and steam rooms - breaks through the roof terrace of these glass-clad lower floors, rising a further level to an open-air pool right on top, high in the air, with views to the wooded hills surrounding Bath.

Since the whole cube is perched on four fat mushroom-shaped columns rising from a larger main pool at semi-basement level, this is a building sandwiched top and bottom by massive quantities of hot water, and there aren't many like that. Glowing on a winter's night, with the steam rising from the rooftop pool, it ought to be quite some sight. "It is modern," says Grimshaw with quiet satisfaction, "but it is not minimalist. We're getting across the idea of it as a pleasure, a place where you go to disengage, to contemplate."