Grimshaw is keenly aware that his firm is still seen by some as an edge-of-town, supershed kind of outfit. The new £20m Bath spa, supported by the Millennium Commision and bang in the middle of an official World Heritage Site, is his riposte to that. "If you're any good as an architect," he remarks in his diffident way, "you should be able to do a modern building in any situation. It would have been very easy to do a sham Georgian building. It would have walked through the planners." As it is, the thing that caused most head-scratching among conservationists was not the new building, but the fact that the entrance to it involved replacing an old shop front (in an existing derelict house alongside) with a curving, all-glass Grimshaw number. That little modification went all the way up to the top of English Heritage, and came back down with gracious assent.

Things were made somewhat easier by the fact that the new spa was built on the site of a disused public pool building of little merit dating from 1927. Plus the fact that the project involved restoring and re-opening two gems of Georgian architecture alongside - the Hot Bath and Cross Bath. Grimshaw has taken the dimensions of his big stone cube from the square layout of the 1775 Hot Bath next door, which is linked by a glass arcade to the new building and now contains treatment rooms.

The Cross Bath, a tiny little oval building, sits jewel-like in the little square outside, as it always has, going through various transformations over the centuries: the baths were built in this part of town because here is where the springs rise. Like the old Hetling Pump room to one side of the square (named after a German doctor who used to practise there), the Cross Bath has been discreetly modernised and put into use as part of the overall complex. The spring that gurgles up there into a basin designed by water sculptor William Pye is still regarded as sacred by the new-agers who tend to congregate in Bath - most days you'll find posies of flowers there as an offering to the gods. Grimshaw worked with conservation architects Donald Insall and Associates on the historic work, and the two firms, coming from utterly different ends of the architectural spectrum, have got along rather well.