Nicholas Grimshaw himself still sits in his glass-walled office on the first floor, overlooking the street at the front. His fellow directors wander in and out to discuss the course projects are taking, he heads many of the design meetings. Out of, say, 30 live projects, perhaps half a dozen at any one time will be at the active design stage: a comfortable number for Grimshaw to be involved in directly alongside his colleagues, together with an undercurrent of competitions.

As a result, things are all still surprisingly intimate. In the very volatile world of architectural employment , the modus operandi of the practice is an expression of two-way loyalty - directors to staff, staff to directors. Nobody can wholly escape the boom-bust cycle of the design and building industry: before the commission for the Waterloo International Terminal in the late 1980s, for instance, the practice was down to 20 or so people. Having subsequently reached a peak of 80 there was another disconcerting dip around 1994, towards the end of the UK's last recession, before the present wave of commissions came on stream to carry the office to new heights. This switchback ride is normal enough, though NGP, having not been particularly exposed to the most volatile sector of commercial office and retail design, had an easier time of it than many during the last two turns of the economic wheel. This is not known as a hire-and-fire firm. It prefers to hang on to people it knows and respects, even to the extent of establishing working collaborations with those who, in the normal course of events, leave to form their own practices. Work tends to be referred their way. Grimshaw himself is keenly aware of his responsibilities as employer but he is also a man of routine, who builds up working relationships over time, and likes to see them mature. That way, the work of the practice can develop. It is notable how the present generation of directors, though younger, is battle-hardened in the office.

Periods of expansion, in contrast, bring in fresh blood to rejuvenate the body of the firm. In early 1999, for instance, after 32 years of practice, the average age of the Grimshaw office - including him and his directors - was 31, so making it a younger concern than many brand-new practices. "I'm proud of that," he says. "We've always fed from the bottom. It leads to liveliness, a good level of debate in the office. We have this idea of chucking ideas on the table and letting the best one win."

It is indeed strikingly apparent that this is not an office where the man with the name on the letterhead makes all the decisions. On the contrary, the other directors are a strong and independent-minded bunch. Sometimes Grimshaw would do a detail differently or choose a different material: but he respects the reasoning of his colleague, if sound, for doing it another way. Then again, he has longer experience on his side. Which may be why he remarks, as an afterthought to the above: "A lot of people say I find ways of pushing things my way. That may be because I've learned how to argue for things rather well over the years."

There is a touch of Wright in that afterthought, and this is no co-incidence. He always acknowledges that architecture is hard work, says that he is still learning his trade. By extension, that applies to the whole practice. In different conversations at different times over the years, Grimshaw has referred time and again to this business of continual learning. For him - as a British and European architect now working on commissions in the United States, such as the $40 million Donald Danforth Plant Science Centre in St. Louis, Missouri - the American architectural culture is fascinating. Given their background in highly detailed design, it comes as no surprise to find that he and his directors incline not to the commercial side of the business where architects do broad-brushstroke designs that are detailed up by others. Instead they lean towards the parallel American craft tradition, the tradition exemplified in its varying ways by both Wright and Mies, the tradition of the exact detail.