But now the old shoebox is just a patch of rubble. It didn't take much to push it over. A few yards north, a much bigger theatre has taken its place. In fact it is four times the size. Again, it's a box. Again, it has a smallish-feeling auditorium, which is a bit of an illusion as we'll see. But this theatre has anything but a provisional feel to it. It has been built for posterity. This is potentially dangerous.

What you fear, when such places mature and grow, is the dead hand of the institution. Theatre is about chasing rainbows: impermanence comes with the territory. So by building a £16m, Lottery-assisted, permanent new theatre at Swiss Cottage - and now on a site directly opposite the Central School of Speech and Drama, so creating a thespian hamlet - there is no doubt that the Hampstead Theatre has changed its nature. It is now part of the establishment, even if it is still anything but West End. It has all the foyer and bar space, the scenery and rehearsal rooms, the offices and computers, the flytower and truck loading bay, even the landscape masterplan around it, that you associate with something that leans away from the edgy and towards the corporate. It is inevitable. Successful theatres breed demand for more seats and more creature comforts, just as a small football club does when it makes it into the Premiership. There's always that danger of over-ambition leading to subsequent relegation.

But Rab Bennetts of Bennetts Associates - who has had nine long fund-raising years to refine his concept since winning the competition in 1994 - plays it very straight. He has built a simple rectangular box, nicely detailed with dark blue ends, its flanks clad in well-proportioned panels of dark timber slats and glass. Into this box he has dropped the curving, zinc-clad form of the auditorium. This rises through the roof and merges neatly with a modest flytower, so avoiding the square-lump syndrome that afflicts too many over-endowed theatres.