There's also been an architect-artist collaboration going on. The outside walls have tall slots sunk into them for the light sculptures of Martin Richman. Inside, the sloping curve of the back of the auditorium - again, intriguingly lit by Richman - continues down through three levels, with foyers spanning the full-height space. At the bottom, the auditorium sits on raking struts enclosing a semicircular boardroom. Very corporate. Then again, those struts make a miniature stage-house of their own: you can well imagine this part of the lobby being taken over by performers, especially since a big education room, doubling as a studio theatre, is also down at this level.

You get to the auditorium up gently sloping ramps. Striving to keep the audience close to the action while also packing in more seats, Bennetts has adopted a croissant-shaped plan - a flattened horseshoe or an ellipse if you prefer - where the seating not only curves round, in stalls and gallery, but is also tilted towards the stage to maintain good sightlines. For me, this gives the place a slightly unsettling double-curved effect. Then again, who in a theatre ever looks at the seats?

The key is flexibility. The old auditorium had 175 seats. The new one can take from 150 to 325, depending on whether you have a thrust stage or not, and how you set out the seating. The space can be closed right down for intimate, small-audience productions, or opened right up for the Hampstead equivalent of a blockbuster. Despite this versatility, it accords with the current orthodoxy of clearly being a designed theatrical room rather than an anonymous "black box" of the kind favoured by an earlier generation of directors.

It is all very logical. Bennetts, a wry Scot and a good architect who has never been remotely fashionable, has always eschewed the fanciful in architecture in favour of a subtler approach. At the time he first designed the Hampstead Theatre, you could have accused him of being dull. Now, after a global surfeit of Guggenheim-induced architectural fantasy, it looks like he was right all along. By chance the Hampstead Theatre has arrived just at a moment when that kind of restrained, god-is-in-the-details architecture is roaring back. Even the trendies are designing buildings like this these days.