Architects and theatre directors have a lot in common: they're both used to dealing with abstract spatial qualities as well as nuts, bolts, pounds and pennies; they're both notorious for working right up to the deadline; and above all, they're both used to running the show. No wonder they rarely get along.  The granddaddy of modern theatre, Peter Brook, threw down the gauntlet with his 1968 tract The Empty Space, which begins with the lines: "I can take any empty space and call it a bare stage." Brook stuck to his word and based his operations in a ruined 19th-century music hall outside Paris; 30 years later, he still maintains he's never found a modern architect-designed theatre he likes. Many a director since has either followed suit (Ariane Mnouchkine's Cartoucherie complex, for example, built in a former munitions factory) or railed against the architectural burden they've been lumbered with (Richard Eyre's diaries are full of his struggles to fill the impractical expanse of the National Theatre's Olivier auditorium). What other area of activity would sooner dispense with architects altogether?  "All the interesting practitioners say the same thing about buildings: 'Why is it that we can't build permissive, exciting, beautiful, available theatre space? Why is it that so many of our new buildings feel corporate, overmanaged, overfinished, icily perfect and therefore alienating to a process which by its nature is exploratory and provisional?'" That's not Brook talking: it's Steve Tompkins, an architect. He's one half of Haworth Tompkins, a young firm who seem to have mastered the art of designing theatres that theatre people like.  Their remodelling of the Royal Court, in London's Sloane Square, set things off five years ago, with a universally praised combination of unpolished practicality and sensual urban chic. They've since installed the Almeida theatre's exciting temporary homes - a disused bus depot in King's Cross and a hulking former power station in Shoreditch - and renovated the Regent's Park open-air theatre. At present, they're halfway through rebuilding another London institution, the Young Vic; meanwhile, their Egg children's theatre opens next month in Bath. Beyond that, the theatre-related work continues: a research lab for the National, a school theatre in Oxford, the V&A's Theatre Museum.  "We would never describe ourselves as theatre specialists," says Tompkins. "You do one building and it seems to work out, but we found we had a taste for it, a passion." The passion goes beyond building theatres: a theatregoer since childhood, from a family of amateur performers, and married to someone in the business, Tompkins already had a foot in the theatre world.  "I feel a lot of empathy for what theatre-makers are trying to do," he says. "A lot of it is counter-intuitive to people trained as architects. Often, the very things that architects would identify as intensely problematic turn out to have been the things that were sustaining the creative life of the organisation. One has to be careful not to argue for poor theatres, but there is something inherently creative to theatre-makers - certainly ones that I've come across - about raw, unfinished spaces."  So far, Haworth Tompkins' theatre projects have all dealt with "found space", and they've become used to making the most of what's already there. At the Royal Court, for example, the architects juxtaposed ancient surfaces and new elements rather than smoothing them into an "icily perfect" whole. Tompkins retained the 19th-century auditorium but peeled back some outer layers to reveal the riveted ironwork holding it together. On the outside wall of the auditorium drum, meanwhile, artist Antoni Malinowski applied a vibrant vermilion coat that shines like a beacon into the square.  "Even though it is a carefully wrought scheme, hopefully it has that sense of being able to be knocked, kicked, scuffed," says Tompkins. "The whole palette of the building is about patination and the evident passage of time, because practitioners use spaces they like again and again, and they're aware of performances and performers who've used the spaces before. There was a myth about Olivier's handprints being on the back stairs, for example. I don't think they ever really were, but there was concern over losing them. Mind you, we still demolished them."  At its extremes, there's almost an element of set design to this type of work - the distinction between theatrical and architectural space starts to blur. Especially in cases like the temporary Almeida spaces, where Tompkins had just a few months and £700,000 to transform empty shells into practicable, commercial venues. "The theatre technicians and set builders worked alongside our carpenters and builders, and there was a pretty seamless relationship," says Tompkins. "We were finishing the theatre as the dress rehearsals were going on, literally."  The Young Vic redevelopment was a natural fit for Tompkins' approach. The theatre's original designer, Bill Howell, also strove to work with his theatrical clients. The result was an auditorium that's still one of London's best loved, despite the fact that it was designed as a temporary breeze block structure, and, 35 years on, is surrounded by a virtual shanty town of leaking, rotting, cluttered and confined spaces. Just as Howell incorporated the tiled butcher's shop that still stood on the original bombed-out site, so Tompkins is retaining the butcher's shop and the essence of Howell's auditorium, although he's expanded its capacity by 200 people. "The holy grail of auditoria is more people into same space," Tompkins explains. "It's like an electromagnet: the more coils you can get the more current you can generate."  He's also giving the complex much-needed breathing space: two new studios, a double-height foyer, accommodation for performers and staff, and a flexible circulation system connecting them all. There will be a rough approach to the interiors: plywood sheets, concrete floors, unfinished timber and red-primed steelwork. The spaces will acquire theatricality through theatrical means, says Tompkins, such as lighting. For the street facade, he has commissioned artist Clem Crosby to hand paint industrial panels with a pattern of cadmium yellow over black. These will be assembled randomly, behind a giant gauze of aluminium mesh.  Is there a possibility that the thrill of seeing a play in an unconventional place will overshadow the quality of the drama? Found-space theatre is in danger of becoming another exotic bourgeois diversion, a problem that Tompkins acknowledges. But the same techniques Tompkins has used for trendy London spaces, he's also applying in less fashionable areas, such as the Egg, a children's theatre in Bath. Essentially, it's another conversion of an empty, historic structure, in this case a 19th-century cinema. But this is a theatre so small that adults will have to bend and squeeze to negotiate their way around it.  Two 15-year-olds interviewed the architects for the job and told them exactly what they wanted: in a word, playfulness. The auditorium is a horseshoe of red corrugated plastic and industrial metalwork. "An adult client for a 120-seat auditorium would never have come up with this, and I don't think we would have on our own either," says Tompkins. "We'd never have thought of making an elliptical theatre. It has some of the qualities of a Victorian playhouse, and a sense of glamour."  The Egg opens next month. Meanwhile, the Young Vic has already passed its first audition: a visit from Peter Brook himself. The director, who regularly uses the venue for his London shows, reportedly demanded a £10,000 refit of the dressing rooms before he would stage his Hamlet there in 2001. Could this be a purpose-built theatre he might actually like? It ought to be, since Tompkins hired Brooks' design collaborators, Jean-Guy Lecat and Andrew Todd, as consultants. "It was a charged moment, that's for sure," Tompkins admits, "but happily he was very complimentary. I think he had always liked it but found it too low. But now we've raised the ceiling and expanded the capacity, I think he saw a lot more opportunities in the space."  After this next round of curtains has risen, perhaps someone will ask Tompkins to design a brand new theatre. "I wish they would!" he laughs. "We're trying to learn the lessons of these found spaces and these hybrid spaces and see if we can start to develop a thesis for building new theatres. That will be a very interesting project."
If they could, theatre directors would ban architects from coming anywhere near their buildings. Steve Rose meets the one man who has won them over.