Radical revival.

The Royal Court has been revived in a programme that sensitively responds to its pioneering theatrical history. Originally built in 1888, London's Royal Court has been the home of the English Stage Company since 1956. It is one of the most important theatres for new writing in Europe, presenting a consistently challenging mix of avant-garde and experimental new work in the intimate setting of a conventional Victorian playhouse. Yet by the mid 1990s, the theatre had become dangerously dilapidated. Structurally unsound, technically obsolete and riddled with damp, it had reached the point of physical collapse. Successive piecemeal alterations had confused the building's circulation and reduced its efficiency, but there was little apparent potential for expansion on a tightly constricted site. Despite these drawbacks, Haworth Tompkins were commissioned to undertake an extensive renovation that gets under the skin of the old theatre to invest it with an invigorating new lease of life.

From the start, Haworth Tompkins set out to absorb and crystallize the physical and artistic character of the Royal Court, which might easily have been swept away given the necessarily wide-ranging scope of the redevelopment. The architects spent months with directors, actors, designers and staff in order to understand the nature and workings of this stoutly cherished institution and evolve an architectural language that could reflect both its history and continuity.

The result of Haworth Tompkins' immersion is plainly discernible. Their approach is painstakingly archaeological, an imaginative synthesis of excavation and preservation. Existing basement levels have been expanded, circulation rationalized and new parts added, effectively doubling usable space and freeing up the main auditorium and foyers. To the rear, a new nine-storey annexe clad in red-painted steel panels, Cor-ten mesh and cedar planking contains new office spaces and dressing rooms, replacing an Edwardian addition. Behind the cedar skin lies a new service entrance, so that scenery need no longer be sawn in half before it can be brought into the building.

Front-of-house spaces have been replanned to make them lighter, larger and connect more coherently with Sloane Square. Incremental layers of internal plaster have been scraped back to reveal the original brickwork, fragments of tiling and mosaics. These gutsy, scarred remnants are simply left raw, as evocative evidence of the building's former incarnations.

A curved three-storey drum encloses the outer wall of the remodelled auditorium, marking the threshold between the outside world and the inner sanctum of the theatre. Coloured with layers of rich vermilion pigment by artist Antoni Malinowski, the surface of the drum subtly gleams and shimmers in different lights and is a constant reminder of the shrine-like presence of the theatre at the heart of the building. The auditorium has been reconfigured to improve sightlines. The original Victorian cast iron structure has been stripped back and exposed, its riveted, pitted hulk like a decaying ship's hull. Plump new seats in battered maroon leather reinforce a sense of dark, clubby intimacy.

Back-of-house spaces have also been extensively improved. For the first time in the Royal Court's history, there is now wing space, including a carpentry workshop and scenery store. The fly tower has been underpinned, reinforced and extended upward and the stage is equipped with a hydraulic lift and new floor of untreated Jarrah timber.

The Theatre Upstairs (the Royal Court's smaller studio theatre) retains its modest, domestic feel, linked to the rest of the building by a main stair that shrinks in scale as it ascends, like a staircase in a private house. A new, higher hipped roof made of dark reclaimed timber adds space and conceals service ductwork, while a simple rostra seating system offers directors and designers a great range of staging options.

Additional front-of-house space has been created by tunnelling under the road outside to connect through with Sloane Square (an exceptionally complex feat of civil engineering that involved navigating around an Underground line and a main sewer). The new undercroft extends the existing stalls bar and houses a kitchen, restaurant, bar and small bookshop. Polished concrete and Venetian stucco line the bunker-like restaurant, which is also generally accessible to the public and looks set to become a lively new Chelsea meeting place. Pavement lights transmit a delicate luminance into the subterranean depths.

Balancing the demands of historical continuity and experimtal innovation, Haworth Tompkins' intelligent remodelling echoes the essential spirit of the Royal Court; radical yet acutely sensitive, bold but rigorously pragmatic. Like a splendid moth emerging from its constricting, fusty cocoon, one of London's best loved cultural institutions has been gloriously reborn.