When the Chichester Festival Theatre reopened last weekend with its bold, new rear volume by London architecture firm Haworth Tompkins, it was no less controversial than its original incarnation. That first building launched in 1962 with a modernist hexagonal design by Powell & Moya, which lent itself to a pentagonal thrust stage pointing ahead to a centre aisle. 'It divided the audience and it divided opinion,' says Steve Tompkins, who led the redesign of the concrete landmark. 'Some would say it's the thing that will always make it a flawed space for theatre.'

It could have been done away with for good this time, but Tompkins says he and his partners 'liked the eccentricity of it'. ''The correlation between the geometry of the stage and the architecture cannot be changed, lest it destroy the feel,' he continues, so the decision was made to 'ameliorate but not obliterate.'

Instead, the architects took a hatchet to the piecemeal extensions added over the years, as the summer theatre festival grew into a year-round concern. 'By the time we were commissioned to renew the building, a lot of the original clarity had been clouded,' says Tompkins.

In the past decade, Chichester has risen above its regional-theatre origins to produce award-winning West End transfers, so it was clear the organisation needed more space for set-storage and offices. Haworth Thompkins, whose design for the new Everyman Theatre in Liverpool was recently nominated for a Stirling Prize, responded with a back-of-house extension clad in weathering steel. 'We liked the relationship between the toughness of the brutal concrete and the toughness of the Cor-ten [steel],' says Tompkins. 'The latter will become a much more recessive surface, chocolate-coloured shadow version of the original building.'

To boost the gathering spaces, the architects opened up the foyer with two double-height glazed extensions that jut out into the surrounding vista. A pair of hand-painted ceiling frescoes by London artist Antoni Malinowski complement the geometries and link the indoors with the new landscaped grounds.

Of course, the play's the thing at Chichester. Haworth Tompkins' transformative influence in this respect will be seen at the new theatre's inaugural production of Amadeus. In the low-ceilinged auditorium the architects brought back two original side galleries that had been closed off due to fire regulations. The repatriation dramatically improves the balance of the theatre, and even though it has increased the number of seats to 1,300, it creates a cocooning effect, 'so it feels more intimate and intense.'