The English city of York may be steeped in centuries-old history, but one unexpected jewel in its crown is a 1967 concrete creation by Patrick Gwynne. York Theatre Royal commissioned Gwynne to design an addition to its historical home, to house a café-bar area, after the success of his Serpentine Café in London's Hyde Park.

London architects De Matos Ryan were recently tasked with making the Grade II* listed venue – including Gwynne's vaulted umbrella forms – more appealing to more people, with the aim of increasing its revenue. Work has now finished; as a result, the size of the public space has doubled and the layout has improved, to encourage theatre goers and other visitors to use the seating and lounge areas throughout the day and into the night.

Gwynne's extension now houses a bistro, with dining chairs by York furniture designer John Green and pleasing muted crockery from Olympia Kiln. The architects discovered a yellow mosaic back wall and white Formica countertop – both installed by Gwynne – and restored them, as they did its 31 circular skylights.

Meanwhile, in the auditorium, the stalls seating has been raked (by tilting the floor), and the stage's raked floor has been flattened, to improve sightlines. 'There is not a bad seat in the house now,' says Angus Morrogh-Ryan, the architectural practice's director, who has also just put the finishing touches on the firm's revamp of Sadler's Wells in London.

This raking allowed the architects to increase the front-of-house space; and by glazing the Gothic colonnade, which had been open to the elements, they carved out another seating area and furnished it with pieces from HAY.

The theatre's £6m refurbishment is one of a number of boosts for this part of York. The £8m redevelopment of York Art Gallery, to a scheme by Ushida Findlay Architects and Simpson & Brown, opened last year, and De Matos Ryan is currently refurbishing York Mansion House.