In the 20th century several changes took place , with the Ballroom becoming a cinema , until the Society for the Protection of Ancient Buildings became the owners in 1931 . The building was restored by A Mowbray Green in 1938 , with Oliver Messel as the interior designer . During the Bath Blitz of 25 / 26 April 1942 , one of the retaliatory raids on England by the Baedeker Blitz following the RAF 's raid on Lübeck , the Assembly Rooms was bombed and burnt out inside . After the cessation of hostilities in Europe , they was restored by Sir Albert Richardson , with work being completed in 1963 . The ballroom ceiling had to be repaired after it collapsed in 1989 .