Slum clearance and rebuilding began in Leeds during the interwar period when over 18,000 houses were built by the council on 24 estates in Cross Gates, Middleton, Gipton, Belle Isle and Halton Moor. The slums of Quarry Hill were replaced by the innovative Quarry Hill flats, which were demolished in 1975. Another 36,000 houses were built by private sector builders, creating suburbs in Gledhow, Moortown, Alwoodley, Roundhay, Colton, Whitkirk, Oakwood, Weetwood, and Adel. After 1949 a further 30,000 sub-standard houses were demolished by the council and replaced by 151 medium-rise and high-rise blocks of council flats in estates at Seacroft, Armley Heights, Tinshill, and Brackenwood.