Seven out of 10 BTO flats up for sale have waiting times of four years or lower, says HDB. SINGAPORE: The Housing and Development Board (HDB) launched 10,209 flats for sale in July's Build-to-Order (BTO) sales exercise, it said on Wednesday (Jul 23). This includes 5,547 BTO flats and 4,662 Sale of Balance (SBF) flats, it said in a press release. Seven out of 10 BTO flats up for sale have waiting times of four years or lower, including 1,396 units with less than three years' waiting time at