Build-To-Order was introduced in April 2001, currently BTO sale launches are in February, May, August, November. Future BTO locations are announced 6 months in advance. This table shows BTO names, town, launch date, estimated completion date, number of units breakdown by flat type, selling price ranges, etc.
Officially there are 18 HUDC estates, but actually Housing and Urban Development Company built 22 estates for middle-income people, including 3 not managed as public housing and one demolished in 1990s. HDB took over JTC and HUDC in 1982. Phase III and IV are HUDC flats built by HDB, last HUDC was completed in 1987. HUDC was replaced by Executive HBD flats built since 1984.
Selective En-bloc Redevelopment Scheme was introduced in 1995. 71 sites were selected by 2007 and another 11 sites in 2008-2012. This page shows list of SERS sites and (personal prediction) future SERS sites. On 19 August 2018 HDB announced that only 5% of total sold flats are eligible for SERS, of which 4% have been selected already. For all other flats there will be VERS (Voluntary En-bloc Redevelopment Scheme) that will start in ~20 years (probably in 2036 when first leases reach 70 years old).
Executive Condominium (EC) is a form of subsidized housing built by private developers, having same eligibility restrictions like HDB (family nucleus, citizens and permanent residents), you should dispose any other property 30 months before applying, minimum occupation period of 5 years before you can sell to citizens and PR, even single PR. After 10 years since TOP date, all restrictions are lifted and even single foreigners can buy. I made an interactive map with TOP dates.
Design, Build and Sell Scheme was introduced in 2005, giving private developers freedom to build and sell public housing in the way they sell condos. Since private developers are hungry for profit, DBSS flats have smaller rooms and abnormal high prices compared with BTO flats despite of same income ceiling ($8.000 per month, $10.000 per month after 2011). DBSS land sales were suspended in 2011.
Singapore Improvement Trust set up in 1927 and focused on infrastructure, it also built small scale public housing, such as Tiong Bahru (started in 1936) and Queenstown (started in 1952 and completed by HDB in 1960s). In 32 years, SIT built only 23,000 flats, housing 8.8% of Singapore population in 1959.
Sale of Balance Flats, comprising unsold leftover flats from past BTO launches, surplus flats from SERS projects, old repurchased flats (including flats from some divorced or fiance couples), are done “when HDB accumulate sufficient flats“, usually twice per year, but this is NOT A RULE. Since 2010 the SBF exercise is done same day with a BTO launch.