Q10: If I have been given a flat selection appointment, can I still apply for a flat in another sales exercise?
A10: You may apply for another sales exercise so long as you have not booked a flat. Once you have booked a unit, you will not be eligible to apply for another unit. From the Mar 2012 sales exercise onwards, applicants who have booked a flat/Studio Apartment and later cancelled their booking, will have to wait out a one-year period before they can apply again (whether as applicant or occupier) for a subsidized housing-this includes the buying of a subsidized flat from HDB, a resale flat with a CPF Housing Grant, a Design, Build and Sell Scheme (DBSS) flat or an Executive Condominium (EC) unit from a private developer, and a resale flat that has been announced for redevelopment under the Selection En bloc Redevelopment Scheme (SERS).
As quoted in HDB website, can't post URL yet:
hdb.gov.sg/cs/infoweb/results-of-sales-application/get-help