The current regulation does not allow you to buy a public housing or Housing Development Board (HDB) Flat if you own private property in Singapore or any other countries (unless you disposethe private property within six months of the purchase date of a non-subsidized flat).
There is no restriction on the number of Private Properties you can own in Singapore, but there is a restriction to own only one HDB flat at any one time, regardless of whether it is subsidized or non-subsidized. For concurrent ownership of HDB flat and Private Property, you have to own the HDB flat first.
You must ensure that you have fulfilled the Minimum Occupation Period (MOP) of five years before you can buy a private property with concurrent ownership.
By Eileen Tan and Ui Wei Teck, property investors and authors of Enjoying Mid-Life Without Crisis. This tip and dozens more are from their book.