This fucker is again thinking that only he knows the best. What the fuck? Go and read up properly before you give wrong information to the innocent here. WHAT THE FUCK TO YOU MAN?
Now back to the question. A HDB flat owner can buy a private property after completing the Minimum Occupation Period (MOP) and if the loan is fully paid by then. Otherwise, it'll be 10 years from the day of getting your keys if there is still an outstanding loan. Although there is no limit to the number of private properties can be purchased, the owner will however needs to inform and get the HDB's approval before making the next purchase of a private property. This is more for formality because most of the time though, they would be approved. In addition, if the owner has completed the MOP, he can apply to the HDB for approval to rent out the whole unit.
I'd been through this and I know very well. Presently, I have a HDB and 2 private properties, all fully paid.