http://www.hdb.gov.sg/cs/infoweb/reside ... ter-buying
"Conditions after Buying
Buying a Resale flat
Minimum Occupation Period (MOP)
You must occupy the flat for a minimum period of 5 years before you can:
Sell the flat in the open market
Sublet the whole flat
Invest in private property
, both local and overseas
The MOP starts from the day your flat purchase is completed.
Waiver of MOP (only for Top-Up Grant)
If you and your spouse each own a flat and wish to dispose of both flats to get an HDB resale flat using the Top-Up Grant:
The MOP may be waived for 1 flat to allow you to sell it in the open market
The other flat can be transferred to eligible family members or persons
Disposal of existing flat
You must dispose of your existing HDB flat (if any) within 6 months of buying a resale flat.
Disposal of existing ownership/ interest in property in Singapore or overseas other than HDB flats
You, your spouse, co-applicants, and any occupiers listed in the resale application must dispose of any interest or ownership in any private residential property within 6 months of buying a resale flat.