First you check if you could sell through this portol
https://www5.hdb.gov.sg/BP21/ISRR014p.n ... l?OpenForm
Direct from the horse's mouth
Minimum Occupation Period (MOP)
HDB flat owners must have
physically occupied their flats for a period of time before they are eligible to sell it on the open market. In general, the requisite occupation period (referred to as minimum occupation period - MOP) is 5 years for a flat purchased directly from HDB or a resale flat bought under the CPF Housing Grant Scheme; and 2½ years for a resale flat without a CPF Housing Grant. The period will depend on the mode of purchase, the mode of financing and the flat type.
A list of the MOP by flat type is shown below.
1-Room and HUDC Flat
No occupation period for owners owning 1-room flat or HUDC flat
2-Room to Executive Flat
5-year MOP for owners who bought 2-room and larger flats directly from HDB;
2½-year MOP for owners who bought 2-room and larger resale flats from the open market without CPF housing grant
Resale Flats bought under CPF Housing Grant Scheme
5 years from effective date of purchase of resale flat
Flat bought under Selective En-Bloc Redevelopment Scheme (SERS)
7 years from date of flat selection (provided seller has taken possession of the replacement flat) or 5 years from effective date of possession of replacement flat, whichever is earlier and subject to HDB's prevailing policies.
If there is any period of subletting of the entire replacement flat (subject to HDB's prior approval), the MOP will be extended to make up for the subletting period. Flat owners who had enjoyed the SERS benefits more than once are not allowed to sell the replacement flat.
Do I have to fulfill the minimum occupation period of my flat before selling it in the open market?
Yes. One of the eligibility conditions before a flat lessee can sell an HDB flat in the open market is meeting the minimum occupation period. The length of occupation period depends on the mode of purchase and the flat type. In general, it is 5 years for a flat purchased directly from HDB or a resale flat bought under the Housing Grant Scheme; and 2½ years for a resale flat without a Housing Grant . You are advised to ensure that you are eligible to sell the flat in the open market before signing the Option to Purchase and offering the Option to the buyers.