A flat is a home first and an asset second, and HDB enforces that with the minimum occupation period. Until it is over you cannot sell the flat, rent the whole of it out, or buy private property.
How long it runs
5 years for a standard flat and 10 for a Plus or Prime one, counted from the day you collected the keys. Time you did not physically live in the flat does not count towards it, so living overseas pushes the date back.
What Plus and Prime change
These categories cover flats in choicer locations, closer to the centre or to an MRT station. They come with a discount when you buy and three strings attached: the 10-year occupation period, a share of the resale price returned to HDB when you sell, and buyers who must meet the same income ceiling and eligibility rules you did.
That last one matters most for resale value. A Prime flat can only be sold to a household that qualifies, so the pool of buyers is smaller than for a standard flat in the same neighbourhood.
Our MOP calculator takes the date you collected the keys and tells you when you may sell.
There are exceptions on divorce, bereavement and renunciation of citizenship. Source: HDB: minimum occupation period.