You must not be a tenant of a public rental HDB flat or owner of other HDB flats unless you are:
Divorced/ legally separated. In this case, only 1 party can rent a HDB flat from another flat owner
A owner who is eligible to rent out your whole flat. However, you must rent out your own flat within a month, after you have rented an HDB flat from another flat owner
You must not be the owner of an Executive Condominium unit where the 5-year minimum occupation period has not been met.