My husband and I co-own our HDB flat (fully paid). If we buy a private property, can we continue holding on to the HDB flat? Assuming we are allowed to keep the HDB flat, would the Additional buyer stamp duty kicks in should we buy a second private property (ie. our third private property)? If we choose not to co-own the private properties, does that mean the ABSD kicks in only on the 4th property (including the HDB)?