A property is considered acquired by the buyer and sold by the seller when the option to purchase, or sale and purchase agreement is executed, even if the sale has yet to be legally completed or if the property is not completed at the time of purchase.

ABSD liability depends on the buyer's profile and number of residential properties owned at purchase. Assuming you are a Singapore Citizen, ABSD is not payable if the option to purchase for the sale of your HDB flat is executed before the purchase of your private condo and you do not own any other residential properties.