Source: https://www.iras.gov.sg/taxes/stamp-duty/for-property/buying-or-acquiring-property/additional-buyer’s-stamp-duty-(absd)

Liable buyers are required to pay ABSD on top of the existing Buyer’s Stamp Duty (BSD). ABSD and BSD are computed on the purchase price as stated in the dutiable document or the market value of the property (whichever is the higher amount).

All buyers / transferees (i.e. individuals, entities including housing developers) are required to complete the ABSD Declaration Form (PDF, 277KB) to be witnessed by their lawyers (if applicable). The completed form need not be submitted to IRAS but it should be retained for at least 5 years from the date of purchase / acquisition of the property as IRAS may request for it for audit purposes.