Stamp duty is required on any lease, including renting out a room. If it was never paid, IRAS may impose penalties and collect the unpaid stamp duty whenever the omission is discovered. Evasion or late payment can lead to a fine of up to four times the original duty payable (or up to $10,000 and possible imprisonment). You could be asked to settle any outstanding duty and penalties even after you sell your flat, if IRAS uncovers the unstamped agreement.
