If you are a landlord and, during the term of a lease, increase your tenant's rent by the amount of inflation (the maximum allowed by law), you must inform Finanças what the new rent will be. This communication to the Tax Authority is mandatory and can lead to the payment of a 10% Stamp Duty on the amount of the monthly rent increase.
The new value must be communicated through the “AT” website by the end of the month following the month in which the change occurs. Payment can be made at the Local Finanças office, at an ATM or via electronic banking. When communicating the change online, you will be issued a payment note with the multibanco payment reference on the “AT” website.
If the amount due is less than €10, there is a technical exemption, and no tax is due. For example, you charged a monthly rent of €500 in 2017. The inflation adjustment percentage is 1.15% or €5.75. The difference is less than €10 so no Stamp Duty is due.