Rental Income for Non-Residents.
If you rent out your property in Portugal, you will need to declare the income on a Portuguese income tax return. Even if your clients were foreign holiday makers and you were paid outside Portugal, you still need to declare the income to the Tax Office.

Since non-residents are not eligible for the ‘Simplified Regime’, the simplest way to declare rental income in Portugal is under Category F, Rental Income and, as of 2005, non-residents benefit from a special tax rate of 15% for this type of income. Note however that once tax has been paid in Portugal you should be eligible for a foreign tax credit in your home jurisdiction but be sure to consult the appropriate Double Taxation Treaty on how to deal with any potential conflicts.