Tax on non-principal residences, more commonly known as "tax on secondary residences", is applied to anyone who occupies a dwelling in a municipality without being registered with it.
Anyone living in a property as a non-principal residence is required to fill in a declaration form at the municipality.
Make thus sure to register with your municipality when you rent or own an accommodation in the Brussels-Capital Region. The amount varies depending on the particular commune and can rise to +-2700€ a year.