All citizens living and working in Belgium are required to pay a minimum of taxes and most of them also pay social security contributions, the latter being the sums that partly finance the social security system. In general, the fiscal situation of each individual is evaluated in terms of the conventions for avoiding double taxation agreed with Belgium. It is therefore not possible to choose the place where you pay your taxes! Unless proved otherwise, the law assumes that individuals who are registered with a municipality (in the national register) have established their fiscal domicile in Belgium and must therefore declare all their income in Belgium, including both national and international income.
Staff members of international organisations often (but not always) benefit from tax privileges. However, different tax regimes exist and may apply to you depending on your nationality, whether you were a permanent resident in Belgium before taking up your post, your status within your organization, headquarters agreements, agreements on privileges and immunities and double taxation agreements in force.
If you’re an employee on a Belgian payroll, your personal income tax, whether or not in the form of advances, will be directly deducted from your wages and you will receive only a net amount on your bank account. All other type of taxes are to be paid directly to the State, region or local administration.
Trying to avoid paying taxes or contributions will result in fines and interests that can pile up quite considerably if not followed up upon and can unexpectedly arrive after a few years. In any case always make sure to take the right action when receiving your annual tax form, or any other official letter from a Belgian administration relating to a tax due.
The main taxes for foreign residents in Brussels to be taken into account are Personal Income Tax,Tax on a Second Residence, and real estate tax if you become owner of a property in Belgium.