The Region of Brussels-Capital taxes each real estate owner on the value of his property, and sends the tax form yearly, for a period covering the past 12 months. This tax (précompte immobilier/onroerende voorheffing) is due for the property you own and in which you live, of for a property you earn an income from (by renting for example).
The amount is calculated on the presumed annual rental value (revenue cadastral/kadastraal inkomen) that is attributed to the property by local authorities.
Changes in your family situation (as number of dependents), or in the value of the building self (as significant renovation or embellishment works) can impact the amount taxed by the government. You should also check whether you qualify for the BeHome annual bonus.
Your tax situation and declaration can easily be updated and processed on the MyTax platform of the Ministry of Finance of the Region of Brussels-Capital.