apparently my question wasn't very clear....sorry

I know nothing about it but I wonder why rental income would be treated any differently than any other kind of income though there very probably are expenses that one could claim against the rental income.

Because that is, and always has been, the law in Belgium - but note that it only applies to property that is let for strictly residential purposes, which is why it is commonplace for residential rental contracts to prohibit any commercial use of the property (e.g. working from home).

Of course, the fact that the income isn't subject to taxation (other than the fixed property tax) feeds through into achievable rentals - one of the reasons why the rental value/ capital value ratio is lower in Belgium than in most comparable countries.

The first answer is the reason you should ask professional advice rather than ask on a forum, and presumably if you can afford to buy real estate you can go to a notary and get it .

Income from letting unfurnished domestic property is not taxed.
If the property is furnished then the income from the portion which is assessed as being for the furnishings IS taxed.

As far as I know, you do not pay any income tax for a rental house. I think you must be paying cadastral income every year but the rent you get is tax free which is quite unusual. Many countries do not charge you for the place you live (that is not included in the tax) but any rental house 2,3, 4 , whatever the number is, you are charged. However, in Belgium it completely goes to your pocket.

No income tax on a single second residence you cannot live in and the residence can be anywhere in or out of Belgium. Note single second residence

i answered your first question. please check there.