tax on establishment, use of public roads and/or driving force

I suspect that this is a provincial tax (but it could also be a municipal tax).

You should check the text of the relevant tax decree (which can be found on the back of the tax letter or on the website of the authority levying the tax) to see how that surface area should be determined.

For self-employed persons in secondary occupation (without a separate company), this is usually the entire surface area of the dwelling or the part of it that is used for the secondary occupation. In the latter case, you must include both the office area and all storage spaces where you keep materials or the garage where you state your car.

Johan