When you build something, you will need to get a planning permission (most likely). Here are some links I've found:

Belgium is a bit of an odd ball when it comes to getting the permission. Walloons and Flemish have their own rules, and each commune has its own rules too (this would make sense anyway). So, if you're building your own thing, make sure you go to your commune to ask what you can build before you start. You may be in for a surprise :) (or :( )

HTH,