replacing velux in the attic

Assuming that you live in Brussels, then you need to see the "urbanisme" regulations for the specific commune that you live in. The regulations are different depending on where you live. If you are going to change the materials i.e. from wood to PVC, (and possibly even the color) of door and window frames from what was originally agreed on the planning permission of the house when it was built, you need a new permission.

However, some communes don't apply the law, some do, and some have exceptions to the law allowing change under certain conditions.

yes, please do ASAP as Saint-Gilles is since 2 years strictly enforcing buildings regulations!
Friends had to replace within their sales process 3 years old = 'new' windows, which had been fine under the old rules, but not under the newly set up ones.

We were told we by our architect didn't even beed permisssion in overijse to insert veluxes in a previously whole roof so i think it depends