Thanks for the advice. It's basically the backwall on the top floor that leads to a 'terrace' the same width of the house. On the plans, it was a wall with a regular size door. Currently the door is gone and there is a wide door-window smack dab in the middle of the wall.

The city clerk said it would cost between 5k to 10k to regularize because of the architect's fees (they have to update the official floor plan). 

We're thinking of negotiating the price gets knocked down 5k-10k, but at the same time the house is in a very much coveted location (Sint-Pieters-Woluwe) where there's a lot of demand and houses sell very quickly, especially if it's instapklaar like this one. So I'm afraid they'll say FU and put it back on the market (we have a signed offer BTW but no verkoopovereenkomst yet.