Construction of attached house (mitoyenneté)

Hello, has anyone been in this situation? My house (in Flanders) was intended to be terraced (attached on both sides) when built, but one side has always been an empty plot. Now the plot has been sold. I vaguely remember the notaire telling me when I bought the house that if anybody wanted to build they would have to pay me a fee for the mitoyenneté. I've looked on https://www.notaire.be/, but I can't find anything about this, possibly I'm using the wrong search terms, things might have changed (it was a long time ago!), or maybe I imagined the whole thing.

I imagine I will hear from the new owners before they start to build (if they start to build) but I would love to hear from anyone who has experience of this what the exact situation is.

Thanks!