When rolling out their fibre networks operators have to apply for the necessary local and regional permits or authorisations. The exact requirements and provisions may differ depending on the location or region. The Broadband Competent Office (BCO) publishes overviews of the regional reglementation on its website. You can consult these here for the Flemish Region, the Walloon Region and the Brussels Capital Region.
Municipalities and cities in particular play a major part in fibre roll-out. In accordance with Article 43 of the European Electronic Communications Code it should be ensured that these permits can be applied for on the basis of simple, efficient, transparent and publicly available procedures. A decision regarding such an application should in any case be finalised within six months, ensuring a non-discriminatory and fast implementation. In this regard, the BIPT publishes a number of recommendations for cities and municipalities to facilitate fibre roll-out.
Finally you can also find a list of competent authorities here.