The Grootschalig Referentiebestand (GRB) data base contains data regarding buildings, lots, roads, waterways, rail, and public artwork.
The data base is maintained and updated by Flemish municipalities using geodesy measurements and satellite photos. It forms the geographical basis which others can use to inoculate their data.
The data is now freely available. However, registration is required. This decision is taken to make the data more easily accessible, ideally leading to higher re-use. The Flemish government expects a positive effect on the quality of the data through feedback channels. Furthermore, it expects the development of new data sets and the creation of GRB data-based applications.
Because registration is required, the data is not (yet) available via either the Flemish of Belgian Open Data portal. The data is available via https://download.agiv.be/.
Read more (in Dutch): https://www.agiv.be/news/2015/december/vlaamse-overheid-stelt-grb-data-gratis-open