Notre-Dame Cathedral in Tournai by Jarek Pokrzywnicki
Huge cathedral but still covered with scaffoldings. This will last until 2017 - major works, although minor works will be carried out until 2025 (?).
Currently (July 2015) it is not possible to visit main nave but at least you may see some views of archeological findings from different stages of monument construction (even from the time when there were Gallo-Roman buildings).