If the limitation is due to a better performance should directly notify the download, not the number of pieces, but the number of polygons of the set, which is really affecting the performance .But for buildings that have no intention to modify , perhaps automatically calculate the polygons that are inside it perhaps complicated .... but it could enable manually delete the polygons once finalized the blueprint? Or even harder ..... ¿merge all objects and turn them into a single mesh? Conserving the price of the objects used ....
A typical user is not going to know what polygons mean and they would be able to identify with the piece count a lot easier