When a building is present in the open data layer, but missing in the osm layer, this could mean 2 things:
The building is new and the OSM data is not up-to-date.
The building is demolished, and the opendata source is not up-to-date.
The age of the building in the opendata layer can be an indication for which of the two cases above is most likely. If the building is older than say 5 years, the latter case is probably the most accurate, and we shouldn't automatically import the building to the OSM layer.
When a building is present in the open data layer, but missing in the osm layer, this could mean 2 things: