True.  It looks like the shape UrbIS shows is only the inner part. :-(

Here is the solution I found here: keep the official UrbIS shape and tag it as building:part.  Add another building part next to it with the extension (and building:min_level=1 because there is no "ground floor" here).  And add a shape with building=* encompassing both of them.  That way we’ll have correct rendering, it works in 3D models and we keep official UrbIS data on OSM.

Footways added too.