how to deal with objects from special projects

I'd try and contact the original author. Their other edits are in Spain, so if your first language isn't Spanish I'd try and include a Spanish translation of what you write. It looks like it might be an import of some sort, or tracing over some non-Bing or older Bing imagery.

Finding out which would help you determine whether the buildings (which are in some cases different from and in some case about 20m southwest of the corresponding imagery) are correct or whether the Bing-traced roads are.

A little further to the northwest there are some GPS traces on major roads. There Bing does seem to be slightly offset, and these buildings overlap those traces, suggesting that the (imported?) buildings are offset from at least partially wrong.

I would not be pleased if you deleted it, If I added something new that IS there, on the ground,or even planned or under construction and had gone to trouble to trace it from an allowed source. The developers if asked, may be only to pleased to have there plans on the map, but get permission. Most imagery will be at least months and probably years old. If you are still concerned you should survey it yourself or at least contact the mapper for his explanation before deleting someones hard work. EDIT But looking at your link it does seem strange, can you do a survey or get some other confirmation? or does the mapper reply to contacts?