I noticed that if the land was abandoned and there were open projects buildings left on it anyone could come along and open the project and take the materials they wanted from it. You can NOT delete the building unless you take all the materials out of the project.
However, when a building is on your own property and you delete it you don't get any materials back from it. This includes open projects. It just deletes everything. Hey, supposedly a lot of the materials we make buildings out of won't be around anymore with the passing of junk heaps. We need to recycle where we can.
Structures that are not on your property will NOT let you delete them until all the mats are taken out of the project. It's hard to test anything with the constant relentless rollbacks but Im trying to make a fence this afternoon to prove you do not have to take anything out of it to delete it.