Blender

When I import a model from the blender, this warning appears to me and parts of the buildings are deleted

 

Can you share your model file?

 

Geometry can be self-intersecting...however, individual polys cannot. I'm guessing you have some ugly quads and Unity's triangulator can't work around that. Quads should also be fully planar/flat in general.

You might be able to get around it by triangulating the mesh in Blender first. It would then let you see where the issue is so you can fix the geometry. This is assuming of course that I'm right about the cause of the issue.

 

You might have to find the polygon/face, that is intersecting badly with the other parts of the model and fix it, so that it does not intersect with the rest of the model.