Unity Deletes Intersecting Polygons? Blender to Unity

I made a model of a building in Blender and exported to Unity as an FBX. However, upon import, Unity deletes huge chunks of my mesh, with the console error telling me that my model had intersecting polygons so Unity deleted them. My model isn’t rigged, and in Blender it looks fine. Can someone tell me what the problem could be? This is really messing me up!

it might possibly be duplicate vertices in your blender model, or normals reversed in blender.