How to setup custom collision on this mesh

So I bought this building; :///FullPreview/Index.cfm/ID/679276
Now what I thought is, it would come with some sort of custom collision, but it doesn’t. And if I use the collision thing on Unity it doesn’t work, the collision goes to the outside blocks on the building and is just all over the place.

What do I use to setup a custom collision box?

Take a look at this video: The last way will be the best one for your case :wink: