How do I copy my buildings from one project to another one ?
If you find that you've done a lot of work on game objects that you want to copy over (rather than just simply assets), then prefabs is a good start. You may want to export them to a package then import that package to the other project. Something I've taken to, for copying a lot of game objects, is:
1. Make a new empty scene, call it (in your case) Buildings
2. Select your original scene in the Project assets list
3. Use Load Scene Additive http://www.unifycommunity.com/wiki/index.php?title=LoadSceneAdditive
4. Delete everything you don't want to port over (everything but your buildings) from this new scene
5. Save it, export it as a package
6. Load your other project
7. Import that package. It should preserve everything
In the Assets folder of your project, simply copy the relevant files, to the Assets folder in another project.
It is probably better practice to rather create a PreFab of your building object, and then copy PreFab to your other project.
I’ve posted an item in the Asset store which can copy one or more items from a scene into a new scene.