Smooth/Flat Shading Issues

The shading works fine for flat shading, like so:


But I’m having troubles with the smooth shading:


It has worked before, and I was satisfied with the look until I began working on the big window on the front. After I made the window and gave it a material, the whole house mesh just didn’t have good shading. It has the same problem no matter what the material is. Could I have some help?

Delete and faces that are inside your mesh then select all vertices W / remove doubles then Ctrl+N to recalculate normals
You may need an edge split modifier to give the sharp edge required around the window.