How would industry-professionals approach this? I am trying to figure out how professionals in the animation industry would approach this sort of building. If you take a look at the green details of the windows, those are a separate object from the main building, as well as the little lip underneath the windows.

Would this be how they would do it professionally? Or, would they probably make these pieces all out the main building? And if they would use separate objects like I did, is it okay if they overlap, because I've heard mixed opinions on that. I'm just trying to learn the best habits now, so that one day, if I do get a job in the industry, I know what to do. 

I hope this makes sense! Thank you so much!
It depends.

If the final result was to be animated, then they would most likely make any moving part a different object. But on non-moving parts, such as the Door Overhang, it would probably be left to the individual artist's discretion what to do on that individual piece. For your purposes, I would do whatever you are most comfortable with, and whatever you think will be most efficient for that individual scene.