It would be easy (trivial) to reserve specific cells on a plan for towers, it's a bit harder to come up with a simple UI for it - I don't want to invest too much time on menus, dialog windows etc.
About overhangs: I agree that in many cases they look excessive and inappropriate. On the other hand, they make the overall shape of a building look more interesting (on average). Ideally there should be a command to make turn a specific overhang into a normal... wing, but again it's a question of UI.
I'll thinks about it.