Oli, I think I have some advise for you.
Make each floor on a different layer, this will greatly simplify modeling.
Make sure that all the lines that you draw are Exactly on the axis that you want them on.
Occasionally check to make sure that faces that should be coplanar are coplanar.
Always remember computer modeling is mathematical, if you want clean efficient models they must be mathematically correct.
I made this model when I was 13 and I am now 14, so I'm almost as young as you. The roof on that house took me a looong time to fix a 0.1 degree mess up that I made while I was not paying attention.
I'd love to see some more models of yours, and possibly some clean ones once you become a more experienced modeler.