Big Red Box with 5 and 2/3 standing space.
Change color : It makes it look nooby
Make rooms taller : 6 or 7 studs High is a reasonable height for rooms
Fill empty space : The lobby has a lot of empty space, try filling that up with something.
Improve Windows : the Default windows are good when used right but you used them in a way that removes from the overall appeal. Make bigger windows and try not to use the Default one unless you are making a window like the one in the Bedroom.