Resist the urge to build your Sims too large of a house. They'll end up spending too much time walking from object to object. The best tactic for expanding your house is to increase the size of your rooms until the room score is maxed out (including windows, wallpaper, floors and furnishings). This way you'll have all of the benefits of the large house, but won't have to spend half an hour walking from the fridge to the table.