There are other considerations here. What does the rest of the neighborhood have? If your house would be the only 2-story, then give strong consideration to keeping a ranch. If the addition leaves you with no backyard, add a second floor.
Then get an architect to discuss the plans. Good friends of mine are adding on to their house. Their architect made some suggestions that enhanced the project - things for better flow from room to room and more useful space from the same footprint. The extra cost is a fraction of what the total job will cost.