Many contractors and builders figure home addition estimates by the square foot. However, this usually only works if you are adding quite a bit of space. Because home additions require every major component of a home--roofing, walls, insulation, flooring, electrical, plumbing--your estimates can have a huge variance. What's great about having several room addition estimates is that you can better determine the range of costs for this project, and from here you can figure your budget, not matter which contractor your choose.