Building a house or doing addition work is one of the largest single investments a person can make in their lifetime. It's the time for you to do research. Know how much you are willing to spend, how much you can afford, make sure you have the plans in mind for your construction, and know which contractor to pick.
1) Determine how much you can spend. Realize that in all projects, there will be surprise expenses & change orders. Build a buffer into your price.