Building cost per sq. ft. are different in every State. Difficulty depends on your knowledge of construction. I wouldn't do it as a first project without someone very knowledgeable being involved each step of the way. That includes everything from planning, drawings for the building department, getting permits and on through to the end. If you don't, it could cost you a bundle in mistkes. My dad (a general contractor) built the second story on my last house. Before we started I thought, "I can do this", after we got into it I thought "Thank God he's doing this and I'm just the helper". The building department in your state should also be able to give you a general idea on how much construction is costing per sq. ft. Good luck!