A stand alone addition or an extension to existing space is the way to go in today's market if you are looking to add livable square footage.
For an extension either on the first floor or the second floor, you can expect to spend $325 - $450 per square foot with a lead time of 6 - 12 months from permitting to construction completion.
For a stand alone addition you can expect to spend $150 - $350 per square foot. This is dependent on the type of build you are considering.