I think everyone is seeing your question differently...I see it as what is the difference between remodeling your current home or adding a new room to existing construction. Do you need permits???
If you just rehab your existing home you don't have to have permission. If you are building on a room addition to an existing home...or doing a tear down re build...you would need to hire someone that can pull the appropriate permits to do to meet the appropriate county/city code.
I may have assumed too much too...but being more specific with your question would be helpful.