If you have the plan set and all the permits you can dig the foundation and poor the foundation. Next you frame the walls. After framing and erecting the walls one sheets the roof. make sure you get all the required inspections
It isn't as easy as it seems. You need to get an engineer to approve of your addition, then you have to take his approval and fill out the paper work at the building commissioner. Then you have to hire an inspector at various points of the build.