You do not have to work with a contractor to get your project done but sometimes it is the prudent thing to do when you need to get a building permit.
The permitting requirements across the nations 25,000 local jurisdictions vary so widely, that it creates an opportunity to get into a lot of expensive trouble on permitting issues. The following recent example will illustrate the point.
A church had purchased a used factory built structure (mobile home) and had it placed on their site assuming it would meet requirements of the local building jurisdiction. The church planned on doing all the rest of the work themselves to save whatever costs possible.
The information that should have come to the surface but did not, was the church wanted to use the mobile home for an assembly area for meetings. The mobile home carried a B2 occupancy group on its label and the floor plan was engineered for 50# psf. The group A occupancy in the 1997 UBC dictates 100# psf floor load for that type of A occupancy.
The solution to this expensive mistake are still in the negotiation stages and the costs now are probably more than the contractor would have charged to do all the work.
So, find a well recommended licensed contractor if you are not familiar with the permitting process, or use a permitting service or architect to pave the way.