If you don't have the proper permits, there's a good chance that the city or county will slap you with a stop work order, which will only delay your project. Permits take time to pull and they cost money, so make sure you get the permits as soon as possible before construction begins to avoid costly delays in the project or even fines. Your homeowners insurance won't cover unpermitted work if something happens [source: HGTV].
This is an area where communicating with your contractor is really important. Some contractors take care of pulling permits for you, and if this is an option, I'd highly recommend it. Dealing with local government can be a headache, and contractors usually have contacts in the permitting office that make it much easier and faster for them, since they pull permits all the time.
If your contractor isn't responsible for pulling those permits, you'll want to get a jump on this as soon as possible, because you'll inevitably be missing paperwork the first time around, and there's usually a waiting period between when you submit everything and when the city or county issues your building permits. Once you have your permits, find out if you need to post them publicly. Most cities require that you have permits and plans available on-site, so that their inspectors can check in on the work.