self-improvement vs licensed-contractors
It depends where you live if you need a contractor or DIY in my state Oregon and several others I have lived a home owner or close family member can do everything. Non load bearing walls usually do not require permits but any change to load bearing walls, new electrical circuits usually do require permits, in 1 state I lived a permit was required for changes to the plumbing and replacing the roofing. Checking your local requirements is the first step if you post your location I am sure we can find a more complete answer.