Ditto what norman74 said about tangling with the crabby-assed building inspector. I've gone round with mine a lot lately with the rehab project the wife and I are doing and it's a time-consuming headache. HOWEVER, he is a genuine guy who is simply trying to do his job to protect me, my neighbors, and any future residents of the property from damage and loss from faulty construction. While he's been a pain in the complete ass, he's also offered a lot of advice and good ideas for our project. Other municipalities may not be so lucky.
There is another factor to my motivation to play by the rules which is P&C insurance. My wife is our insurance agent and she's got horror stories about policies being cancelled and claims being denied from damage to additions, decks, sheds, etc., that were built without a permit. I would hate to have damage to a laborious, expensive project be destroyed, only to get a letter from the underwriter asking me for copies of the permits.
Every community is different, with varying building and zoning codes. In the long run, it's worth the trouble to just call and ask your local building department what the regs and permitting requirements would be for your project. As sucky as it can be to deal with they exist for reasons that we don't appreciate until there's a bigger problem.