Hi John Generally, if your upgrade is structural, involves electrical, plumbing or HVAC work, your local county will probably require a permit.
Remeber, the Uniform Building Code or National Building Code (depending on where you live) are designed to ensure building remain safe and mostly, safe from fire hazards.
Many people in the building trades (like me) do not get permits when we probably should. This is usually because we know the codes and don't want the bother of arguing with inspectors we often feel don't know as much about it as we do. However, we take the risk that when it comes time to sell, an independent inspector for the buyer may ask to see old permits or certificates of occupancy (for large projects like additions). If we can't produce them, the buyer's inspector will advise the buyer that the work done (most of which can't be seen) cannot be confirmed to be built to code. This could be a big negative in the ability to sell one's house.