You don't always need a permit to remodel the inside of your house but it also depends on the extent of the remodel and your county, municipality etc.
Doing remodeling with only minor cosmetic changes such as painting, replacing flooring, installing new countertops, cabinets, shower etc don't always require a permit.
Although if your county or municipality requires a permit for remodeling it's often for altering of plumbing or electrical wiring and systems, structural changes such as foundations, adding or removing walls, repair structural walls through re-framing etc.
Most often you can get by with remodeling the inside of your home without permits as long as you're not doing any outside structural changes.
Tearing out drywall, remodeling the bathroom, replacing a shower, tiles, flooring etc can almost always be done without a permit.
I've lived in homes that were in places that required a permit for even inside remodeling but I did it without any permits and was just fine.
Mainly any remodeling that involves modifying the home's structure, roofing, electrical and plumbing as well as heating and cooling systems are mainly what requires permits but it can vary depending on your location.