I ran into a fellow (trying to sell his house). He started with a cosmetic remodel, but he started pulling things apart, and next thing you know, with the walls open he had to start redoing the plumbing and electrical. At that point he decided to fix the roof, and while he had the roof off, he changed the roofline of the garage.... you get the picture. Somewhere in there, the house became unlivable and he and his wife had to rent an apartment while they were doing the work. Then.... an inspector was inspecting a new deck across the street and said.... hmmmm, I wonder if they have permits for that.
When I met the owner, he was literally hiding a couple blocks away, and then driving quickly to the place to show it to us since he was trying to avoid the code people. Of course, he was so far underwater, and I was new at the game, so there was nothing I could do even if I wanted to mess with permits. Since then, I've moved out of town where there are no code requirements.
Moral of the story? Depending on how strict your area is on codes and permits, I'd be very careful. On the other hand, I've heard of an investor that got roped into a project that was half done, with no permits.... they hung tarps in front of the windows and pulled long shifts, including getting an electrician to come work in the middle of the night... and got it done.... no harm no foul. If nothing is visible from outside????
Good luck with your project