Quoted from fiberdude120:

In my town I could not sell my last house until I got a permit for my back deck. It was get a permit and inspection or tear it down. And you are correct in one aspect that a permit is nothing its the CO that counts. When the square footage of your living space doubles from when you purchased it and there is no explanation of where it came from and your mortgage lender wants to see COs on all the areas that are finished in your house it could cause the lender to refuse the buyer a loan. In your area gas milage may vary.