A lot depends on exactly where you are. Some places are pretty slack and pretty much do nothing after the fact. Some places are extremely anal and go completely over the top over any excuse to play God.
I would be inclined to forget about it at this point. If they did not raise a big fuss till now, probably will not be any in the future.
You might just ask around in a very gentle type mode and see what other people say usually happens in your location. Usually you cannot get a "Permit" for something already built. There might be a procedure for making things all legal. From my experience money talks and it would be more a paperwork exercise, surely plenty of paper, meetings and extra payments. The tax man will also be very pleased.
Being where I have been, I would be inclined to forget about it. They have rules you never want to know. I just would not open a can of worms if I didn't have too.
Some places come out and remeasure the house and take pixs, etc every so many years as part of reassetment. Again in my experience, nothing happens, they just raise the taxes but they do that anyway. Let sleeping dogs lie. Let the tax man make up the difference, he will anyway.
Problems like that are a best answered by a good shrug and failure to remember.