Just to add what others have said, the consequences can be extreme, or not. It would depend on many factors beyond your control. But don't think that just because you believe everything is up to code, that high quality work was done, that this will be enough.
Look at this from the point of view of the permit issuing authority, and the building inspector who would check to see that work was indeed done to code. One of the consequences of building with no permit is that no inspections were ever done. Can a building inspector go back and be sure that work WAS done to code? Sometimes, not without tearing things apart. And a building inspector will not be happy to see their time wasted. Do you want to make the person unhappy who must choose to pass on your work?
On top of that, a presumption might be that the person who could not be bothered to get the proper permits also might have skipped over something important.
The point is, do the work without the proper permits, and you risk creating a very irritated person, one who has the authority to freely order your fine piece of craftsmanship turned into a pile of rubble.