Only your municipality can answer this for you.

If you have proof the plumbing was pre-existing and approved with the house build, then you won't have to tear that out.

In my experience, most cities just want the money. But it varies by individual inspector as well as by city and area in the city. We avoided permits for 50 years because of pre-existing issues, and when we finally did a major remodel recently we called in the inspector. He was so happy that someone in the neighborhood was trying to improve things that he focused only on the one thing we had pulled the permit for (electrical) and ignored everything else, which allowed us to upgrade each problem one at a time. If you get someone who is reasonable, they will not make you tear anything out. They can also cut a tiny hole in the ceiling and use a dental mirror and a flashlight to see electrical, in some cases. Good luck. I would do whatever will allow you to sleep well at night, even if dad thinks it's silly (spoken from experience, not from a criticism of your dad).