When I built my house, I had permits and did everything well above code.
What I found was that a building inspector will help you out a lot if you are nice to him.
The big advantage of building with permits is that when it comes time to sell, you will get top dollar and not have any legal problems.
In my case, at least, once I got on the inspector's good side by doing everything he wanted the first two visits, he became a great asset, not confrontational at all. After about 4 visits, he wouldn't even get out of his car. I would meet him in the driveway, wave at the new work, and he would sign the inspection form. I had to ask him to get out & look at something if I had a question on what to do next.
I bet that an inspector will want you to correct any safety issues on the old work, which you should do anyway, but will let a lot of existing stuff go.
When we were looking for our property we found a place we liked enough to make an offer on.
Just walking through, I could see a double handful of code violations, and as part of the offer, wanted the worst of them fixed. The seller didn't want to do anything, he was going to Central America as a missionary, and sold to someone else. He claimed that in OR, once something has existed for 5 years, it is "grandfathered" and does not need inspections.
Well, the new buyer wasn't really capable of doing all the repairs & upgrades necessary, and sued the seller within weeks of closing. Now the guy is in Guatemala and having to defend a lawsuit. I don't know what that is costing him, but he would have been far better off to fix the stuff first and then sell. Instead of being able to fix the stuff himself, he is going to end up paying lawyers, paying contractors, and probably paying the buyer's lawyers.
When you really think about it, the purpose of requiring building permits is two-fold.
1. Insure safety, and
2. Increase the tax base. (This is by far the more important to the city.)
Since you are doing the work yourself, you can give a real lowball estimate of the cost and your taxes will not go up that much.