How does that affect the buyer and seller in a transaction? Disclosure, knowledge, and acceptance. That's pretty much it. Well, unless you piss off your neighbor of course. That's further down this post!
The older the home, the more likelihood that some type of change has happened to the original structure. We see it a lot in San Fernando Valley (my 2nd specialty area), because the homes can be much older there. More people have owned a particular home. More people have changed it to suit their wants & needs over the years.
Now, disclosure here, I'm not an expert on when you must pull permits. That's not part of my job. I have to guide buyers on how to determine if they are comfortable accepting a non-permitted something or another, done to the home they are purchasing.
If it's some finagling of existing stuff, most don't seem to pull permits. A new bathroom sink, hardly. A new water heater, supposed to but no one does that I've heard of. A new roof, I would. A room addition, oh yeah!
But, let's say you're madly in love with a home that doesn't have permits on a room addition. Some issues that could arise? Electrical, plumbing, insulation, etc. not done correctly. You can't see that without opening walls. So, you ask seller for details and then take a leap of faith hoping you're not in arbitration with him in the future!
But, and I've seen it happen, you take that leap of faith and you start arguing with your new neighbor about something? Lo & Behold, the Building & Safety inspector knocks on your door and .....well....damn, you can see how this might go!
That's the warning I give clients for obvious additions that could bother a neighbor or that the neighbor decides to report you. Insurance usually isn't an issue, but you should check with your own carrier.
I've never seen a fire start due to inadequate remodeling electrical work. I've heard of that with old electrical though!
It's the buyers responsibility to verify permit status on a property. It's a sellers responsibility to disclose whether things were done without a permit. So, between the two parties, it should be okay. As long as their respective agents are assisting them with the disclosure, knowledge, and acceptance portion. We assist, it's the clients job to disclose & decide.
Just some food for thought for the day!