Here in SF, there's an online permit database accessible from the property lot database [1]. A new or prospective owner can check for permits for past additions. I don't know how far back it goes, but I'm pretty sure it's at least 40 years.
No guarantee of course that there would be something similar in your area, let alone accessible anonymously. Not to mention that maybe it wasn't obvious that the in-law was an addition, and that it's civilized to assume that the previous owners didn't cheat.
No guarantee of course that there would be something similar in your area, let alone accessible anonymously. Not to mention that maybe it wasn't obvious that the in-law was an addition, and that it's civilized to assume that the previous owners didn't cheat.
[1] https://sfplanninggis.org/pim/