Thoughts on Unpermitted ADU

[Jared Aquino]

I have my eye on an older property (1920s), and after touring the place it became evident that a two-story ADU was built and connected to the original structure. I'm not sure when the ADU was built, but I looked on the City of San Diego website database and found that no permits have been pulled in recent years. My RE Agent said that the addition was built roughly 20 years ago, but that info is coming from the listing agent with nothing much to back it up.

For those who've purchased properties with un-permitted ADUs, can you share pros/cons and other things that you take into consideration when analyzing the deal? What should I be aware of, and what kind of costs should I expect to incur if I do move forward with this?

Thanks!