Well, for starters you know the addition isn't permitted, so if anything happens to your tenant, your insurance may not cover it. And if they did, they wouldn't cover the negligence lawsuit.
There are unpermitted additions all over the county. This means you'll never know if a homeowner had a contractor do the work to code, or more likely, gathered some friends and relatives to do the work to save a lot of money.
Also, if this is living space, and you're planning on getting a loan, your lender will want it to at least look like it was done to code, but they could always end up not funding it in the end.