In my area the County Appraisers office has drawings, additions etc on line. Basic permit information is also available on line, so I'm very fortunate. If inspection reveals something in disagreement with the public records, I look for a permit.
It's not always clear what a "remodel" permit entails, and I report that fact as well. Its easy to get this basic information, but generally I dont dig further, and dont value blatantly unpermitted work since it could be flagged for demolition by the city.
An example recently was a garage converted to office space. When the property sold, the city somehow found out about the converted garage and had the new owner remove the office space and return the area to its original use, presumably for life safety reasons. The buyer reported they purchased the property for the expanded office space, and didnt know it was illegal here etc......I imagine the usual law suits ensued, but the bottom line is I disclose any discrepancies with public records that I encounter, and be cautious about valuing unpermitted, possibly illegal additions. We are obligated to research these discrepancies if we encounter them, but only to a point, at which time it becomes a code enforcement issue, a legal issue, but not a market value issue.