Non permitted renovations

[Stephanie Mccolaugh]

Interested in buying a multiplex in a great location. Its an old property that was recently renovated. The price is close to the max of my comfort level, and after doing some due diligence I realized that the renovations (including plumping and electric) were not permitted.
-Liability wise- is this worth messing with?
-Would an inspector be able to see if things are up to code with drywall in place, or would I have to get a permit compliance specialist on site for something more invasive?
-If I make an offer, I want to low ball it to accommodate potential repairs...would a 20% reduction be reasonable? 

For context- this home has been on and off the market for over a year, and one of the units has been vacant for quite some time.