Plumbing issues and permit

[Vandana Sharma]

I bought a multifamily property 7 years ago and after making some upgrades, I have been renting it.  Recently, a disgruntled tenant started  calling the City for several repair issues.  Most of them were not legit, and others were fixed in a timely manner. But, then she  complained of some sewer smell in the basement (which is unused and she is on the second floor), and while I was still investigating/fixing it, she called the City Plumbing line and filed a complaint.  They have since inspected the basement and the apartments and opened up a Pandora box of violations and permit issues.  This will require a master plumber to basically assess and fix the plumbing in the entire building.  Huge bills, for sure.

Since this is older building (almost 100 years old), what is the best way to handle the situation and is there any abatement of code for older buildings?