Zumba class should stop, at a minimum you need her to have an insurance policy with you as a named insured.Once there are three steps, the building code requires a handrail.

Thereby if your code violation was done 30 years ago and the city found out last week, you would’ve one week into the statute of limitations.

You can also check quick zoning guide here: https://www.phila.gov/documents/zoning-code-information-manual-quick-guide/However, zoning doesn’t always = current use.

Or are most of you strictly relying on your CPA?

You'd be forced to bring the house up to new construction code if you did this. 

What differs by state is if he is going to keep this as a rental  there may be more restrictive laws when they issue lead certificates, how they define as lead free, what they considered abated and how strictly they enforce.

But local building codes and unions are opposed to it.

Call you local zoning or code enforcement office to find out for sure.

High level market research has been completed and I have narrowed to about 20 zip codes in my area, which is still a lot... 

Those can can come with strict regulations so do your research.