Zoned SF Residential but rented as two units

[Greg Betts]

Hello BP community, I need your help. I'm new to all of this, even more so new to zoning and planning rules. Please share advice or expertise. 

I've come across several SFH's zoned SF Residential but the property is being used as a multi-unit dwelling, an example being the basement and upstairs rented out separately. Zoning/planning says the units would be grandfathered if the dwelling was being used as a multi-unit starting in the early 1970's. They won't file a case unless a neighbor complains or requests an investigation so...

Are these situations worth pursuing, should I walk away, does anyone have experience or some guidance?

Thanks!!