Chicago Zoning Question

[Long Yang]

Hi BP, 

I'm currently looking at a 8 unit all residential multifamily building. The owner has operated this building for almost 20 years. But it is currently in a B1-2 zone. What documents do I need from the seller to verify the current usage is grandfathered in and in conforming status? And also assure that the current residential usage will not change with new ownership? 

Thank you!