Multi-family grandfathered on commercially zoned lot (B3-1)

[Kavan Kucko]

Hi, I am looking at purchasing a multi-family 3-flat in Chicago. The house sits on a lot that is currently zoned as B3-1 ("Shopping centers, large stores, and retail storefronts, often along major streets. Allows more types of businesses than B1 and B2 districts. Apartments permitted above the ground floor.")

The current seller says that the house is grandfathered as a three flat and is a legal non-conforming property. Public tax records support this, but I haven't seen the zoning certificate yet. Let's assume for now that they are right, and this is a legal non-conforming multi-family sitting on a commercial lot.

My question is, is there anything I need to consider in purchasing a legal non-conforming multi-family that sits on a commercially zoned lot? 

I am aware that I would not be able to do any construction that would increase the square footage, but I assume I'd be able to pull permits to maintain the property, say if I needed to rebuild a deck or work on the electrical. 

Does anyone have experience with something like this? Do people think that this could be an advantage, if say, a developer became interested in purchasing the property to build a storefront years down the road?