Zoning issue: How to prove our duplex has always been a duplex?

[Robin Johnson]

Looking for advice on how to prove our property has been a duplex. Any help would be much appreciated! 

Here's the full background: We recently bought the duplex in South Salt Lake, UT and through the refinancing process, the appraiser flagged that it's zoned incorrectly. Now, we have to prove to the city that the property has been a duplex as far back as we possibly can in order to get a non-conforming use designation. The home was built in 1900 and, as far as we can tell, has been a duplex this entire time. 

They've been extremely vague in letting us know what proof is required to make our case—but say we need to account for every year and the more information we can provide, the better.

Here's what we're looking into so far: 

-Tax records: The county was able to provide these as far back as 1997.

-Utilities: Power company says they can provide something in writing that says the house has had 2 meters since 1970. Gas company can provide similar information, but only as far back as the late 90s. 

-Building permits 

-Zoning changes

Does anyone have experience dealing with anything like this and the types of documents we might be able to find or request to get this designation? Any advice or ideas would be hugely appreciated. Thanks!