Non conforming back house - no city records - no problem???

[Isaac A.]

I'm considering a property that has second home in the back, that is technically on a separate yet connected lot.  The small 2 bedroom 650 square foot home is nowhere to be found in the city records, yet has been around for 50+ years, and used as a rental property at the rear of the property, behind the primary house.  

I'm curious how local jurisdictions look at homes such as these, that we might think of as being "grandfathered".  Any experts out there with some insights on non-conforming, grandfathered, weird properties...that might not have similar comps, hence making it harder to find a loan, etc?

We would rent the front house out, and live in the back house, which I was hoping would possibly help us qualify for a larger loan. 

Any insights or wise souls out there with some advice...I'm all ears.