Mortgage for SFH with grandfathered accessory dwelling

There is a property (SFH) I am looking at that has a rental property (2br apt) in the back yard sitting on top of a garage- the entire property is actually zoned as a single family dwelling, but I was told by the owner that because that rental unit/building was built a long time ago, it was grandfathered in as a legal dwelling without special permits. I haven't verified this yet with the local town.. but assuming it is true -

Does anyone have any idea how banks would qualify this property as for a mortgage? A single family home or a multiple familiy home? How would insurance etc treat it?

Anyone ever face this unique situation before?