Let me first respond by saying that "Illegal" wasn't the correct wording for what I was getting at. The home appears to be unpermitted. It is untaxed. There are other outbuildings on the subject site that are currently being taxed, but nothing residential. The current owners have absolutely no idea what the original age of the home is, since they purchased it from the bank in 2014 after a foreclosure. They paid cash for it, so no appraisal or home inspection was done. Then they basically gutted it and remodeled it completely, WITHOUT any permits. The only thing I found inside the home that had any sort of date on it was the electrical panel which was inspected in the 90's, however the address on the panel does not match the address for my subject property.
I completely agree that there are many, MANY homes throughout the area that were built in the early 1900's and before the practice of requiring permits. Based on the minimalistic design of this structure, I'd guess that it was built somewhere between the 20's and the 40's, but that's a total shot in the dark. The biggest concern is that in order for the County to recognize it as a residence and tax it appropriately, it would have to be brought up to current code and an after-the-fact permit would have to be issued. That is a lengthy and complex process, and can be a costly undertaking.