Buying house with unpermitted addition?

[Jack B.]

House is listed as 1,300sf 2 bed, 1 bath but the description talks about the 900 square foot addition, which is:

-A MIL apartment with full kitchen, living room, dining room, laundry room, and full bath, with separate entry.

-A single car garage converted to a bedroom, with it's own full bath and walk in closet.

The work LOOKS professionally done, but I suspect the additional square footage is not listed in the details, just the description verbiage, because it is unpermitted. So what's the likely scenario buying a house like this and what is the worst case scenario?

I don't think it would be an issue for me to live there or rent it out, but if I try to get it permitted and they want everything redone and it costs a ton of $$$, it could be an issue for me when I sell? Even then, I think for the price the house is worth just the permitted square footage. The MIL apartment, (which again, is totally self contained, even have their own laundry and separate entry) seems like a bonus to me, and judging by the look of it, it seems like I have a good chance of passing inspection with only minor hassle, if any.