not the same issue but some guidance
You are correct; not the same issue.
But I don't know what guidance one can glean from it to use for the OP's situation?
The GSE excerpt refers to an illegal unit.
The OP's situation refers to a non-permitted bath.
This thread may quickly turn into another:
Does an alteration without permits create an "illegal use" even though the alteration is consistent with the zoning ordinance, but in the specific case, is not permitted?
Normando: I agree with Randolph's advice; the market will dictate if it has value or not.
A client may have a guideline not to value a non-permitted bathroom, but if the assignment is
as-is market value, then I consider such a guideline to be contrary to the definition of an as-is market value.
Better, if the client doesn't want to loan on a property that has a non-permitted bath (which they are certainly entitled to do), its guideline should be to have the appraisal completed "subject to permits": that way, it is a lending requirement (and an acceptable assignment condition) for that specific lender, and not an appraiser's arbitrary decision.
Otherwise, as-is means as-is.
Good luck!