Think HBU. If it is not legally permissible, the HBU is a SFR no matter the configuration or number of meters. Practically speaking, no reputable lender would fund an illegal use. So if this is for a refinance/purchase with a standard lender, the request does not make sense. I would appraise as a SFR with necessary functional / cost to cure adjustments to make property functional as a SFR.
(my bold)
I don't disagree with you (on the bolded part) because if the requirement is to provide an opinion of market value "as is", that analysis will value the property at its H&BU and make whatever appropriate deductions are necessary for the illegal use. So, while a lender may not fund a loan based on the value of a property as-if the illegal use were legal, a lender might and can fund a loan of a property that has a current illegal use if that value is based on market value.
Further, another reason for a lender to request such an appraisal is to document why the loan is not being made. The appraisal will need to disclose that the current use is "illegal" and value it as-is. The lender then, in turn, has a reason to tell the borrower why, as-is, it won't make the loan or to suggest that if the borrower wants to return the property to a legal use, it will then consider the loan.
These kinds of situations are complex. In most cases, for a basic residential mortgage assignment, the "illegal use" will stop the process (and, my practice is to contact my client when I run into this situation to get instructions of how they want to handle it). Nonetheless, there are some legitimate reasons for a lender to request the appraisal to be completed, as-is, fully disclosing the illegal use and analyzing how that illegal use impacts the market value of the property.
I would appraise as a SFR with necessary functional / cost to cure adjustments to make property functional as a SFR
This would seem to me to be the correct process to value the property, market value, as-is. Implied in the analysis of the functional adjustment is whatever additional EI is necessary to add to the C2C to bring the property into compliance at its H&BU.