Interesting, but deadly for you. Is it up to a 4 unit code now. Here, a 2 unit here can have smoke detectors, but a 4 unit must have a hard wired fire alarm system. Big difference. Also, the fire insurance policy will not pay if it wasn't done with permits and zoning allowed. Insurer will use any excuse not to pay. Around here if you didn't do it right and someone dies in a fire, the city will arrest owner, so good luck.
This is a slick owner trying to skirt the law. And you are going to help them, think about it.
Now the lender may say, yea everything ok. But that loan goes south, or a fire, and the now unhappy investor holding the loan, will not know you one bit, and sue you.
So don't be stupid for how much. You get anything wrong, and the state will find 10 other things you didn't do properly to burn you with. And the happy do it lender will become the suing lender because you missed whatever. And saying i didn't know that is not an excuse. This is one of the most obvious do not do appraisals. And some of you are suggesting to do this.