What if the appraiser found a vacant lot at the address given at the assignment? Per the logic of the reply, the appraiser should just complete the report, maybe add an EA or two, if he sees dead grass and smells fuel oil or notes cracked lead-acid battery casings.
This is a completely different situation as the OP was hired to appraise a property improved by a residential structure and that is what the property actually is. In your example, the property is a vacant lot, the appraisal of which is completely different from an improved property (in my case, I would turn down the assignment at that point since I do not appraise vacant lots - they are simply too much liability for what clients here want to pay). Regarding finding potential environmental hazards, yes, I would complete the appraisal report subject to inspection of the property by the local/state environmental agency or an environmental engineer as long as that was acceptable to the client. If the client insisted on an "as is" appraisal in that situation, that would be an unacceptable assignment condition at that point and I would decline to complete the assignment, but would still bill them my full fee and send them a written report as to what I had observed about the property.
Also, what if the subject turns out to be incredibly more complex than indicated by the order or other communications about the assignment? I guess it would be a violation of the management section of USPAP ethics rules for the appraiser to negotiate a higher fee? In each case, the initial SOW rests upon a premise discovered to be faulty.