We would not create this problem on our own. Yet, it's a problem we have to solve or walk away from.
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In this case the correct action (IMO) would be to walk-away.
If I take measurements of an improvement, use those measurements to draw a sketch of that improvement, and then use that sketch as a basis of my valuation, that is an approximation. But it is an approximation of what is.
If I'm appraising vacant land. And, although the parcel is sufficiently defined on a piece of paper, there are no clear markings on-site (say, open field). I meet the contact on site, and she tells me that it is bounded by the road North, somewhere close to a grove of trees East, about 20' from the seasonal creek South (which, when I get there, is out of season), and more or less by the fence line to the West. I can use this general description (depending on the assignment and its intended use/SOW), to approximate the subject parcel. Again, I am approximating what is.
In Doug's post, the suggestion is one can approximate what isn't, using an EA, and it can be done since the client can impose by virtue of a supplemental standard. There is a lot of misinformation in that suggestion (if I understood it correctly); much of which I use to believe myself; so Doug is not alone here, nor is his belief make him the outlier.
So, I agree with you that an appraiser (hopefully) wouldn't create the type of subject-identification problems on her/his own. But one of the most important "relevant characteristics" (that Greg highlighted in his excerpted comment) regarding a non-existing parcel is that
it does not exist. And, if the appraiser properly identifies that it does not exist (is not factually a parcel), then the next step would be to immediately eliminate the option of using an EA, and going straight to the option of using a Hypothetical Condition. When I say, "Going to" the option of an HC, that doesn't mean that option will work either. since it has its own set of requirements ("thresholds", if you will) to insure that the results are not misleading.
For the intended use of a mortgage finance transaction, if being made by a Federally Regulated Institution, or by any lender that sells to the GSEs or Secondary Market, or by any lender that requires those guidelines be adhered to, an HC that the land doesn't exist when it does is not acceptable, because the valuation is not "as is". So, for the mortgage finance transaction, it cannot be done and produce credible results.