OK, let's use the OP. Suppose I have fee simple ownership of the rights to the property discussed in the OP. Does my ownership not also include the rights to the underlying land? Sure it does. So, those rights (the rights to the underlying land) already exist, and they can be sold separately.
Reasonable question. Only I feel you should edit that statement to "might be sold separately." I own the air rights over my house to a certain extent, subject to governmental and public good easements of course. I'll consider placing them on Ebay later today and see if I get any takers.
If we take this to reality, instead of theory, and toss in a lender asking for an appraiser to determine the "Market Vaue" of those air rights, for a mortgage intended use, and report it on certain forms with perhaps preprinted SOW with a prohibition of modification of that SOW on said forms, and demanding only a CB1 use to boot (I know, said information not in the original post. But what the hey, I'm just a dumb residential appraiser versus all the superior commercial ones of the world!) we have a horse of a different color regarding the true nature of the problem identification.
I guess that is the thing about many forum threads. I don't assume the O.P is trying to pass a state licensing test involving only questions meant to represent hypothetical situations or ones that can, or should, be answered only from a "Pure" USPAP standpoint. And also, as you well know about me, I also tend to always question if the job really got completely addressed by TAF / ASB responses or if they left the questions begging for more answers. All too often the job is left intentionally vague, in my opinion, to try and make the answers work for 100% of all possibilties, commercal or residential. Personally, I find that approach typically leaves the real world issues for residential appraisers in a really nice mess. Just exactly as we often do here on the forum. Because we drift off on pure theory and stop asking such questions as:
What about the septic?
What about the well?
What about the fencing?
What about the................................................
But I've posted that stuff already. My own final decision on this topic is not fully decided. It's an interesting topic. I will say this, no matter what side of the coin anyone is on, such an assignment is FAR, FAR, more complicated regarding complying with the SOW Rule than hardly anyone is bothering to post about. And I believe, in my skeptical way, that well over half of the real estate appraisers out there, both commercial and residential, would royally scew up the assignment no matter how much reading of this thread they did before completing it. Or anything TAF/ASB has put out to date on it.