As is v. as if vacant? As is contains an obsolescence if not ideal to site. As vacant there is no obsolescence...An ideal site ? If zoned for either SF or MF, is there an impact?
A house on a lot zoned commercial will generally bring much the same as one zoned residential (value in use). But the C lot is usually more valuable...So the improvements are less valuable, effective age increases. To me reflects an obsolescence related to the change in land value.
Then perhaps I did misunderstand your post?
I thought you said that FO is not addressed in the H&BU.
(I know you know the following, so it isn't directed at you...) since FO affects improvements and not the site, its fundamental analysis begins in the H&BU as-improved component. Indeed, even if an appraiser misses it in the H&BU analysis, once discovered, the appraiser should go back to the H&BU analysis because depending on the specifics, that FO might trigger an action to the improvement (renovate, remodel... which is the issue here at hand, where "remodeling" is a change-in-use, or demolition). The level of FO may also change the likely buyer; another item that is addressed in the H&BU analysis.
I think the fundamental difference you and I have in these discussions (and, it isn't about the analysis, it is about the conclusion) is this:
I say this: Concluding a H&BU of a property other than what exists doesn't require any change in the property. The value is what it is, as-is.
I infer you say this: Concluding a H&BU of a property other than what exists implies that the property must change to what the appraisal's H&BU.
A conclusion of something other than what exists doesn't (IMO) require the property owner to do anything. The value is what it is based on what exists (for an as-is appraisal). That valuation is impacted
if whatever exists isn't what should be there. Your example of the existing SFR on a C-zoned land is good example (IMO). While that property's selling price may be the same as a house on SFR-zoned land as-of the effective date, one thing will be affected and one thing might be affected:
1. The remaining economic life (as you point out vis-a-vis an increase in effective age) will shorten.
2. The likely buyer may change. The likely buyer might become an investor rather than an owner-user. A home located in a commercial-transitioning neighborhood might not have its rent-potential affected negatively but it could affect the owner-user appeal of the property. Even if the transition to commercial is slow or comes to a stop, the subject's investor appeal might be greater than its owner-user appeal. And, I'm not talking about an interim use here; transition might take 10-years. The buyer isn't purchasing the property to convert; the buyer is purchasing the property for its rental income as an SFR.