I wouldn't use "interim use" for our situation.
As you note, the use is the same (residential); it is the improvement that is not maximally productive due to its configuration. The site (presumably) supports two townhouse units, but the current improvement is a single unit. If we assume that the analysis shows that conversion should take place, there is no "interim" (why wait? Change should occur now).
Value in Use or "Use Value" (Dictionary of Real Estate, 4th ed.): the value a specific property has to a specific person or specific firm as opposed to the value to persons or the market in general.
From The Appraisal of Real Estate (12th ed.): Use value is the value a specific property has for a specific use.... Real property may have a use value and a market value...
Use value has a different definition than market value.
We assume (in this scenario) that the townhouse improvement as-is (2-units combined into one) does contribute value to the underlying site. Therefore, there is use value (it is worth something, more than the vacant site). As-improved, its worth whatever a 3,600sf townhouse would fetch. I can appraise it that way, but I cannot call the value "market value" because if my H&BU analysis is correct, the subject would sell for more as-is to an investor who will remodel vs. to an owner-user who will occupy.
The investor is the likely buyer; because the investor will out-bid the owner-user.
The value the owner-user would pay is less than what an investor will pay; the value of the larger GLA for an owner-user utility is less than the value of that GLA reconverted to the original, 2-unit configuration (after all costs and EI are accounted for).
Don't confuse "as is" with H&BU.
As-is, the subject is a large, single-unit townhouse.
If the H&BU is to remodel, then the as-is value (market) reflects the as-is configuration and accounts for all the costs to convert. As-is, this will sell for more than the townhouse to an owner-user.
There is nothing hypothetical or extraordinary about the valuation. The valuation reflects the "market value".
Appraising the property as-improved (when the as-improved configuration is not H&BU) and not considering the H&BU and what impact that has is an "as is" valuation as well. But the value is not "market value".
There is nothing hypothetical or extraordinary about the valuation. The valuation reflects the "use value".
As the text states, "Real property may have a use value and a market value".