Couple of points: (1) the fact that there are two separate lots does not absolutely imply that there are two different H&B uses (e.g. that there is excess land). Determination of whether or not there are separate H&B is the job of the appraiser, who may, OR MAY NOT, conclude that the vacant parcel is excess land. (2) IF the appraiser determines that the 2nd lot is NOT excess land, then the guidance is to invoke a HC (namely that the appraiser is valuing the property as if it were one single lot).
Obviously two parcels with the same current use can have the same HBU whether vacant or improved. And equally obvious that HBU is an opinion to be developed (not assumed) in conjunction with all 4 tests, not just zoning. But as I keep saying "HBU as improved" is the thing when there are existing improvements.
Illustration:
Site is zoned for 2 units but is currently improved with a single residence. In terms of the HBU as improved there are basically 3 alternatives; either
1 - the property is worth more in its existing configuration than the underlying site value as if vacant, or
2 - the property is worth more as conversion bait to a different use
3 - the property is worth more as land value (arguably, less the cost-to-cure to make return it to vacant condition).
If the property is worth more in its current use then THAT value will represent the property's MV and those are the comps we're looking for.
So with the excess lot that's the question - if it's worth more in its current use as additional lot area to the improved parcel then that's the HBU for that parcel. If it's worth more if sold off individually to a different buyer then that's the HBU. There are no existing improvements to convert so that alternative doesn't apply.
As always, the definition of MV includes the explicit assumption of a hypothetical sale as of the date.
We also sometimes overlook marketability and supply/demand issues. What is the current value of this parcel if nobody is buying such parcels at this time? We can come up with a number, but can we actually expect it to sell within a reasonable exposure time as of the date, or a reasonable marketing time into the future? If the typical marketing time (relative to the eff date) for a particular property type is measured in years instead of weeks we might have to consider the effects on value of the market conditions that may change during the interim. As a buyer you may be looking to what you think the value of the flip will be in the future, not necessarily today.