Personally, I do not see that the HBU is different. The HBU "as is" is the same as "as if vacant" (because it is vacant) for a lot. An improved lot "as if vacant" is the same as the HBU of the vacant lot...a potential for building a new house. BUT improved, it is "as is" with the improvements. So both the vacant lot and the improved lot have the same HBU....vacant for construction, "as is" - what it is today. The vacant lot remains vacant as an interim use. We can speculate about what the IDEAL improvement would be. We can speculate what if. But HBU test does not specify the necessity of improvements for a vacant tract to be at its HBU. Both both it is a reasonable use (vacant lots are common). It is the probable future use for the foreseeable future at least. Therefore it is physically possible, supported, financially feasible (or people would not buy vacant lots), and results in the highest value as is...we don't need to speculate about a future use.
In terms of farm land, numerous farms are composed of smaller parcels which operate as a single economic unit. But certainly what farm couldn't be divvied up into smaller parcels? But that is a future use, not "as is". As is they operate as a single unit, just as most houses with an adjacent lot, excess or surplus.
In the case below these lots were platted in the late 1900s, Lord only knows why. It was all orchards then. and they range from 5 to 20 acres in size and cover over 1000 acres. These are part of only 2 farms, and one property had maybe 20 lots or more. They are nonsensical subdivisions who certainly do not deserve their own individual HBU.
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