My thoughts are...what is the present use? Lets say the property is zoned commercial but is, and has been, used as residential. If the highest and best use was something different...then why hasn't it already been changed? Thus my theory the highest and best use is, as improved.
Zoning, in and of it's self, does not necessarily create a "higher use". There is more to the test for highest and best use than just "legally permissible".
Lets see if my memory serves me....
Legally permissible
physically possible
financially feasible
maximally productive
I think that is correct. We have some older homes on the city's westside where the city changed the zoning to C-6, commercial. Single family residences are mixed in along with some small commercial usages like a legal office or beauty shop. Since there is really little demand for more commercial space, the existing use (could be interim too) is the highest and best use.