This is where the confusion about these types of assignments start. Let's break it down:
Your subject sale contract and thus the property client engaged you to appraise is the subject house AND the 2 vacant parcels as an assemblage.
We agree the two vacant parcels can have their own separate HBU - which may or may not be the same $ amount if they are sold together with the subject dwelling. While we need to do an analysis of the HBU of the excess lots, the assignment is NOT to appraise the excess lots as stand alone lots . The assignment is appraise them TOGHETHER with the subject (on its primary site ) The assignment is not to appraise the subject just on its primary site, since the SC is sell it with the excess lots.
As part of analysis we might develop aa value estimate of the lots if sold alone and their contributory value if sold assembled with the subject dwelling . If they are worth more alone, the assemblage diminishes the value of the lots, but the WHOLE assemblage can still have a HBU as improved ( the way I see it )- with an interim use for the vacant lots to hold as an investment or keep for large site use .
However, IF the house contributes so little to the assemblage that your opinion is tear down the house, then the HBU as improved is NO .
Definition of the problem.
Subject sale contract and engagement only specified one parcel, lets call this parcel "A". The county GIS site shows the dwelling's improvements straddle parcel A and parcel B, (which has a separate tax ID number). Not a big deal at this point, typical for this part of town since these parcels are small and most homes straddle two parcels.
At the inspection the seller tells me about three other parcels which are going to convey as well, parcels C, D & E.
I go back into tax records and identify each parcel by owner name and tax ID. I can see that A, B & C contribute to the improvements. D & E are literally a vacant lot adjacent to A & B screaming "Build a house here"... (don't worry about C, it is not located along the street and is to the rear of A & B).
So the question is excess or surplus, or neither as leelansford pointed out. I have ruled out surplus. Parcels D & E don't actually need to be split off, they can be sold as is. To build would require vacating the lot line between parcels D & E, $50 application fee and a survey.
Now, they might be separate parcels and therefore not excess, however that is going to look strange as even parcels A, B & C are separate parcels. What do you want, this is old town USA? I spoke with a kind woman at a local title company to make heads & tails of the situation and she told me that her property (same county) is a quarter of an acre and is seven parcels (with seven tax ID numbers).
So I'm leaning towards excess, HBU analysis, and lots of explaining. The client suggests I include the value for lots D & E as a line item and go across the board with it in the grid, but I'm not sure about that.