My point is simple and I will explain it with an example Lets say the DSCA yields $125,000 for 2 units. I normally adjust gross GLA irrespective of the number of units plus +/- bathrooms from each unit So if the final opinion of value is $150,000 that will give $75,000/unit
Now my subject has 3 units Should I leave the computation per unit blank ? Or should I place $225,000 and say that the opinion of value ;per unit is not considered due to differeces from subject?
The appraiser need to make sure s/he is comparing apples to apples.
Assume that the lot size of the duplex and the subject (3-unit) are the same.
Assume that the zoning allows for 3-units on both and assume the H&BU of the sites, as vacant, are the same.
The duplex is $150k, or $75k per unit. If the two units are identical (or sufficiently similar to one another) then the per-unit indicator reflects each unit's configuration (bed/bath count and unit size) as well as a pro-rata share of the site value (50% of the site value for each unit).
Now, let's assume the subject's 3-units are identical to one-another and reasonably similar to the unit-configuration of the duplex (such that any adjustments for size/configuration are not really materially).
While the duplex sold at $75k per unit, the triplex is most likely not worth $75k per unit. Because in the per-unit value of the duplex, 100% of its site is accounted for. If the appraiser adjusts the subject up $75k for its 3rd unit, the analysis has inflated the value by adding an additional site-value component to the subject.
Land Value + Contributory Value of the Improvements = Market Value
Land is worth $90k for both the 2-unit and 3-unit site.
Duplex: Contributory value of the Improvements = $60k. Total value for two unit property = $150k. The unit adjustment for improvements is $30k, not $75k.
Triplex: Site value is $90k. Adjusting $75k per unit to value the property means: Value ($225k) - Land ($90k) = Improvements ($135k). But the contributory value of the units for the duplex is $30k/unit. 3 x $30k = $90k.
In this simple example, if the appraiser does not address the land-value component in the $/unit, the appraiser is adding an additional $45k to the subject.
Real life isn't as simple as the example. But real life needs to be analyzed similar to the example. Otherwise, relying on a $/unit adjustment could lead us down a wrong path.
Your bigger question (I think) was if I make a per-unit adjustment do I need to screw around with the individual configuration adjustment. I would say, "yes"; compare what exists on a one-to-one basis, and evaluate the additional unit based on its contributory value; but that "contributory value" of the 3rd unit may not be a simple price-per-unit of the duplex properties.