Adjust the comparable's sale price to estimate what that comparable would indicate if it matched the subject. The subject is the common reference point. Changing the subject for every comparison would move that reference point instead of putting the evidence on a consistent basis.

Worked example: The subject has a one-car garage. Comparable A sold for $410,000 and has a two-car garage. Assume market analysis supports an $18,000 contribution for the extra garage capacity, with all other relevant factors equal. A is superior on this feature, so subtract $18,000: its adjusted indication is $392,000.

Comparable B sold for $380,000 and has no garage. Assume the market supports a $12,000 difference between no garage and the subject's one-car garage. B is inferior on this feature, so add $12,000: its adjusted indication is also $392,000. The subject's garage never changes. The comparison prices do.

Notice that the two adjustments are different. A second garage bay need not contribute exactly as much as the first. These figures are invented teaching assumptions, not local valuation evidence. If the extra bay cost $30,000 to build, that cost alone would not establish its contribution to value.

Fannie Mae's comparable-adjustment guidance requires market-supported adjustments rather than arbitrary rules of thumb and calls for explanation of the evidence.

Pitfall: Superior subtract, inferior add is useful only after identifying which property is superior and confirming that the comparison is to the subject. Also, two matching adjusted figures in this simplified exercise do not establish a real appraisal. Other differences, sale conditions and the reliability of each comparable still require analysis.

Use the Chapter 17 appraisal guide and Chapter 17 practice.