Final answer:
The appraisal adjustment for the subject property with a pool is an increase of $21,500, to account for the value of the pool compared to the comparable property without one.
Step-by-step explanation:
The appraisal adjustment refers to the process of modifying the value of one property based on differences between it and a comparable property. In this case, the subject property has a pool, while the comparable property, which sold for $253,500, does not. Considering the pool has been rated as having a price differential of $21,500, this amount should be added to the value of the subject property to make a proper comparison.
So, the appraisal adjustment would be an increase of $21,500 to the value of the subject property. This reflects the additional value that the pool contributes to the property when compared to the comparable property that lacks this feature.