The text book answer is ..."adjustments are the results of paired sales analysis". The fact of the matter is...."adjustments are made from the appraiser's experience and knowledge of the market as supported by data in the appraiser's work file and other appraisal work files".
Adjustments are "market perception of value rather than cost"; however, they can be supported by cost information such as builder's price lists of options. Many times buyers shopping for homes compare items such as fireplaces, central air conditioning, patios, and decks from what they see offered in new homes.
I am always ask by REALTORS® for a list of adjustments for certain features. I always reply...there is no magic list. Classic example is ... what is the adjustment for a two car vs three car garage in, say, a $185,000? After they answer, I say...."then would you use the same adjustment in a $350,000 to $400,000 home? Same with fireplaces. Is there one standard dollar adjustment for a fireplace? Of course not! As an example, in my $175,000 ranch style home, the builder charges $3200 for a gas log fireplace. My pairs analysis of similar sized and featured homes indicates a market perception of $2,500. In a $400,000 home the adjustment might be $5,000 to $6,000 for a two story stone front fireplace.
Adjustments are the appraiser's interpretation of the market perception of value. As long as your adjustments are reasonable...and supportable, you should not be required to "prove" them in each and every appraisal report. Just be careful because if you say...."as a result of pairs analysis" the underwriter might just call you on it and want to see that "pairs analysis".