Appraisal techniques

[B K]

In a comparable sales approach, you select comps. Then you do adjustments to comp prices depending on their differences with a subject property.
Where do you go to determine the adjustments for a particular neiborghood location. For example, additional bathroom will be worth $6,000. Or adjustment for square foottage, for example, will be worth $30 square per foot, and so on. Which organization do you go to find these typical adjustments?