One nice little thing about using MARS and the RCA approach, is that asking the appraiser to use different comps isn't going to change the value. All he can get are some new explanations about why different comps had different sales prices. MARS will be run on all frigging possible comps - hundreds or thousands if you want, and adjust all of them to the exact same price to the penny. The appraiser will have to go and break down the new comparables residuals into value components, but whatever he does, however he breaks down the residual for each comp, because the sum of the values of the residual value components (e.g. condition, quality of construction, functional utility, etc) must always equal the residual for the comparable, the adjusted sales prices will always be the same, and likewise the value conclusion from the SCA. So, much of your work is already done for handling any new comps the client might want. And, why would he want them if the values are not going to change - and you certainly are going to charge him for the extra work.