I'm glad we have this forum, and for Richard's positive comments about the pole building valuation evaluation I recommended.
My point about not relying only on the Cost Approach is that if the loan is sold to Fannie/Freddie, they won't take it. They rely only on Sales Approach. However Richard has a positive way to adjust for quality which is reasonable and supportable. Keep your notes, and describe your adjustment procedure in the addendum. Marshall & Swift is a good source for cost info.
Regarding the garage/storage space: On oversize garages, I normally adjust for the number of side-by-side vehicles which can be stored using their own doors, and the rest of the space is allocated to "shop" space as a separate adjustment at the bottom of the grid, with the amount based on the space size and finishes/amenities. I've never been challenged by an UW for doing it this way, and it shows them that some value is attributed to the extra storage/shop space.
Other appraisers will make a larger garage adjustment to account for the extra space. Either way works as long as it's reasonable and explained.
I'm told it is improper to allocate vehicle space adjustments for extra vehicles one-ahead-of-another as total garage parking, since vehicles must be moved out one at a time.
By the way...another important element to inspect: Is the separation between the garage/shop/storage and living space protected by non-combustible wall & ceiling surfaces - typically 5/8" sheetrock, and a self-closing fire door into the living space? If not, do report 'subject to' repair and call for this to be done. On one pole building/residence I did, the entire underside of the floor above the garage had no firestop ceiling, which I reported. It had to be installed before the loan closed.
Dave Towne