First, no appraisal is ever as thorough as it could be. It just isn't practical to survey every site, check for every detail of zoning and building code compliance, and test every roof for leaks.
So, appraisal inspections are intended to reveal the most important determinants of value, including things like condition and quality, but there are numerous items that would be impractical to check for and/or that probably wouldn't significantly affect value anyway.
Regarding curbstone appraisals, for either a $20,000 house or a $20,000,000 estate, it is up to the appraiser to decide whether the significant value determinants can be reasonably verified from the curb. This afternoon, I played golf in Santa Barbara. Before I left a friend told me that tract homes in Goleta (a suburb at the north end of the city) are now selling for $800,000 - $1,200,000. These are run-of-the-mill (for example) 1,500 sf homes built in the late 60's, a few with cars parked on the front lawn. With values increasing every month and very few sales, the opinion of value isn't going to be changed much by the hole kicked in the bedroom door. One ad I saw for a modest $895,000 home said something like "kitchen curtains not included". Whew, glad we got that straight before it became a deal killer. Anyway, I think a reasonable opinion of value could probably be established without knowing whether the bathroom floors were tile or vinyl.
I'm thinking out loud, now. The other criteria is the use of the appraisal. If the Client specifically requests a curbstone appraisal, I believe that two things are implied in the request. First, that the Client will be satisfied with a reasonable, but less precisely developed value opinion (a slightly wider range of value) and second, that the appraiser will notify the Client if a reasonable value opinion requires a more detailed inspection, for example, if the Subject is missing a large portion of its roof.
Anyway, I don't think the size or value of your Subject precludes an exterior inspection. But, if a reasonable opinion of value can only be developed by inspecting the interior, get back to the Client right away. The Lender's point of view is obvious - if they can get by with a 2055 exterior, why make life any more difficult?