I have seen this type of request from lenders before. In my area of coverage this would almost never apply.
That being said and assuming that there is a dwelling involved, I think that I clearly indicate that the subject is on what ever acreage there is. I would note any adverse or positives about the entire parcel. Then I clearly would indicate that at the lenders request no adjustments were made for the excess land over 10 acres. If the comparables had 8 aces I would adjust to 10 aces. If the comparables had 10 or more aces there would be no adjustment and a comment as to why there was no adjustment.
Would you accept the lender telling you that you must use GLA adjustments of no more than $50/sf because anything else exceeds their guidelines, and they don't want to loan on "excess house"?
Edward-
My post, although a little sarcastic, is made so to emphasize this point:
An as-is value is an as-is value. An as-is value is dependent on (among other things):
A. What exists.
B. How the market reacts to what exits.
The purpose of the appraisal report is to accurately describe "what is" and then appraise it "as is", or as something other than "as is".
In the case where 15 acres exists, but only 10 acres are being considered, that is appraising something as it isn't. You can report that it exists, but that doesn't solve the development (valuation) issue of not appraising it as it exists.
In the case where something is being repaired, one appraises it "as-is, subject to this repair being made". Once the repair is made, the Extraordinary Assumption (that it was going to be repaired) is met, and the "as is" value = the "as is" condition.
In the case where something is being built, one appraises it under the Hypothetical Condition that it is already built. This is not an "as is" appraisal. The appraisal is worthless until the the house is built. Then, the as-is value = the as-is condition (on an HC, it is a little more complicated than that because of the difference in timing between appraisal and completion dates. But this is is the gist).
In the case of the 15 acres being appraised as 10, it is not "as is". There is no mechanism to make the 15 acres into 10.
So, the "as is" value is not based on "what is", but based on "what isn't".
Do you see how this conflict is irreconcilable?