Marc,......At some point in time we all might find ourselves appraising that one property which could present itself as being among those highest appraised value(s) in that immediate neighborhood. It happens. I would chuck that notion about ....value can not be higher than the highest "comp" (sold). Perhaps your subject here is on the verge of taking such a prominent position in the local neighborhood, and the addition built there and the imminent purchase will help to establish a value-high. To secure that position this subject truly needs two more neighbors to re-sell in the coming weeks at appreciated prices so that the three of them can keep that local value raft floating through the summer.
About the addition, off the lower level.......and your saying that "the sq.ftg. does not count".........what exactly are you meaning ? You say also "sub-grade" and I assume you mean that this addition is adjoining the home's basement level. I say that too with perspective of walking into the front door of the home and then going down (stairs) from there to this new bedroom. Is that the case ? And, again, what are you meaning about "sq.ftg. does not count" ? Of course it counts. Is there now walk-out access to that rear yard area where there may not have been before ? Please don't say that one walks out from this new bedroom into the yard !
Present your total property with discussion of enhanced (new) living area, an additional bed and bath in a level that might not have had those before, quality of workmanship in that addition, possible walk-out there to privacy setting upon some patio or deck and neighbors not being able to see the owner basking naked on a lounge chair on a sunny summer day. (O.K., don't describe it that way, but make a generalized remark to that effect.) I have often used the "Functional Utility" field to show the TOTAL finished sq.ftg. in the home, and TOTAL beds in the home, and referred the reader to look at selling prices of comps at the top of the column. I make NO $-adjustment in the Functional Utility field when I show those data-#'s there, they are there just for visual impact on the reader's eye and I explain my use of that field for that purpose.
Sometimes total house function (finished living area amount, and bedrooms), buyer demand and the purchase price paid show a nice corelation guiding my final value opinion for the subject, especially when the subject has some element of uniqueness......like your addition on the basement level (?).
I have let it become very routine to NOT make hunches, statements or prognostications with interested parties .......before I have seen the property......and easily displace any such discussion by excusing myself from pre-judging or assuring this interested party of anything......as I have not yet seen the place and brought together all the benefitial data it offers. Follow that statement with.....and, I'm sure you understand what I mean......and 9 of 10 will say "yes, sure".
Just tell the story about the subject. Give the client a true feel for what they are about to invest their money (or somebody else's $$ !) in. Give them added photos, and use those best comps the market offers. The "value" will sell itself, and may or may not match the purchase contract amount. So what ! Buyers, sellers and lenders throw all kinds of money-numbers around like confetti at a birthday party.......and yet sometimes it is the appraiser that might feel un-invited to that gala event. It happens. Tell the story as the local market dictates. If they still like you, you might get another order from that client. No promises though.