Take a deep breath everybody, take a deep breath.
O.K. Lee Anne, I did one a while back that sounds somewhat similar in complexity, although at the other end of the financial spectrum. It was a refi of a condo in a complex of only three units, and none had sold since being (custom) built a few years earlier. To top it off the units were the only condos in the exclusive suburban city, and were built in the footprint of a historical barn (Yes, the turn of the century barn (stable?) was 15,000 SF so you can see that the neighborhood had been high end for quite a while). While I had no “Comps” I was able to reference Competitive sales and address marketability issues by referencing comments from the Town Planning director (That they were making it easier to get zoning changes due to the good response the builder had with this project), and by referencing the increase in high end cluster homes (the owners had “Downsized” into this 5,00+/-SF $X million home :roll:, as you guessed they were not appraisers.) :lol: I also clearly disclaimed that there was no reliable market data, that I made assumptions, and that the reliability of the final estimate of value would naturally be less reliable.
But,
The most important thing I did was talk to the local banks chief review appraiser throughout the process and keep him up to date. If you have a knowledgeable end user, who can read and understand your report, and they know about the situation and want to proceed, then it can be fun, because you really get to stretch your legs and do something you can be proud of. If you are doing this for a broker that wants to try to put it into a conforming pigeonhole, it will be nothing but trouble for you and them. You said it may be better to go a head and do it so you can at least be looking at the same page. Thats ideal, and works if the reviewer/underwriter knows and respects you, but will kill any future if you don't, and subject yoou to a lot of liability you dont need.
Good luck, and if you continue, think competitive, not comparable, and disclose,disclose,disclose.