There are 2 ways to look at this:
1. What's the DOM for a spec house before it becomes C2? Every 'depends' one lists (weather, roof type, etc) is more unnecessary subjective opinion that comes into play, whereas UAD was intended to eliminate a lot of that. If one is using their forms, go by their definition. Keep it as objective (read supportable) as possible. One can more easily support a never-lived in 9-month old house being called C1 than they can C2. What is the opposite of new? Old? Used? Pick one, and apply it to a house. If a house has not been used, it is new, and per UAD, is C1.
2. The gray area is not "lived in or not", but rather how much wear and tear pushes a never-lived-in home to C2? Houses depreciate the second they are finished. Imperceptibly, but still something. There is no such thing as NO depreciation, so the definition itself is worthless (big surprise). A spec house that has sat for 10 years, still never lived in, would probably be C3, if not maintained. Exterior facing elements depreciate whether lived in or not, so time on the market vacant can matter if it gets large enough. If a newly built house goes through a hail storm, it clearly is no longer C1, because there is (likely) noticeable depreciation from the hail. It may even be C6 if the damage is severe.
The problem, as usual, is UAD (like ANSI) is clear as mud regarding stuff. We are supposed to have distinct buckets every house fits in, with numerous gray areas comprising the rating. 80% of the houses I have appraised in my career have been C3 or C4. That is MOST houses, with 2 ratings to choose from. No granularity at all. 95% have been Q3, Q4, or Q5. 3 quality ratings, from the 800sf shack to the 6000sf brick home with 10' ceilings, granite tops, and complex roofline. Wondering out loud if the new forms will include new UAD ratings with more granularity.
If you inspect a house and cannot detect any visible depreciation, and the house has never been lived, I would call it C1. Just clearly state how you arrive at that though if the house has sat vacant for awhile.