Re read the condition ratings and definitions (I add the UAD definitions to every report from my software.
UAD is NOT about our opinion, it is about certain facts about the property. To answer your above question built in 2013, the answer is right there in UAD italics under C1, where they explain ( Note: Newly constructed improvements....that have not been previously occupied are not considered "new" if the have significant physical depreciation ( that is, newly constructed dwellings that have been vacant for n extended period of time without adequate maintenance or upkeep)
The answer to dwelling that was occupied/used as a sales office use, C1 says it : "The improvements have been recently constructed AND never occupied. " If your C2 house used as a sales office is in pristine condition, then explain as such. , if market shows no adjustment between C2 and C1 then make no adjustment, but the dwelling that was occupied as a sales office is still C2.
If a sales office model is in BETTER condition than a C1 house, explain, but the model sales office is still C2 because it has been occupied. Just the messenger here.