Well Eric it just got a lot more complicated then.
What' you're dealing with is an inherent difference related to initial build, not just conversion.
As you stated, the builder built it that way. There could have been structural differences in assembly and weight bearing, etc. Normally with a conversion, you'll just see one wall knock out, a see through knock out, several doors, etc. Builder could have built it that way as well, but it appears they went all out for a major value in use increase. That may not be the same as market value, but heck... Ever seen that movie with JackBlack & Stiller - ENVY? The dude built a mansion in front of his pals house. "Everyone told me I was crazy to build a mansion in the valley, but I wanted to stick to my roots"
Sounds like you're missing the equivalent of two whole sides (each interior sides) of framing and finishing and structural support.
Don't just run with reconversion, first you must perform the permisibility analysis. only when you can honestly claim its' not recreatable or permissible or present anywhere else in near or semi near semi comparable markets, can you present the opinion it must be restored.
You'll be accused of missing valuation if you subtract before you try to add. So you must prove the case of recreatable, permissible, commonality first. Call the zoning department correctly and the assessor. "I've got this house...." Don't tell on this one, because if it's not permissible, you'll get in hot water for sharing info. Find out if it is permissible first, then go from there.
If it is permissible, it can be considered an anomaly a market frankenstien that the right mad scientist would find more value in use. So maybe value is simply a combined value, and the added value in use would be offset by the tremendous longer marketing time to sell the unit?
Remember the basics, value in market, value in use, cost to cure, functionality.
Sounds like a tremendous home that someone would love to live in. But can someone just peruse down the road a few miles, and find a typical all single family larger home neighborhood? What is the locational factor that would drive someone to need to live right there? Or was it just a fluke? You see they get off combining condos because the substitutability factor is limited if not nill in certain markets, especially city where there is no more room to build out, and they instead started the trend of building up. What was that book called? The water tower, the water fall... About the first skyscraper in America.
You must opinion the value in use, the value in market, the offsets, the substitutability, the permisibility, the feasability, the commonality.
Yep, you've got an honest to god complex order. Love those orders, they're great learning experiences, no matter how experienced you are already. Have a good one, you're the appraiser, you make the call!