I did an appraisal for 1/2 of an attached duplex. It's zoned in such a way that each half can be sold separately. I did not develop the cost approach because it is non sensible to consider building 1/2 of the whole building when that does not represent what was actually built. The builder has the economy of scale when building the whole building- the builder doesn't build "1/2" of building separately and place it on the market. These two units share a common wall. Who pays for the common wall if you are only looking at building only 1/2 of the building? Also, that common wall is supported on each side by intersecting walls that further support the common wall. If you were to look at building 1/2 of the building then that common wall would probably need to be reinforced above and beyond its current specs. Also, they share a roof with a ridgeline in the center. If you were to only build 1/2 of a building then the ridgeline would have to be moved to the center of the one unit. Obviously, lots of problems... Even if I were to calculate the cost of the entire building (2 units) I would be missing important data from the other attached unit because I obviously didn't inspect it. The client is insisting on me developing the cost approach. What do you think?