Appraisals for Zoning-Violating Properties?

[Russell Howes]

I'm considering an offer on a triplex; when I called the city to ask more about the zoning I was told that the lot is only approved for two units (one of the units is violating). They say an owner (not sure if current or previous) had tried to get a variance but for whatever reason it wasn't doable, and that any future construction would be difficult to get approved.

I'm evaluating whether to proceed with an offer, but this at least impacts the price I'm willing to offer. I'm curious how a zoning issue like this would affect an appraisal. Would an appraiser typically know the city/zoning rules well enough to be aware of this issue? In either case, would the property be appraised as a duplex or a triplex? Any other factors I should be aware of? Thanks!