A property must be appraised in terms of its highest and best use. The definition of highest and best use is as follows: The reasonable, probable and " legal use of vacant land or an improved property " which is physically possible, appropriately supported, financially feasible, and that results in the highest value.
The highest value is only one part of th analysis , when a lender makes a loan on a property, their collateral and security is based on what it is - NOT - on what the appraiser or owner want it to be. In this case as of the effective date of the appraisal , it's a legally permitted duplex, On commercial properties, often there is a future potential higher or better use, but most often its current use stays in effect until its financially feasible to develop either a new structure or convert the existing use. This is why you see SFR on commercially zoned lots or land and often the use never changes. Its like in Los Angeles, I get reviews where some Dunderhead appraiser, has a 1,500 Sq.Ft. home on a 3,000 Sq.Ft. lot. he thinks because its zoned commercial thats its H & B use. But the idiot has never built or developed anything and he didn't spend 10 minutes, thinking about the permit process or feasibility of a commercial building on it. With set backs, ADA approved on site parking, there is no room for a structure, so thats why we have all these 90 year old homes, with old rental houses on them.