I am a residential appraiser and occasionally have to deal with HBU in regards to residential properties that are on business zoned sites. However, my question is kind of the opposite of that situation. My hypothetical problem/situation is as follows: Assume land in an area is worth twice as much when it is zoned commercial as when it is zoned residential. A business is located on residentially zoned land and its use is only allowed because it is grandfathered. Current zoning would require this business to be located on a commercially zoned site if it were to be built new. So the question is what type of vacant land (residential or commercial) should be used in developing the cost approach? My thoughts are that since the cost approach is based on the principle of substitution, commercial land would be appropriate. Thanks for any input you all can provide.