short answer: find sales
long answer:
in my market if the property is located in a small town with a population of 1500 to 3000 and is a regular rectangular lot the adjustment is typically 20¢ to 40¢ per sf depending on the size. i have support for this with lot sales and place those in the site value of the cost approach.
there are some neighborhoods with higher values, and in those neighborhoods its not a per sf adjustment. for instance there is a river neighborhood with higher grade homes and those lots sell for around $25,000 whether its a 1/2 acre or 3/4 acre lot. when using neighborhood sales i dont adjust for the site. when using out of neighborhood sales i find the value of the comparables site and adjust it against the subjects $25,000 site, its not a per sf adjustment. then theres the river view/location adjustment but thats another topic.
many of my assignments involve rural residential properties of 5 to 40 acres surrounded by 1000's of acres of farmland and forest with no neighbors for miles. these are always adjusted at a price per acre. i have sales supporting adjustments for small acres of mostly level to slightly rolling with few trees, wooded with hills and possibly a creek (good recreational hunting ground), and larger tracts of 20 to 40 acres of mixed use tillable/recreation/residential. Each category has its own set of sales that are placed in the site value and is used to derive the site adjustment.
then there are villages that literally give away vacant lots. these are depressed small towns with populations ranging from 30 to 500. there is no economic activity, no school, no church, nothing except residential property, and maybe the occasional grain elevator. there was at one time a small business district with a post office, bank, restaurant, bar, and a few shops; however, these have closed down 20 or more years ago. the buildings now sit vacant and are either falling down or have been razed. the youngest homes are 35 years old with the majority being over 80 years. transfers of vacant sites are common but are typically for $0 up to $500. in these areas i dont make site adjustments unless there is some negative external influence like located next to a dilapidated building or grain elevator.
there are more but those are the most common that i can think of right now.
