In the early 1900 hundreds it was not uncommon to have residential areas subdivided in small lots as low as even 15-25 ft lots street frontage. This was done in the small towns all round the nation. The purpose was pretty smart when you think of who was going to live in these small town. Think of a small pond with a few large fish and many small fish. So if a person wanted to build a housei n town he could buy the number of lots he needed but not more than needed to build. Outside of these towns and villages was farmland or just vacant land. There was no such thing as a suburb.
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OK, i have one issue with your valuation problem as you have described. "I am working on a 4,000 sf+
historic home built in 1921 located on a larger-than-typical lot of 19,700 sf (it is actually six 25' x 125' lots + 1/4 of a 25' X 125' lot.) The current improvements are located on the four northern lots (+ 1/4 lot), and the two southern lots (total of 6,250 sf) are vacant."
I suggest before you go though all these Gyrations with the owners is you first must determine what "Historic Home" actually means to your Valuation Problem. If this property is registered with a Historic Society of some sort:
https://www.nps.gov/subjects/nationalregister/index.htm
The reason I am bring this up may help you solve just what you can or can not do. Its very possible that you don't have excess or surplus property. In other words the owners may not be able to severe any portion of the total property because of some Historical Society Agreement that may limit the Use of this Subject site.
Just a thought to consider.
My second thought/recommendation is this/ Abandon the FNMA Series Format. Go with a some other format. Either complete Narrative or a mix and match of GP or AI Forms for grids pages and photo addendum.
Again get rid of any reference to lending if that is NOT the purpose and intended use of the Appraisal report.