If the county doesn’t show the conversion you have a can of worms in terms of un-permitted work, I’ve seen the county require the work to be re-done under permit.

Is anyone familiar with the zoning rules in neighborhoods inside the permitted around converting a single family home to a duplex?

I hope your designs are already done and submitted for permits.

It appears that government-backed mortgage entities such as Fannie Mae and Freddie Mac now permit short-term rentals as of April 2019, so long as you don't turn the property over to a management company or timeshare company.https://www.wsj.com/amp/articles/second-home-rider-rewrite-11555599871https://therealdeal.com/2019/04/19/fannie-mae-issues-ruling-on-renting-newly-purchased-second-homes-on-airbnb/amp/

I produce drawing sets for permit on a regular basis. 

Consult with the historical society and be sure to pull every permit.

In other words make sure the work you do is fully permitted and performed by licensed and bonded workers.

I don't have hard numbers, but even the cheaper price of land may be completely washed out by the fact that a builder in Springfield can buy a platted subdivision for less cost per lot than in a rural town. as a new home builder myself.. the reason most of the mid west smaller areas has little to no production building is as you so accurately point out you can buy an existing home for less than replacement costs.. so until that flips.. the only new construction is usually upper end  those that have the money and want to build themselves a nice home to live in.. as a point in fact. in our market by the time you buy a lot and pay for permits etc your at 200k  before you build anything.. median price point is about 400k.. 

@Joshua Zastrow, I'm working on a new spreadsheet that analyzes more recent data -- specifically, monthly employment numbers from the Bureau of Labor Statistics and the Census Bureau's monthly report on issued building permits.

Is there a lengthy review process for permits in that area?