Zoning ordinances, building codes, and ADU rules are all hyper local, so I would suggest you get an architect involved to help sort through everything.

GIS property tax map or records.  

The Seller is not interested if there is immediate tax payment.   

At the same time you can go to the local building departments and find the plans for the projects and get the GC info from that as well as the architect.

This MFH property is 50+ years old and the City wants to enforce recent building codes on this property.

I need to figure out a base operating cost so I can consider this when I assess land purchase price, but I"m finding it very difficult to asses things like insurance, property tax, utilities etc. on something that isn't build yet.

And I started in the land business in 75 and my dad started in 65 its a fun bizz.. in our day the land was bought generally at tax sales.. its only in the last 20 years that these marketing techniques and the how to folks started educating the mass's..

This book got me off the sidelines and in the game: Build a Rental Property Empire by Mark Ferguson and this book elevated my landlording methods: Managing Rental Properties by Heather & Brandon Turner~ Lindsey

House hacking can be a powerful strategy to build wealth and financial independence. 

Then I work out an estimated rest of life (the building inspector did this as part of his report) and then I put in an estimated replacement cost.