Building will be 80k-200k depending on size and square footage.

An example problem statement that I may have stated (and built the software for) in 2003, when I started investing full time: Build a tool for lead generation that collects county records from the county recorder's office (and property card data), finds the recent first mortgage defaults  from trustees websites or public filings for properties that are between 1000 and 1700 square feet, in a neighborhood of 90% like-kind properties with greater than 20 parcels, houses built after 1978, that can cash flow at current rates when fully leveraged. 

A super ball number to get you going would be at least $190/SF-$220/sf ish... to figure out square footage it's probably best to connect with a Chicago architect like @Samuel Pavlovcik or @Prashanth Mahakali both can talk through the zoning and plan development process.

I was thinking it may be closer to a "wash" or, maybe only slightly more only bc w/ a duplex at least we aren't paying for two exterior wall materials for two sfh?

Does anyone know what’s the average cost for cosmetic/ full gut rehab in Seattle Washington per sqft.

Well @Sam McCormack, @Russell Brazil hit nail square on the head for impact in my team. 

SFH in Tacoma WA, gas utility quoted free to bring a gas pipe from the street to the house and a meter.

Looking at a property built in the 1980s that the seller says is a 2/1 1606 square feet but all data bases I look at (Zillow, MLS, Propstream) show it as a 1/1 1070 square feet. 

And you could probably still rent it out for $1500-$1800 a month.When we lived in Burbank, California, we rented out our 300 square-foot studio guest house for $1500 a month.

Seems renovating this part of the attic would require ceiling insulation, drywall, flooring, electrical outlets, & heating source.My concern is that a reappraisal wouldn't add anything to legal square footage.