Literally tens of thousands of very similar duplexes were built in Cleveland and the inner-ring suburbs the two decades preceding the Great Depression, typically with two full-floor flats of nearly identical layouts on the first and second floors.

Inventory decreased 18.5 percent for Detached homes but increased 7.8 percent for Attached homes.Median Sales Price increased 1.0 percent to $274,823 for Detached homes but remained flat for Attached homes.

She was also able to rent the house out for me for a flat fee to a great tenant, for more than I thought it was worth. 

I own a two flat now in my name.

Yes, I think I understand what you mean about internal fairness between the two partners.However, there's still a long term capital gain on the cashflow in excess of profits once the basis is "used up" right? 

Well, since you can't rip the walls to look at framing, wiring, and plumbing, look at what you can see:1) Level of trim, appliances2) Finish carpentry or gaps in joints3) The flatness of floors and squareness of rooms. 

We'll make enough in June and July in excess of last years numbers to offset lost revenue from Covid. 

Is there a Chicago specific directory of average costs for things, like price per sf of replacing existing decking or flat roof replacement cost per sf?

If you are doing buy and hold then compensating a high flat fee amount might be more appropriate.

Flat roof?