Here is the link https://www.boston.gov/departments/new-urban-mechanics/addition-dwelling-unit-pilot

Going forward if the situation arises again(hope not), I would explicitly state that the window units draw more energy and the tenants are responsible for any increases.I don't know what type of AC unit you have, but when you say it's "running rough"...rough as in mechanically or it's just not cooling after the repair?

it's hard to hedge against a specfic industy in momentum, especially when you don't know what the potential mechanism for failure will be  to short against.

The siding has some serious issues and some of the mechanicals are at the end of their lives.

I always ask if they are willing to sign a mechanics lien and make sure they "never get ahead" of me. 

Maybe a mechanics special multifamily in the Valley.  

Our framer frames for two of the bigger builders in town, we have to pass a framing inspection with the city, each mechanical trade has to be independently licensed and has to pass (and schedule) their own inspections, I have COI for each sub, etc. 

Going into business with family can be very problematic.Assuming you still want to, the mechanics are pretty easy.  

This scenario would also provide an opportunity to get to the remaining outdated mechanics and prevent the eminent emergency repairs on the 100+ yr old plumbing.

If not, search the forums for "IRR" and "Cash-OnCash" and you'll find entire threads dedicated to explaining the mechanics of calculating those two metrics.