We have 80 or so listings with a wide mix of properties ranging from more urban in Tampa/St Petersburg and then beaches and everything from studios to 6 bedroom houses. 

One is a garage conversion studio, a second is a pool house conversion, and last is a 2br/2ba garage conversion.If your strategy is a buy and hold, I haven’t found that much benefit in getting them formally permitted.

I did a search on this site, Google search, Amazon search and it's difficult to find books on this topic.  

It's a single family residence with a garage that has been converted into a studio apartment.The garage conversion is not a legal ADU, but it is partially permitted.

Scale can change any game (Amazon is an example) and some providers will to bring to market things that were unthinkable a few years ago just to reach enough scale and market share to disrupt their competitors.

Yes, your existing furnace is way too large to heat your basement if it was used to heat 4 units, probably even if it did one unit depending size.Checkout on Amazon, for example, "natural gas space heater." 

It’s available here and Amazon.

It also shows rental rates for different sizes studio, 1br, 2br, etc. 

Columbus overall has seen great appreciation with many new jobs being created from large expansions by companies such as Intel, Honda, Amazon, Nationwide Children's hospital, and Wexner Medical Center.

He currently owns a couple of business (Liberty tax and amazon store front), however does not make as much as I do on his W-2, for various reasons.