At the end i will add a list of items why adding a single ADU in single family zoned areas may not be the best RE option.The most common value add is via a rehab.  

Insurance is expensive, there is a lot of extra maintenance like painting or electric in common areas that don't apply as much with smaller units. 

Quote from @Nathan Gesner: Unkempt lawns, trashy or broken down vehicles, mattresses and trash on empty lots, boarded up houses, bars on windows, dealers and hookers on the corners, the lack of stores or gas stations, occupied properties are dilapidated, etc.Or when Google maps shows stuff like this: If those are drug zombies, as they appear to be, that's an "F" class neighborhood where nobody pays rent and structure burnouts are common.

What is more common is someone saying they have everything, calling it "easy" and it's all just BS sales pitch.

If you have hired a specialist to perform a cost segregation study of your real estate property, they will likely use one of the six most common methodologies recognized by the IRS.

Another case where common sense isn't so common.My $0.02 ...

You're facing a common investor challenge: learning from experience (and sometimes mistakes).

The common feedback I hear typically revolves around your accounting preference. 

A rising tide lifts all boats. 

Lack of provisions for dispute resolution is a common oversight.