Mostly people with minimal RE experience are adding a single ADU and they often make that choice based on cash flow without taking into account the negative equity position.  

Note: This could happen NOT by your choice with a layoff.

This can be very expensive and is usually the worst choice because you can't justify the cost.2.

There are more details here if interested.I will not tell anyone to do or not to, that is a choice that you will have to make and also it is a choice to work it or not, like any system.

So, an area might have been D class 20 years ago and is C class today in terms of what the property condition, crime etc show you BUT if the tenants still stigmatize the area in their mind then mostly D class tenants will be applying there because they have less choices.

I'm getting asked constantly about different neighborhoods and cities in Cleveland so I decided to make the most up-to-date graded map of Cleveland to serve as your ultimate investment guide.

Yes they can use a realtor of their choice.

How does one define design choices, etc?

Artwork is often personal choice, but you definitely need some stuff there.

Start with BiggerPockets Ultimate Beginners Guide (free).