I have single-family rentals and I would say that the biggest missed opportunity I see from other landlords is general laziness - not painting worn paneling; refusal to re-work odd spaces or configurations; not putting in a dishwasher; etc. all things that in my area limit the desirability or marketability of the property.

what you can do is justify your offer with the same configuration house that has similar PSF like the one that you described.

Without knowing how the house is configured it's difficult to answer.

Most properties in those locations won't fit this criteria, but many will - you'll need to understand the (1) types of property configurations (2) locations  and (3) the market rent / types of tenants that will work for this criteria. 

I generally try to install a micro-hood if the cabinet configuration allows. 

The lot isn’t really configured well for an addition to the existing building...plus that would be much more expensive that converting the existing garage.

To my knowledge, there is no jurisdiction in Utah that prohibits your live-in mother-in-law from having her own refrigerator, bathroom, and separate entrance.That being said, the configuration you are exploring is called an accessory dwelling unit (ADU).  

If you have many comps that are different configurations, the Comp analysis will still allow you to examine Price/SqFt.3.

I am looking at a new roof, internal configuration changes and addition of 1500sq.ft.

Find a place you like, then check it against all the other places nearby that are close in configuration.