The more common one is a flat value for the ADU, like they would for a pool or another bonus.

Have you had it evaluated by a Realtor, or are you relying on Zillow or other online algorithms to estimate your property's value?

Are you referring to small multifamily deals or commercial properties with five or more units?

Flips or Buy and hold or a  hybrid of both?

Every youtube video I find when searching this problem involves someone replacing or moving an electric box that either hasn't been wired yet or is re-wired.

Finding contractors in Florida after 2 hurricanes wont be easy or inexpensive 

The problem is mainly a concern for people with a net worth or reasonable income. 

I was also looking at whether or not they were someone I wanted to work with.

I don't think there is a right or wrong here.  

I wish you would have told us the year, it makes a huge difference.Anyway, everything you mention, plus 1) get a camera run on the sewer line, 2) make sure the Elec panel is newer and sized correctly (a lot of HIs will just say if it's functional or not) 3) Foundation ( is it slab or raised?)