I recently had to get some garage door springs fixed on my home and nobody would give me a quote or even  come look at it without charging me $75+.For all this I pay 8% of rent collected, $300 to place the tenant and $150 for renewals.

The guest had brought additional guests than they listed on the reservation, doesn't respond to my messages/concerns, left my front door wide open when they left. 

I compared other properties in terms of age, sq ft, acres, beds, baths, basement, garage and a subjective measure of interior condition.I have a huge problem with the automated comps (Zillow, RPR, etc) because they could not account for the interior condition. 

For example we usually budget about $2,500 for kitchen appliances including dishwasher, because we rent to families and a full-size french door fridge alone will be $1200-$1400.And we usually have two bathrooms, and they take a lot of material and labor. 

Appraiser used comps 10 miles away, different school district, different sizes, I think all had 2car garages, subject had 3. 

A hands-off garage conversion ADU is over $100, even doing much of the work yourself, it will be much more expensive than your estimate.Single ADU additions in single family zoned areas are typically one of the worse re investments. 

Quote from @Anthony Pitruzzello: Please note in my question: "and an apartment over a large garage/workshop in a separate building (for me)." 

It has two 2-bed/1-bath units and a detached garage apartment (ADU) that needs to be fully gutted and redone.

It was a single family  with a garage under.

Also, by utilizing the field in the MLS for garage apartment or mother in law suite will help pick up in the searches online too.