As a custom builder for decades, I dealt with this way more than once, especially with new subs that had not worked for me before.Back in the old days we used to just go with a verbal ok, but in the last 20 years or so, it is imperative to have a written CO (even an email sent from the jobsite) from the Sub and then I would add 20-25% and submit a CO of my own to the customer.

Rent out part of the space at least to help pay for mortgage and utilities.  

Also, how do you deal with utilities?

This step would include understanding City zoning & platting, locating utilities, researching City codes for development requirements (big ones include detention), etc. 

A realtor who used to have a PM company called "JBS Realty" until this month.He used to manage 8 of my properties and is currently refusing to pay out multiple 5-figures worth of rents/property reserves/security deposits/unpaid utility bills owed. 

My question is - what is the best strategy for utilizing my home equity and buying my first rental?

Especially if you get a long term resident in the ADU and then you might can utilize them to help on your turns of the MTR. 

I see a lot of posts for furnished monthly stays where owners are fretting over utilities and how to ensure a tenant doesn't abuse their being provided.

I hired a GC to manage the build out, but there has been no consensus on who is responsible for the utility payments (Water and Energy). 

Utilities are typically $350-450/month for a 3/2.If you have any more questions feel free to reach out!