One thing to note- the building is older (built in 1922), is considered in a historical district, and although each unit has been fully renovated, I have not upgraded all the electrical/roof and plumbing, so that's on the older side which I know is more challenging to get insured... 

It wasn’t easy—there were tons of repairs needed, from plumbing and electrical updates to cosmetic fixes like flooring and painting.

Old skool K&T systems are simply not designed for the electrical loads that houses have today.

Make sure there are no Federal Pacific electrical panels. 

Things they did bad which I wish I had asked more information about:Never involved in the screening of tenantsThey recommended keys for cash by waiving the rent amount buy only collecting fees which they would get not meThe cost to evict is 750-950 yet other companies told me they will include eviction in the priceThe tenant didnt pay in full last month and I was the one calling out to them and asking questions then a few days later I get an email saying something happened with the tenant and they are investigating.You get passed from person to person every week or so.No one told me when the construction was complete and so the house was ready for a week and the post to zillow and other sites was not done.It took them another week to take pictures and then another week to upload it.I was paying 200 dollar a month electric bills while the house was vacant because they left the AC running in the empty property.

This house had a working HVAC, working electric, and all plumbing was functional, so all it needed was cosmetic work.

They'll steal your copper, hot water tanks, furnaces and electrical wiring.

I agree with Nathan (don't forget to price running extra water, sewer and electrical lines).  

The main issue is that to do the ADU you have to spend so much bringing other items up to code like electrical, plumbing, lighting (windows) and egress.

An example is on one permit I had plans for electrical, plans for gas, plans for AC condenser.