However, it can legally be used as a bedroom/extra room for the main house.

I have seen on this website answers both ways regarding this (the site administrator saying leave it in Sch A, and his colleague (also a enrolled agent) advising that it can be written off in Sch E, but that there are certain restrictions mainly -- you must have passive activity gains where the deductions are covered (no passive activity loss carry over allowed.

Are you investing mainly for cash flow, appreciation, or long term equity build?

Main issue: if you add an ADU the property becomes a 2 unit bldg., which means it’s under rent control for both units.

I can't afford the Boston area, so I've invested in property in Maine. 

The main aspects of real estate that I am interested in would be owning rentals, and working as a RE agent. 

Again, more options to attract good tenants.  

Second, a stable asset that cash flows predictably attracts buyers who want relatively hassle-free stream of income and have a long term time horizon.

One thing that attracted me was that everyday is different. 

My main goal now is to put together a team for when I really get started investing.