Accessory dwelling units (ADU), as they are called in Fulton, are fine, all other muni code aside, as long as they don't have a stove.

Should I just build a second Accessory Unit in the back and rent that for another $1,200/mo? 

The markets are very very similar from price point major difference is down here zoning pretty much does not allow duplexes, triplexes and accessory dwellings so your multis are larger complexes of 10+ units

@Susan JojolaIf the basement unit is an accessory apartment (check your zoning), you could probably advertise the main house as a house rental if you feel that is beneficial.

The other option is to build an addition with an ADU (accessory dwelling unit) finishing the house and adding another 650 sqft to the main house ( 3 bed 2 bath ) and a 550 sqft attached ADU (1 bed 1 bath). 

@Ed AllenMatt Lesher at The Agency PDX is a fantastic broker, and also an investor himself.Another idea you could look at is buying an SFR with an Accessory Dwelling Unit (ADU) on the property, which could be detached or within the main house i.e. basement as you said.

This was to be my forever home with an accessory unit on the property.

The in-law unit is a legal Accessory Unit.  

In certain zoning districts the city will allow an accessory structure as long as it doesn't have a kitchen.

@Bryan Covet, let me introduce you to @Joshua Dates who does some wholesaling in the area.Not sue why this did not link up to Joshua but look hid up and I will try again from his site.Cheers,Buddy