I am considering a 20 foot extension to my cabin - a new master suite with bathroom and fireplace, that would take me to around 2900 feet. 

The house is a 3/2 and has an ADU with a kitchen, and bathroom.

If the kitchen needs an upgrade or the bathrooms, or flooring then you could add value to the house and potentially cash out refinance or get a HELOC based on the new value and use that for more upgrades.

The dog uses the bathroom indoors on those pads.  

Try looking into zillow/Redfin and see what your potential rents you can get near the neighborhood by filtering the bedrooms/bathrooms of the intentional property that you are planning to buy.

A quick synopsis: We got our zoning approved through the city, we then had to get a variance because we want to run an 8 bed facility but the International Building Code would have had that be a "Commercial" property and would've needed to be compliant with all of the ANSI requirements like having an accessible main floor bathroom and 2 accessible main floor bedrooms, this would've been cost prohibitive to us.

I’ve done Zillow, filtered it based on Bathroom/bedroom size, home type, etc.I’ve found a few but in the event where houses for rent don’t show up (due to time of posting ?)

Now if I remodel the kitchen, bathrooms, doors, windows and some other stuff I could probably rent it out for $650-$700, increasing my cash flow, specially when I move out.

I get the bathroom it should have an exhaust fan.

It was built in 1944, there are 2 bedrooms 1 bathroom.