If there are 2 separate stand alone units, such as a primary house with a basement/carriage house/detached garage with it's own kitchen and bathroom, then I would simply list them as 2 separate units on the OTAs.

Then simply do a cash out refinance after the 6 month mark if there is equity and build the Carriage house/Guest house.

FNMA Appraisal Guidelines for 2-4 Unit properties require “The building sketch shall include all completed levels above grade within the dwelling, all basement area (both finished and unfinished) and any finished living areas that were not included in gross living area, but were given value by the appraiser, such as detached accessory units, carriage houses, living area above detached garages/barns, pool houses, game rooms, studios, etc. · A floor plan/building sketch with exterior dimensions is required on all transactions. · If the floor plan is atypical or functionally obsolete, thus limiting the market appeal for the property in comparison to competitive properties in the neighborhood, a detailed floor plan sketch showing interior walls, doors, and interior dimensions is required. · The appraiser must also include calculations to show how they arrived at the estimate for gross building area.”Okay, that is long enough.

Consider ADUs, MILs, carriage houses and Casitas.   

Look at any investment in any time and you will find people who look like Nostradamus, and people who missed the boat for whatever reason - fear, horse-drawn carriage syndrome, mistaken assumptions, etc.The bottom line is that you'll never know for sure except in the rear-view mirror.

I'm expecting $40k - $50k in repairs to get rented.There is a carriage house in the back which is a yuuuge 1 bedroom, all utilities are separate.

You can certainly find them, but be ready to  buy a dumpy place and put in sweat equity or be ready to put down good money for something more ready to go.Two other options we see working for our clients in Denver (and Colorado Springs) is either A) buying a bigger house up north or west and renting by the room; or B) buying a home with a basement apartment or carriage house and doing Airbnb in that other unit.

(According to two people, this is for carriage houses that were built well before any current lot-coverage/setback limits were in place.) 

@Dave Meyers based on 1 & 2 in your list I roughly estimate 250K for a 2bd 1ba "Carriage house."

They recently announced some "ADU" trial run which is great and all, but like we have carriage houses and ADU's already in the near east side and near west side neighborhoods, so it has always blown my mind that there is no clear zoning direction on them.