Instead, I'd plan to either invest the money and just let it grow so that when I retire, I can pay off this house or I'll use the cash to build accessory dwelling units on my four other rental properties.

Love- and drug-, apologies, cigarette/vape “addicts” (fans) will pay for their “freedom”.

No dice here though because Roseville does not allow accessory dwelling units (mother-in-law suites count for this as well as walkout basements) to be rented for anything less than 30 days. 

You got your answer more than once - use comps for a 2-unit, BUT i'll just add that if you are looking to get started and cashflow is more important than paying over a 2-unit comp price may not be the worst idea.I'd also double-check to see if the unit happens to be in Chicago's pilot accessory dwelling unit (ADU) area....if so then you could legally convert a unit.Lastly, it might be worth doing some zoning calculations to see how many units you could have by right, so that in the future if you wanted to you could re-zone to more units if you had the square footage.

Your goals and story: My wife and I want to grow a rental portfolio in Colorado.Type of property: Single family (preferably with accessory dwellings / carriage houses)Location of property: Colorado SpringsPurpose of financing: PurchaseType of financing sought: Preferably conventional since we don't have a big complicated portfolio yet.

In my area these carriage houses, detached apartments, Detached Accessory Units, are sold and marketed including these units. 

Do some research on local zoning regulations and explore possibilities for accessory dwelling units. 

An accessory dwelling unit shall be located in a principal dwelling unit that is located on a lot of at least 14,000 square feet.

Accessory buildings and guest quarters.(1) Accessory buildings and guest quarters shall be located behind the front wall plane of the home and in the side yard or in the rear yard of the principal building and shall not occupy more than thirty (30) percent of the rear area. (2) Accessory buildings shall meet the minimum side and rear yard setbacks for the district in which it is located.

I wanted to share my experience planning, permitting and now building a 3 bed / 2 bath 800 square foot detached accessory dwelling unit (ADU) / back house / laneway house at one of my single family rental properties.