How to value a studio addition over a garage in Denver

[Jake Adams]

I am currently under contract for a house in Denver. The current house is a 2 bed 2 bath with a detached 3 car garage. The property is in a great neighborhood in Denver, new community projects with retail and dining within walking distance. Also very close to a hospital.

The plan is to add a 400 sqft addition over the detached 3 car garage. It will be a studio apartment (bed, bath, small dining area). Thinking it will be a good addition for Airbnb, or short term rentals. Could also make a great long term rental since it will have its own access and garage space.

The problem I am having is how to value the addition. There is nothing in the neighborhood I can find that is similar. 

Would you value the addition with the same price per square foot as the main house, is it more valuable, less valuable?

Any help or insights are appreciated! TIA