The law states that products are covered if "such component is installed in or on a dwelling unit located in the United States..."
Detached structures may be considered part of the "dwelling unit" if you use them as part of your living space, for example, as a workshop, or an extra bedroom.
https://energystar.zendesk.com/hc/en-us/articles/212109607-Do-detached-garages-barns-or-other-struct...
