It can, with the right capacity and a realistic load, and it is easy to get wrong. A garage or a sunroom is often an unconditioned or semi-conditioned shell: thin walls, a large door or a lot of glass, a slab on grade, and a design temperature equal to the outdoor air when the door opens. A ductless mini-split is a reasonable way to heat and cool that kind of room without extending ducts, but the load can be much higher per square foot than a living room of the same size. Manufacturer starting points of about 20–25 Btu/h per square foot, already labeled as a rough rule on this site’s sizing page, can undershoot a west-facing sunroom or a leaky garage in a cold climate.
Ask for a room-level load (Manual J or an equivalent room calculation) that uses the actual insulation, glazing, orientation, and how the space is used — workshop, occasional sitting room, or overnight guest space. An oversized head that short-cycles can control temperature poorly; an undersized head will simply run out of capacity on the design day. In a garage, also plan the electrical circuit, condensate, and whether the unit is listed for the location.
This is not a whole-house heating plan. If the question is actually whether a mini-split can be the only heat in a cold climate, see that page. For a back-of-the-envelope range only, use the 400-square-foot page’s labeled rule of thumb, then have it checked. Confirm the circuit on the panel calculator if the house is on a modest service.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.