I wasn't thinking of separate structures specifically, but rather a setup where the "house" portion of the equation is more conventionally sized rather than an apartment and possibly does not have its footprint based off of the garage dimensions. My previous calcs around this same question indicated that build costs are very non-linear with respect to living space size; and to build a ~900 square foot living space is not much less expensive than to build a ~1500 square foot living space to the same standard; but this will of course vary with time, location, and construction method. As far as heating cost is also non-linear as space increases. The surface area to volume ratio is more favorable for a larger structure; and if your HVAC is zoned you can just not heat/cool the unoccupied rooms, at which point they just become more insulation for the occupied part of the house. Unless the small living space is being dictated by zoning, land availability, preexisting structures, or some other practical concern I was never able to build a solid case for it to be a more-optimized option than a conventional house with large attached garage; especially when resale is included in the analysis. I'm trying to determine if my analysis missed something, if the results differ in this case due to location etc, or if this garage apartment approach is more of a lifestyle choice than a practical choice - which is fine if that's the case.