I briefly considered adding a 2nd floor to a 1-story home, and the 2nd floor would be a self-contained passive house, effectively, because for environmental reasons I’m unwilling to build anything that’s not passive (that needs heating and cooling). The new floor would need its own support system (could not simply be supported by the existing structure). So the cost came out to the same building costs as it would be for building a whole new home. Upon realizing this, I scrapped the idea. I might as well be building a separate home on an empty lot at that point.
One factor to consider: you never want to be the best house on the block. The low value of other homes will pull yours down while your higher value will prop their values up. Not sure if your plans are significant enough for this to matter.