That's only true when you can ignore building codes on things like loft stairs and other space-stealers. ADUs in my city must be permanent structures that meet all codes and be fully inspected throughout the build. Anything under 500 sq ft / 46 sq m under north american building codes is a tough scene for livability.

There is a related conversation that our building codes have gone too far and are driving up costs due to the space and materials required to comply, and yet I also like bathrooms with grab handles and wide doorways for accessibility. Tricky problem (though we need to kill the 2 stair rule for low and mid-rise apartments asap. There are tons of modern ways to prevent and suppress fires that don't require anywhere near the expense or form-constricting compliance).