I don't see this particular cottage as an ADU... I assumed it was just a smaller-than-usual house. Maybe I don't know the definition of ADU? As you mentioned, if someone is planning on building something like this as a guest cottage, local zoning laws would definitely apply. I've read that in many places ADUs have a 12x12 maximum footprint, or even smaller. One of my outbuildings still has the address of previous owners on a plank above the door, because that qualifies it as an ice-fishing shanty, which is permissible on Vermont residential properties. We use it as a shed, and we use its twin as my husband's man cave.

On a separate but related note, one of my favorite books is Twelve-by-twelve: A One-Room Cabin Off the Grid & Beyond the American Dream--a true story about a tax-resisting doctor who moved into a 12x12 house. Great book by William Powers.