These do not make sense if you are looming for similar build quality to match your home. If you have been inside of a pre manufactured home (a “mobile home”) then you’ll know how everything has to be made of oddly hollowed plastic in order to keep shipping weight down.

The frame is the cheapest part of a permanent structure, and a 300-500sqft ADU is not going to have much in the way of plumbing, electrical, and hvac. Paying a plumber, electrician, and hvac tech for new construction on a tiny building is not going to represent a significant chunk of your budget for what amounts to 1-3 days work maximum.