Great news of legal ICC definition of "Tiny Houses" From
Tiny House Build - In 2017, Andrew Morrison and Martin Hammer wrote International Residential Code (IRC) Appendix Q: Tiny Houses, and after intense vetting and a three stage voting process for which we had an extraordinary team supporting the effort Macy Miller, David Latimer, Jeremy Weaver, Meg Stephens, David Eisenberg, James Herndon, Zachariah Giffin, Beth Ann Norrgard) it was approved by the International Code Council (ICC). After months of effort, advocacy work, and overcoming a lot of obstacles, we are VERY pleased to reveal the official definition of a tiny house:

TINY HOUSE. A DWELLING THAT IS 400 SQFT (37 SQ M) OR LESS IN FLOOR AREA EXCLUDING LOFTS.

https://tinyhousebuild.com/what-are-tiny-houses/