TinyHouseBuild. "What Are Tiny Houses?" November 2, 2017.  https://tinyhousebuild.com/what-are-tiny-houses/

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, 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 square feet (37 sq m) or less in floor area excluding lofts.