It’s a measure of how thoroughly I had purged Tiny Houses from my mind back in 2015 that I missed this milestone back in 2016 and accidentally (via YouTube Recommendation) learned of it in 2018:
2018 International Residential Code for One- and Two-Family Dwellings: Tiny House Appendix Q
The first link above discusses the hurdles still to come, such as zoning, every locality adopting the code, and Tiny Houses that are on wheels (this Appendix is for non-wheeled ones).
But this is a significant step towards making the environment favorable for Tiny Houses.
This video also contains excerpts of the International Code Council proceedings about Appendix Q:
There is no part three of this series — yet.