The new Oregon Reach Code is a voluntary set of standards with a section dedicated to tiny houses. These are can be utilized in place of state building codes, by a DIYer or pro-builder. It requires building officials to accept structures built to it.
The Reach Code includes an option for following the 2018 IRC Tiny House Appendix Q, for tiny houses built on a foundation, which allows for lower ceiling height limits and sleeping loft access by ladder. It also includes a section for movable tiny houses, aka tiny houses on wheels, allowing for temporary RV-type utility connections.
Perhaps the best bit of the new code is this: local building officials have the authority to waive or reduce some of the requirements, when deemed reasonable and when there is no imminent threat to public safety.