Tiny home on wheelsPortland specifically legalized a 'tiny home on wheels' as one of several housing types in its 2021 Residential Infill Project, which implemented the state's middle-housing law — one of the more concrete THOW legalizations of any major U.S. city. Other cities vary and wheeled units generally still sit outside standard residential occupancy rules elsewhere in the state.
ADU / accessory unitOregon's ADU mandate is Senate Bill 1051 (2017), codified at ORS 197A.425, requiring every city over 2,500 population to allow at least one ADU per detached single-family lot. This correction: an earlier draft attributed this mandate to House Bill 2001 — HB 2001 (2019) is a separate, related law covering 'middle housing' (duplexes through fourplexes, cottage clusters, townhomes) in cities over 10,000. The two laws work together to make Oregon one of the strongest infill-housing states, but they are different statutes with different thresholds.
Minimum square footageNone at state level
Foundation requirementFoundation-built ADUs follow standard code; several cities, especially Portland, offer pre-approved ADU plan sets to speed permitting