California has current building-code provisions for tiny houses and a strong statewide accessory-dwelling-unit framework, but a foundation dwelling, factory-built home, trailer coach, recreational vehicle and park trailer are not interchangeable classifications.
“Tiny house” is a lifestyle description. Regulators care about the legal building or vehicle classification.
The 2025 California Residential Code includes Appendix BB for tiny houses of 400 square feet or less, excluding lofts. Appendix provisions apply only when adopted by the relevant state agency or adopting ordinance, so the local building department still needs to confirm the pathway for a specific project.
California Government Code defines an ADU as an independent residential dwelling on a lot with a proposed or existing primary residence. State law constrains local ADU rules, but the unit still must satisfy the applicable approval and building requirements.
California HCD regulates factory-built housing manufacturing; installation is subject to inspection by the local building department. A factory-built tiny home should not be assumed to follow the same pathway as a vehicle-based THOW.
California separately defines trailer coaches, recreational vehicles and park trailers. A park trailer is for recreational or seasonal use under the state definition, so vehicle registration or ANSI construction does not automatically make it a permanent dwelling on a residential parcel.
This page is research and planning information, not legal advice or a permit determination. Rules can change and local officials determine how they apply to a specific parcel and project.