Straight answers
Can you put a prefab ADU in California?
Yes, and California is unusually specific about the minimum your city has to allow. Since 1 January 2026 a local agency must ministerially approve any combination of a converted accessory dwelling unit, a junior accessory dwelling unit and a detached new-construction unit on a lot with an existing or proposed single-family home. It cannot impose a height limit below 16 feet on a detached unit, or below 18 feet where the lot is within half a mile walking distance of a major transit stop, and cannot prohibit a unit of at least 800 square feet of interior livable space with four-foot side and rear setbacks. The prefab half of the question runs on a second rulebook: a factory-built home sold here carries an HCD insignia of approval, placed on the unit at the plant before it ships.
Figures verified against the sources below · September 10, 2026
01 — The longer answer
The two rulebooks a prefab ADU has to satisfy
What the state protects, and what stays local
State law sets a floor, not a ceiling. Government Code 66323, as amended by SB 543 with effect from 1 January 2026, makes the agency approve any combination of the units it lists without a discretionary hearing: one accessory dwelling unit and one junior accessory dwelling unit created inside a new or existing home, where the accessory unit may instead convert an existing structure such as a garage, and one detached new-construction unit with four-foot side and rear setbacks, which the agency may cap at 800 square feet of livable space. Government Code 66321 stops a local agency from imposing a height limit below 16 feet on a detached unit, or below 18 feet where the lot sits within half a mile walking distance of a major transit stop or a high-quality transit corridor, with a further two feet allowed so the roof pitch can match the main house. The same section stops the agency from prohibiting a unit of at least 800 square feet of interior livable space with four-foot side and rear yard setbacks. Above that floor the local ordinance governs, and where a city adopts one under Government Code 66314 the total floor area of a detached unit may not exceed 1,200 square feet. Front setbacks and design standards stay local throughout.
The HCD insignia is the part prefab buyers miss
This is what decides whether a factory-built unit can lawfully be sold to you here. Under Health and Safety Code 19980 a factory-built housing unit carries an insignia of approval, and the Department of Housing and Community Development issues it only through an HCD-approved quality assurance agency, only for a unit that already has plan approval under Title 25 of the California Code of Regulations, and it goes on the unit before shipment. None of that is retrofittable. A local inspector cannot add it later and a buyer cannot apply for it. So before a deposit changes hands, ask the manufacturer, ours included, to show you the HCD plan approval covering the unit you are buying. The other permanent route has no insignia in it at all, because it is a site-built accessory dwelling unit under the California Residential Code.
Prefab ADUs in Southern California and everywhere else
The statute is statewide, so a prefab ADU in Los Angeles, San Diego or the Inland Empire starts from the same state floor as one in Sacramento. What differs is everything layered on top. Seismic demand governs lateral design across the developed parts of the state, typically Seismic Design Category D and rising to E near major active faults, and wildland-urban interface rules under Chapter 7A of the California Building Code drive exterior assemblies in fire-prone areas. A handful of cities also run their own movable tiny house ordinances, Fresno, San Diego and Los Angeles among them, which is a separate track from the ADU one. Our California guide carries the code in force with the source each figure was checked against.
A home on wheels is not an ADU here
A tiny house on wheels registers through the DMV as a park trailer or a travel trailer, which makes it a vehicle rather than real property. That is an occupancy and financing distinction, not a quality one. The vehicle route does not produce a dwelling permit, and outside the cities with a specific movable tiny house ordinance it does not give you a permanent primary residence. If the goal is a backyard home that appraises and finances as part of the property, the accessory dwelling unit route is the one to plan for from the start.
One date worth carrying: the 2025 edition of Title 24, California's building standards code, was published on 1 July 2025 and took effect on 1 January 2026, built on the 2024 model codes. Guidance written before that is worth re-checking at the source rather than relying on, which is why every figure above is linked to the statute or the agency it came from.
03 — Sources
Each figure above links to the authority it came from.
California Government Code 66321: a local agency may not impose a height limit below 16 feet for a detached accessory dwelling unit on a lot with an existing or proposed single-family or multifamily dwelling, or below 18 feet where the lot is within one-half of one mile walking distance of a major transit stop or high-quality transit corridor, plus an additional two feet to align roof pitch with the primary dwelling; and may not prohibit a unit of at least 800 square feet of interior livable space with four-foot side and rear yard setbacks
leginfo.legislature.ca.govCalifornia Government Code 66323(a), as amended by Stats. 2025, Ch. 520 (SB 543), effective 1 January 2026: a local agency shall ministerially approve a building permit to create any of the listed units or any combination of them: one accessory dwelling unit and one junior accessory dwelling unit within the space of a proposed or existing single-family dwelling or an existing accessory structure, and one detached new-construction unit with four-foot side and rear yard setbacks, which the agency may limit to 800 square feet of livable space
leginfo.legislature.ca.govSB 543 (2025), Chapter 520, approved 10 October 2025: the Legislative Counsel's Digest states the bill requires a local agency to ministerially approve a building permit application for a combination of the specified accessory dwelling units or junior accessory dwelling units
leginfo.legislature.ca.govCalifornia Government Code 66314: where a local agency adopts an accessory dwelling unit ordinance, the total floor area of a detached accessory dwelling unit shall not exceed 1,200 square feet
leginfo.legislature.ca.govCalifornia Department of Housing and Community Development, Factory-Built Housing Insignia of Approval: issued only through an HCD-approved factory-built housing quality assurance agency, only for units with prior plan approval under California Code of Regulations Title 25 Chapter 3 Subchapter 1, and placed on the unit or component prior to shipment (Health and Safety Code 19980)
hcd.ca.govCalifornia Department of General Services, Building Standards Commission: the 2025 edition of Title 24 was published 1 July 2025 and is effective 1 January 2026, incorporating the 2024 ICC model codes, the 2024 IAPMO mechanical and plumbing codes and the 2023 National Electrical Code
dgs.ca.govCalifornia DMV Vehicle Industry Registration Procedures Manual: park trailers and travel trailers are registered as vehicles
dmv.ca.gov
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