California ADU Law Guide · 2026–2027
Can a manufactured home be an ADU?
In California, yes.
A plain-English guide to the state and Southern California rules for manufactured homes, modular homes, park models and tiny homes on wheels, including the new laws that take effect January 1, 2027.
The statewide rule
California law expressly includes qualifying manufactured homes in the definition of an accessory dwelling unit. HCD confirms this in the March 2026 ADU Handbook, printed page 30.1, 3, 4
Read this first
Executive overview
California permits several kinds of factory-built and site-built ADUs, but “manufactured,” “modular,” “park model” and “tiny home on wheels” are not interchangeable legal categories. This guide separates statewide entitlements from optional local programs.
Yes, statewide
HUD-code manufactured home
Health & Safety Code § 18007; federal manufactured housing rules
An ADU if the site and installation qualify under state ADU law.
Yes
California factory-built / modular
Health & Safety Code § 19971 and applicable California Building Standards
An ADU when approved as a permanent, code-compliant dwelling.
Local pathway only
Park model RV / tiny home on wheels
RV/park-trailer standards; often ANSI A119.5 or NFPA 1192
Not automatic statewide. The city or county must offer an applicable local pathway.
What changes January 1, 2027
AB 956 expands the state-required detached-ADU allowance to two on qualifying single-family properties. SB 1117 changes qualifying impact-fee calculations above 750 sq. ft. AB 1621 constrains repetitive housing plan checks, with exceptions and denial rights.6, 7, 8
Key takeaways
- A city cannot refuse a qualifying HUD-code manufactured ADU merely because it is manufactured. Building, installation, fire, utility and site approvals still apply.
- Local movable tiny-house permissions are not a blanket exemption from ADU limits, zoning, access, fire protection or wastewater rules.
- A City of San Diego movable tiny house can be allowed in addition to locally allowed ADUs and JADUs under Municipal Code § 141.0318(b), subject to that section.
- Land inside a city is not governed by the adjacent county's unincorporated zoning rules. Verify the jurisdiction before choosing a unit.
1 · State law, current through October 2026
California ADU rules summarized
The March 2026 HCD Handbook describes statewide ADU requirements in Government Code §§ 66310–66342. State rules generally override conflicting local ADU restrictions, subject to statutory exceptions and project-specific health and safety conditions.1
Permits
The agency generally must give a written completeness determination within 15 business days and approve or deny a completed qualifying application within 60 days (with specified exceptions/tolling).
Lot coverage & size
Local standards generally cannot prevent at least 800 sq. ft. of eligible ADU interior space with four-foot side/rear setbacks. For locally limited ADUs, minimum size caps are generally 850 sq. ft., or 1,000 sq. ft. with more than one bedroom.
Setbacks
For eligible new detached/attached ADUs, no more than four feet at rear and side lines under applicable provisions. Existing conversion and fire-safety exceptions need separate review.
Parking
No new parking for many qualifying ADUs, including specified transit locations and conversions; garage-conversion parking usually need not be replaced.
Sprinklers
Generally not required for an ADU when not required for the primary residence. Movable-home local fire ordinances may add requirements.
Impact fees in 2026
ADUs of 750 sq. ft. or less are exempt from covered impact fees. Larger units follow the pre-2027 proportional framework. Plan-check, permit, water/sewer capacity and other valid charges still apply.
Renting & owners
State law generally restricts rentals under 30 days and bars local owner-occupancy requirements for standard ADUs. Special JADU rules and separate-sale exceptions apply.
Existing multifamily
Up to eight new detached ADUs, not exceeding the existing unit count, plus qualifying conversions (at least one and up to 25% of existing units).
Important: buildable area, fire access, coastal rules, easements, septic sizing, underground utilities and site-specific code requirements may constrain the approved project. HCD ADU Handbook, March 2026, pp. 16–17, 21–23, 30–39.1
2 · Primary legal authority
Manufactured homes are allowed as ADUs
This is not an inference or a local exception. It is expressly stated in California statute and confirmed by the state housing regulator.
Official HCD Handbook · printed page 30
“Can a manufactured home be used as an ADU?”
Yes. HCD identifies Government Code § 66313(a)(2) and Health & Safety Code § 18007 as the controlling definitions.1, 3, 4
What qualifies as a manufactured home?
Health & Safety Code § 18007 describes HUD-code manufactured housing: transportable in sections, built on or after June 15, 1976 to the federal manufactured housing program, and designed as a dwelling when connected to utilities. In 2026, SB 996 amended the state definition and directed HCD to adopt regulations by July 1, 2027 for specified chassis-free manufactured homes; that does not make ordinary park trailers into statewide ADUs.4, 9
Why this matters for Riverside County
In a May 14, 2025 ordinance-review letter, HCD found Riverside County's ADU definition incomplete because it did not expressly include manufactured homes, and instructed the county to amend it. The same letter distinguished post-1976 manufactured homes from pre-1976 mobilehomes.12
For homeowners and plan checkers
The right question is whether a particular manufactured home and its installation meet state law and the applicable objective site, utility, anchoring, fire and safety requirements, not whether factory-built housing is excluded as an ADU category.
HUD-code, modular and park-model differences
| Category | Typical compliance framework | Key permitting point |
|---|---|---|
| HUD manufactured ADU | HUD manufactured-home certification; California Title 25 installation requirements | A statewide ADU housing type, if qualified. |
| Modular / factory-built ADU | California factory-built housing program; applicable Title 24 building standards | Different construction approval track than a HUD-code home. |
| Park model / THOW | RV / park model certification (ANSI A119.5 or NFPA 1192) plus local conditions | Local entitlement must be confirmed; not automatic under Gov. Code § 66313(a)(2). |
Federal HUD label, HCD registration/titling, site installation permits and, where applicable, a foundation/real-property recording must be addressed on each project. See HCD and California Energy Commission guidance.1, 31
3 · Upcoming state law
New 2027 laws and project impact
AB 956 Ch. 791 (2026)
Beginning January 1, 2027, requires ministerial allowance for two new detached ADUs on qualifying single-family lots; each qualifying state-exempt unit may be capped at 800 sq. ft. A JADU need not also be authorized if the full two-detached allowance is used.6
SB 1117 Ch. 809 (2026)
From January 1, 2027, where the lot has no more than two ADUs and statutory conditions are met, covered impact fees on an ADU over 750 sq. ft. are based on the portion above 750 sq. ft., not the entire ADU area.7
AB 1621 2026 enactment
Generally limits housing building-permit plan-check review to two rounds; specified additional reviews may be justified and deficient applications may still be denied. It does not require approval of noncompliant plans.8
SB 996 Ch. 995 (2026)
Updates manufactured-housing conformity and directs HCD regulations for qualifying chassis-free manufactured homes by July 1, 2027. New product types need regulatory verification before sale/installation.9
AB 1815 / AB 2058 2026 enactments
Reform aspects of California factory-built housing standards, local conditions and installation inspections, including future third-party inspection pathways. Not blanket changes to the federal HUD program.10, 11
800−750=50 sq. ft.
Impact fee example · 800 sq. ft. ADU
Under the 2027 SB 1117 rules, an 800 sq. ft. ADU on a qualifying lot uses 50 sq. ft. as the area in the modified impact-fee proportionality calculation. That does not mean all project or utility fees fall to zero.7
4 · Density rights
How many dwellings can be added?
| Property situation | ADU potential | Important qualifications |
|---|---|---|
| Single-family parcel before 2027 | A qualifying new detached ADU; potentially a conversion ADU and/or JADU | Combination restrictions, siting and JADU eligibility apply. |
| Single-family parcel beginning 2027 | Two new detached ADUs under AB 956 | State-exempt units can be limited to 800 sq. ft.; a JADU on top is not assured. |
| Existing duplex | Up to 2 detached ADUs plus qualifying non-livable-space conversion | Existing dwelling count caps new detached multifamily ADUs. |
| Existing 8-unit apartment site | Up to 8 detached ADUs plus qualifying building conversions | Detached count must not exceed the existing eight units; site standards and codes apply. |
| Proposed multifamily development | Up to 2 qualifying detached ADUs | Other state and local eligibility rules determine the total project. |
| Eligible San Diego bonus project | Potentially up to the local ADU bonus cap | Requires deed-restricted affordable units, zoning, access, physical caps. |
State-exempt ADUs generally do not count against a lot's underlying residential density. Some city bonus programs allow more units than the state requires; SB 9 urban lot splits and two-unit projects are a separate entitlement with their own rules.1, 5
San Diego's ADU Home Density Bonus (city limits only)
4ADUs/JADUs on lots up to 8,000 sq. ft.
5on lots 8,001–10,000 sq. ft.
6on lots over 10,000 sq. ft.
2026 program maximums for single-family zones, not guaranteed approvals or automatically market-rate units. Additional market-rate ADUs are tied to eligible deed-restricted affordable ADUs and site criteria; all ADUs allowed under state law count toward the cap. Verify Coastal Zone implementation separately.16, 17
5 · Movable homes
Park models and THOWs: where are they permitted?
Park model RVs and tiny homes on wheels (THOWs) generally are not HUD-code manufactured homes under § 18007. They are usually certified to RV/park-trailer standards, so owners must rely on a specific local authority. The word “ADU” in a local handout does not make the unit a manufactured home under state law.
Confirmed, adopted13, 14
City of Los Angeles
Movable tiny house as ADU under LAMC § 12.22 A.33(f); 150–430 sq. ft. with ANSI/NFPA certification.
Confirmed, adopted15
City of San Diego
One movable tiny house as a limited use under SDMC § 141.0318, in addition to qualifying ADUs/JADUs.
Confirmed, permit program18, 19
Unincorporated San Diego County
2025 PDS director interpretation permits a THOW as a primary or accessory dwelling when county standards are met; no added density.
Confirmed city program20
City of Riverside
Movable Accessory Dwelling Units (MADUs), 150–430 sq. ft.; certain zones and districts excluded.
Local temporary pathway23
City of San Luis Obispo
Limited-duration THOW program (3–5 years, possible renewal), not equivalent to a permanent ADU entitlement.
Parcel-specific verification1
San Bernardino, Orange & Ventura Counties
The statewide manufactured-home ADU right applies; no general countywide THOW-as-ADU pathway verified.
Do not assume permitted12
Unincorporated Riverside County
HCD requires recognition of qualifying HUD manufactured ADUs; a general park-model/THOW ADU entitlement is not verified.
Do not assume permitted21
Unincorporated Los Angeles County
County zoning enforcement generally prohibits occupied RVs/trailers on ordinary residential parcels, with specified exceptions.
Elsewhere in California: Fresno allows eligible THOWs as ADUs with certification, engineered foundation and skirting; San Jose has a special THOW ordinance; Oakland recognizes vehicular residential facilities. Santa Cruz County currently cautions that THOWs may not be processed as ADUs under HCD guidance. These are outside USModular's principal Southern California service area.24–27
6–7 · Southern California local code
City and county rules up close
LAMC § 12.22 A.33
City of Los Angeles
- Allowed use: movable tiny houses are a locally permissible form of ADU; an existing or proposed dwelling on the parcel is required.
- Product: generally 150–430 sq. ft., DMV registered, non-self-propelled, ANSI A119.5 or NFPA 1192 certified, with third-party inspection documentation.
- Site: generally one movable tiny house per lot, undercarriage screened, permitted water/sewer/electric hookups, local appearance, setback and access standards.
- Caution: proposed 2026 “Missing Middle LA” updates are not rules in force. Check the governing code as of your application date.
LAMC § 12.22 A.33(f); LA Building & Safety movable-house checklist.13, 14
SDMC § 141.0318
City of San Diego
- Additional unit: § 141.0318(b) permits one movable tiny house in addition to otherwise permitted ADUs and JADUs, where zoning lists it as a permitted limited use.
- Location: adjacent right-of-way generally at least 20 ft. wide; no more than 150 ft. from the road with a 5-ft. access path; behind or beside the primary dwelling.
- Separation: at least 5 ft. from lot lines and 10 ft. from other structures; not in a brush-management zone or Multiple Habitat Planning Area.
- Certification: DMV registration and ANSI A119.5 or NFPA 1192; water/sewer/electric hookups; no natural gas; wheels and chassis screened; fire requirements in higher hazard areas.
- Rental: no rental period shorter than 30 consecutive days.
SDMC § 141.0318, September 2026 code publication. Information Bulletin 403 (2020) may be older than the current code; follow the code if they differ.15
PDS determination · Sept. 4, 2025
Unincorporated San Diego County
Compliant THOWs may qualify as a primary or accessory dwelling under the county's zoning interpretation. The determination expressly does not increase allowable density or the number of units on a parcel.18, 19
- Under 430 sq. ft., one story with an optional loft up to 150 sq. ft., at most 16 ft. high.
- ANSI A119.5 plus county safety standards, certified by an approved agency; noncombustible exterior, roof, fire-safety, utility and inspection conditions apply.
- Permits, plans, sewer/septic capacity, grading, stormwater, road access, fire district review and final occupancy still required.
- Residential THOWs are distinct from recreational park trailers in RV/mobilehome parks, which may be limited to recreational or seasonal use.
ADU / JADU / MADU FAQ
City of Riverside
A movable ADU (MADU) pathway for qualifying single-family parcels: transportable units on a chassis, DMV licensed/registered, movable on roads but not self-powered.20
- Size: 150–430 sq. ft. by exterior wall measurement.
- Place: on a lot with an existing or proposed primary dwelling, not between the house and the street.
- Excluded: multifamily zones, historic districts, neighborhood conservation areas and designated cultural resources.
- Appearance: window/roof/siding standards, screened wheels and undercarriage, no separate street address.
Not the same jurisdiction
The City of Riverside MADU program does not govern unincorporated Riverside County or other cities in the county. Likewise, the City of San Diego's additional-THOW rule does not automatically apply in unincorporated county areas.
8 · Regional opportunities
Local density and manufacturing opportunities
| Area | Locally relevant opportunity | What it means for your project |
|---|---|---|
| City of San Diego | ADU Home Density Bonus: up to 4 / 5 / 6 ADU/JADU units on qualifying single-family parcels by lot size; affordable deed restriction and other conditions. | Consider paired market-rate and income-restricted manufactured ADU packages.16, 17 |
| Unincorporated Ventura County | A non-state-exempt local pathway to certain larger ADUs, up to 1,800 sq. ft. under specified standards. | Larger HUD-code multi-section ADUs may warrant a feasibility review.28 |
| Unincorporated Riverside County | Additional Residential Accommodations ordinance recognizes several forms of supplementary housing. | Don't combine categories without a county zoning analysis.12 |
| San Bernardino County | The county's 2026 manufactured housing guide acknowledges manufactured homes as ADUs on eligible lots. | Point to the HCD handbook rule in consultations.22 |
| City of Los Angeles | Movable tiny house ordinance exists; proposed “Missing Middle LA” additions were not adopted as of Oct. 24, 2026. | Separate current movable-ADU rules from proposed changes.13, 29 |
| SB 9 sites statewide | Some qualifying two-unit projects or urban lot splits can be paired with ADU rules within statutory unit limits. | Check primary-unit approval, lot-split restrictions, separate utilities and resale planning.1 |
What this means for prefab ADUs
The 2027 rules can change the most efficient product mix. On many single-family lots, a pair of right-sized detached manufactured ADUs may be feasible; on larger sites, local bonuses or multifamily conversions can support an entirely different project. The right choice follows zoning, setbacks, installation method, flood/fire conditions, access, utilities and financing, not a blanket promise about unit counts.
9 · Permitting playbook
Pre-permit checklist for California ADUs
Identify the agency
Confirm parcel address, APN, city vs. unincorporated county, base zone, overlay districts and coastal/historic/hazard restrictions.
Determine the unit type
HUD-manufactured, California factory-built/modular, site-built, or a locally allowed movable tiny house/park model. Get the exact certification and dimensions.
Calculate the lawful density
Count primary units, conversion ADUs, new detached ADUs, JADUs and any affordable bonus; flag the January 1, 2027 AB 956 change.
Validate buildable space
Check 4-ft. state ADU setbacks where applicable, fire separations, drainage, slopes, easements, road access and utility routes.
Screen utilities
Confirm sewer or septic capacity, connection rules, fire flow, electrical service, water meter, stormwater and potential upgrades.
Budget honestly
Separate the home price from transport, crane, foundation, engineering, plan check, installation, utility trenching, taxes and impact fees.
Submit the correct package
HUD labels/manufacturer certifications or local ANSI/NFPA third-party documents, plot plan, installation/foundation plans, energy and fire materials.
Require written correction citations
If an agency refuses a qualifying manufactured ADU solely on construction type, ask for the exact code basis and cite the HCD Handbook and Gov. Code § 66313(a)(2).
Apply 2027 provisions on the right date
A signed law and an operative law are different. Verify dates, local implementation and phased regulations before relying on a specific entitlement.
A useful reference for permit discussions
Beyond the HCD Handbook, HCD's Riverside County ordinance-review letter expressly directs inclusion of manufactured homes as ADUs. It's a useful example when a plan-check response confuses HUD-code manufactured homes with park trailers or pre-1976 mobilehomes.12
Let us handle the rules for you
MyADU4Less, a division of USModular, Inc., handles home selection, ADU planning, design, permitting, site preparation, foundation, utilities, delivery, installation and occupancy. Tell us your address and we'll check what your lot allows.
10 · Sources
Statutes, agencies and local codes
Numbered references in this guide link to these official sources.
- California HCD: ADU Handbook, updated March 2026
- California HCD: ADU regulations and handbook landing page
- Government Code 66313: ADU definitions
- Health and Safety Code 18007: manufactured homes
- Government Code 66323: exempt ADU pathways
- AB 956 (2026), Ch. 791: two detached ADUs in 2027
- SB 1117 (2026), Ch. 809: proportional impact fees in 2027
- AB 1621 (2026): post-entitlement plan-check reviews
- SB 996 (2026), Ch. 995: manufactured housing conformity
- AB 1815 (2026): California factory-built housing standards
- AB 2058 (2026), Ch. 783: factory-built housing installation inspection
- HCD May 14, 2025 letter to Riverside County: manufactured homes as ADUs
- Los Angeles Municipal Code 12.22 A.33: movable tiny houses
- LA Department of Building and Safety: MTH requirements
- San Diego Municipal Code 141.0318: movable tiny houses
- City of San Diego Information Bulletin 400: ADUs and bonus program
- City of San Diego regulatory updates: ADU bonus caps
- San Diego County: tiny homes on wheels permit page
- San Diego County PDS director determination, Sept. 4, 2025
- City of Riverside: FAQ on ADUs, JADUs, MADUs
- LA County zoning enforcement: occupied RVs/trailers
- San Bernardino County: 2026 manufactured-home quick reference
- San Luis Obispo: tiny homes on wheels guidance
- Fresno Municipal Code 15-2754: THOW as ADU with conditions
- San Jose ADU and THOW guidance
- Oakland: RVs and THOWs as vehicular residential facilities
- Santa Cruz County: THOW status advisory
- Ventura County ADU ordinance and expanded size allowances
- City of Los Angeles: Missing Middle LA proposal (not adopted Oct. 2026)
- City of San Diego ADU density bonus information
- CA Energy Commission: factory-built vs. manufactured energy requirements
- USModular, Inc.
Legal notice. This is a general educational summary of publicly available law and agency guidance, not legal, design, engineering or entitlement advice. Rules change and agency web pages can lag new legislation. No guide can establish site-specific ADU eligibility without a parcel and plan review. Local ordinances may be more permissive than state minimums but may not unlawfully restrict state-guaranteed ADUs. Verify current adopted law and permitting standards with the agency having jurisdiction. This guide is not issued or endorsed by HCD or any city or county.
GC Lic. No. 943047 | CA HCD Dealer Lic. No. DL1241679
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Full-service design & build firm for prefabricated ADUs (Granny Flats) throughout Southern California.