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Permits & Code

ADUs and tiny homes

State law makes accessory dwelling units a ministerial, objective-standards permit. Tiny homes on wheels are generally recreational vehicles, not housing, under HCD rules.

State ADU law (Government Code §66310 and following)

  • ADUs subject to state law must be approved or denied ministerially, without discretionary review or a hearing, using objective standards (§66317(a), §66314(b)(1)).
  • The agency must tell the applicant within 15 business days whether the application is complete, listing anything missing (§66317(a)(2)).
  • It must approve or deny the application within 60 days from receiving a complete application if there is an existing single-family or multifamily dwelling on the lot. If it does not act in time, the application is deemed approved. If the ADU is filed together with a new home, the agency may hold the ADU decision until it decides on the home (§66317(a)(3)).
  • A denial must come with a full set of written comments listing the defects and how to fix them, and the applicant can appeal (§66317(b), (d)).
  • Local limits on lot coverage, floor-area ratio, open space, front setbacks or minimum lot size cannot prevent an ADU of at least 800 sq ft with 4-foot side and rear setbacks (§66321(b)(3)).
  • Detached ADUs get a base height limit of 16 feet, rising to 18 feet within a half-mile walk of a major transit stop or high-quality transit corridor (§66321(b)(4)).
  • An ADU cannot trigger fire sprinklers in the existing main home (§66314(d)(12)).

Energy code for new detached ADUs

New detached ADUs are newly constructed homes under the 2025 Energy Code, including the solar PV rule and its 1.8 kW exemption. See energy code.

Tiny homes on wheels

HCD Information Bulletin 2016-01 explains that a tiny home on a chassis is generally a recreational vehicle or park trailer, not a dwelling under the housing codes. That matters for permits, financing and the energy code, and a 12-inch hempcrete wall (about 6,800–13,600 lb for a 400 sq ft home) is heavy for a trailer. MicroHom plans for permanent foundations.

Sources

  1. California Government Code §66317, as amended by SB 543 (Stats. 2025, ch. 520), effective Jan 1, 2026 (Justia, 2025 California Code). Checked Oct 18, 2026.
  2. HCD, Accessory Dwelling Unit Handbook (March 2026 update). Checked Oct 18, 2026.
  3. HCD, Accessory Dwelling Units fact sheet. Checked Oct 18, 2026.
  4. HCD Information Bulletin 2016-01, tiny homes. Checked Oct 17, 2026.
  5. California Energy Commission, 2025 Energy Code guide for ADUs (Apr 2026). Checked Oct 17, 2026.

Last updated Oct 18, 2026. Each source above was checked on the date shown.