Orange County

Unincorporated Orange County ADU rules

Research summary of local ADU rules on file for Unincorporated Orange County in Orange County. Not a determination for a specific lot.

Last checked October 19, 2026.

City or county ADU page

Unincorporated Orange County ADU page

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Unincorporated Orange County rules on file

Local rules recorded in this project for this jurisdiction. Where a city figure on file is stricter than the state baseline on file, both are shown.

County programs
The County page offers “pre-approved” ADU plans (apply as "Accessory Dwelling Unit (Standard)") and an AB 2533 program to legalize ADUs and JADUs. County rules are in Zoning Code section 7-9-90.
Source: OC Development Services ADU page, Orange County Zoning Code section 7-9-90, accessed 2026-10-18.

Fire hazard zones in the County ADU section
Zoning Code section 7-9-90 does not mention Fire Hazard Severity Zones and sets no fire-zone ban, road-width rule, or extra parking for ADUs. Its only fire wording is subsection (c)(4): “All side and rear setbacks shall be sufficient for fire and safety.”
Source: Orange County Zoning Code (County PDF, text dated November 4, 2025), Orange County Zoning Code section 7-9-90(c)(4), accessed 2026-10-19.

Parking in setbacks
The County's ADU flyer (revised January 2026) says parking may be placed in setback areas “unless it is not feasible based upon topography, or fire and life safety.”
Source: OC Development Services ADU flyer (revised January 2026), Parking, accessed 2026-10-19.

County fire hazard map and construction
On August 26, 2025 the Board of Supervisors adopted Ordinance No. 25-015, which repealed Ordinance No. 12-001 and designated the Local Responsibility Area Fire Hazard Severity Zones from the State Fire Marshal's March 24, 2025 map. The County page says buildings in Very High zones must use the fire-resistive features in California Building Code Chapter 7A and/or California Residential Code section R327. The ordinance's Exhibit A map, linked from that page, also shows State Responsibility Area zones mapped by CAL FIRE, effective April 1, 2024.
Source: OC Development Services, Orange County Fire Hazard Severity Zones Map page, Ordinance No. 25-015 (adopted August 26, 2025), accessed 2026-10-19.

Orange County Fire Authority review
OCFA lists when an ADU plan goes to OCFA. A Fuel Modification plan (PR125) and Accessory Structure plan (PR182) are needed when the property is “in a State or Local Responsibility Area (SRA/LRA) Fire Hazard Severity Zone (FHSZ) as defined by the State,” or is next to slopes, open space, or wildland vegetation. A Residential Site Plan (PR160) is needed when the farthest part of the ADU is more than 140 ft from the fire access roadway along the walking path. Sprinkler plans are needed for an ADU on a property whose house already has, or will get, fire sprinklers.
Source: OCFA Planning & Development Services page, When is a plan review required?, accessed 2026-10-19.

Fire hazard zone for this lot
Not confirmed — check with OC Development Services and OCFA. This summary does not look up whether this lot is in a Fire Hazard Severity Zone or the State Responsibility Area. CAL FIRE's FAQ says the Chapter 7A wildfire building codes for roofing, siding, decking, windows, and vents apply “throughout all state responsibility area regardless of the fire hazard severity ranking.”
Source: CAL FIRE / OSFM, 2024 Fire Hazard Severity Zone FAQs (December 18, 2024), What are the wildland urban interface (WUI) building codes in State Responsibility Area?, accessed 2026-10-19.

California state rules on file

The statewide baseline recorded from state law and the HCD ADU Handbook.

Detached size cap described in state law
1,200 sq ft. Recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Side and rear setback cap for a new ADU
4 ft. Recorded in this rules file from Government Code section 66314, which describes a cap on side and rear setbacks for a new ADU.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Setback for a conversion of an existing structure
No additional setback, as recorded in this rules file from Government Code section 66314 for a conversion of an existing structure.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum lot size
A city may not impose a minimum lot size for an ADU, as recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum unit the handbook says cities must allow
At least an 800 sq ft ADU with 4 ft side and rear setbacks. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(3).
Source: HCD ADU Handbook, 66321(b)(3); handbook updated March 2026, accessed 2026-10-16.

Detached height described in the handbook
16 ft, or 18 ft or 20 ft in some cases. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(4).
Source: HCD ADU Handbook, 66321(b)(4); handbook updated March 2026, accessed 2026-10-16.

Permit decision deadline
60 days. Recorded in this rules file from Government Code section 66317(a)(3): the permitting agency must decide on (approve or deny) a completed ADU application within 60 days when an existing single-family or multifamily dwelling is on the lot. Section 66317(a)(1) says the application is considered ministerially, without discretionary review or a hearing.
Source: California Government Code section 66317, 66317(a)(1), (a)(3), accessed 2026-10-17.

Owner-occupancy requirement
None. Recorded in this rules file from Government Code section 66315: a local agency may not impose an owner-occupancy requirement on an ADU. It may require that the property be used for rentals of terms 30 days or longer. The HCD ADU Handbook (updated March 2026) describes the same rule.
Source: California Government Code section 66315, 66315; HCD ADU Handbook updated March 2026, accessed 2026-10-17.

Parking near public transit
No parking standards within one-half mile walking distance of public transit. Recorded in this rules file from Government Code section 66322(a)(1). Section 66322(a) lists several other cases where parking standards may not be imposed.
Source: California Government Code section 66322, 66322(a), accessed 2026-10-17.

Ministerial categories
The HCD ADU Handbook (updated March 2026) states that Government Code section 66323 sets four ministerial categories. This rules file does not restate those four categories.
Source: HCD ADU Handbook, 66323; handbook updated March 2026, accessed 2026-10-16.

Fire Hazard Severity Zones
If your lot is in a Fire Hazard Severity Zone, the CAL FIRE Office of the State Fire Marshal says in Information Bulletin 25-004 (issued March 3, 2025): “Fire Hazard Severity Zones (FHSZs) should not restrict the development of Accessory Dwelling Units (ADUs). However, additional building standards may be necessary, including but not limited to California Fire Code Chapter 49 and California Building Code Chapter 7A.” State ADU law does not itself bar an ADU there. A city or county may still apply its building code, including wildfire-resistant construction (Gov. Code 66314(d)(8)); require side and rear setbacks “sufficient for fire and safety” for a unit converted under 66323(a)(1); and decline parking in setbacks on specific fire and life safety findings (66314(d)(10)(B)). It may also consider public safety when it designates where ADUs may be permitted (66314(a)). Sprinklers can't be required in an ADU or JADU if the primary residence doesn't need them (66314(d)(12), 66323(d)). Some cities and counties still apply their own fire-zone limits. Where this summary has one, it is listed with the local rules. This summary does not look up your lot. Check the CAL FIRE Fire Hazard Severity Zone map viewer linked from the State Fire Marshal's Fire Hazard Severity Zones page: https://experience.arcgis.com/experience/5065c998b4b0462f9ec3c6c226c610a9
Source: CAL FIRE Office of the State Fire Marshal Information Bulletin 25-004, page 4, issued March 3, 2025; Gov. Code 66314, 66323, accessed 2026-10-20.

State baseline on the directory → · State ADU rules on the homepage →

Can I build an ADU in Unincorporated Orange County?

State law sets a baseline for ADUs in Unincorporated Orange County, and the local standards on file are listed on this page. Whether a specific lot can add an ADU depends on zoning, utilities, and other site facts this page does not measure. Confirm with OC Development Services, or run a free address summary.

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