ADU Rules: Altadena vs Downey
How do adu rules rules compare between Altadena, CA and Downey, CA?
Altadena and Downey have similar restriction levels.
Altadena, CA
Los Angeles County
Altadena is an unincorporated CDP in Los Angeles County governed by LA County Code Title 22 §22.140.640 (Accessory Dwelling Units and Junior Accessory Dwelling Units), most recently amended April 2024. Single-family lots may add one ADU plus one JADU; multifamily lots may add up to two detached new-construction ADUs plus conversion ADUs in non-livable existing space.
View full Altadena rules →Downey, CA
Los Angeles County
Downey regulates ADUs under DMC §9414.08. Detached ADUs up to 1,200 sq ft allowed. Max 2 bedrooms. Height: 16 ft detached, 25 ft attached. 4-foot setbacks. Pre-approved ADU plans available through the city.
View full Downey rules →Key Facts Comparison
| Fact | Altadena | Downey |
|---|---|---|
| Governing Code | LA County Code §22.140.640 | - |
| Single-Family | 1 ADU + 1 JADU per lot | - |
| Multifamily Detached | Up to 2 new (20 ft max) | - |
| WUI Fire Zone | Chapter 7A applies (hillside) | - |
| Side/Rear Setback | 4 ft (new construction) | - |
| Max Size | - | 1,200 sq ft (detached) |
| Max Bedrooms | - | 2 |
| Height (Detached) | - | 16 ft |
| Code Section | - | DMC §9414.08 |
Highlighted rows indicate differences between cities.
Altadena FAQ
Who regulates ADUs in Altadena?
Altadena is unincorporated, so LA County Regional Planning regulates ADUs under LA County Code Title 22 §22.140.640, not a separate city ordinance.
Do hillside or fire rules apply to Altadena ADUs?
Yes. Altadena's hillside neighborhoods are in LA County's Very High Fire Hazard Severity Zone, requiring Chapter 7A ignition-resistant construction and Fire Code §325 brush clearance. Hillside-grading provisions of Title 22 apply on slopes steeper than 10%.
How big can my Altadena ADU be?
Under §22.140.640, a detached ADU on a single-family lot may be up to 1,200 sq ft; multifamily-lot ADUs have no maximum size cap. JADUs are capped at 500 sq ft.
Downey FAQ
Can I build an ADU on my property in Downey?
Downey regulates ADUs under DMC §9414.08. Detached ADUs up to 1,200 sq ft allowed. Max 2 bedrooms. Height: 16 ft detached, 25 ft attached. 4-foot setbacks. Pre-approved ADU plans available through the city.
What are the size limits for an accessory dwelling unit in Downey?
Max Size: 1,200 sq ft (detached). Max Bedrooms: 2. Height (Detached): 16 ft. Code Section: DMC §9414.08.
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