Accessory Structures in Altadena, CA: What Residents Actually Need to Know

If you live in Altadena or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. Altadena has 4 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

ADU Rules

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.

Key details: 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).

Unpermitted ADUs in unincorporated Altadena are violations of LA County Code Title 22 enforced by LA County Public Works and Code Enforcement, with stop-work orders, daily-accruing fines, and required permitting. WUI-zone violations carry separate fire-code citations.

Garage Conversions

Garage conversions to ADUs in unincorporated Altadena are governed by LA County Code Title 22 §22.140.640 and California Government Code §65852.2. No replacement parking is required when an existing garage is converted, and existing setbacks are grandfathered. Properties in Very High Fire Hazard Severity Zones also trigger Chapter 7A wildland-urban interface standards.

Key details: Governing Code: LA County Code §22.140.640. Replacement Parking: Not required. Existing Setbacks: Grandfathered. Fire-Zone Construction: Chapter 7A in WUI hillside. Permit Required: Yes — LA County building.

Converting an Altadena garage to habitable space without a building permit is a violation of LA County Code, subject to stop-work orders, fines, and required after-the-fact permits or restoration. Non-Chapter 7A construction in fire-hazard zones may also be cited by LA County Fire.

Carport Rules

Carports in Altadena are accessory structures regulated under Altadena CSD 22.306.070 and LA County Code 22.110.040. They count toward the lot-coverage formula and the 2-space minimum parking requirement.

Key details: Code Section: LA County 22.110.040 + Altadena CSD 22.306.070. Min Setback (R-1): 5 ft side/rear. Height Within 15 ft of PL: ≤23 ft. Counts Toward Lot Coverage: Yes.

Non-compliant carports are zoning violations under Title 22; building permits required for structural carports.

Shed Rules

Sheds 120 sq ft or smaller and ≤12 ft tall typically don't need a building permit in Altadena, but they still must comply with Altadena CSD setbacks. Anything larger needs a permit.

Key details: No-Permit Threshold: ≤120 sq ft. Authority: LA County Title 26 + CBC §105.2. Min Setbacks (R-1): 5 ft side, 5 ft rear. Lot Coverage: 0.25 × lot + 1,000 sf (CSD 22.306.070.A.4).

Sheds over 120 sq ft without a permit are a building/zoning violation — can require retroactive permit, removal, or a Variance.

If you are coming from a city with tighter rules, you will find Altadena gives residents more flexibility on shed rules.

The Bottom Line

Altadena's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Altadena is broadly strict or permissive.

Keep in mind that Altadena can amend these rules at any council meeting. For the most current version of any rule mentioned here, check the specific ordinance page, where we track updates as they happen.