Key Rules At A Glance
- Maximum size: 1,200 sq ft (detached or attached, or 50% of the size of the main dwelling)
- Side/rear setbacks: 4 feet
- Two story: Yes, for both attached and detached ADUs
- Building separation: 6 feet
- Soils Report: Possibly, especially in areas with grading or on hillsides
- Building Verification Survey: Yes
- Coastal permit (in some areas): N/A
- Stormwater Mitigation: Yes
Number of ADUs Allowed in Altadena
In Altadena you can build one attached ADU, one detached ADU, and one JADU on single-family property.
Multifamily property owners in Altadena may convert at least one ADU, and additional ADUs not to exceed a maximum number equal to 25% of the existing units out of existing uninhabitable space. Ergo:
- From 2 to 7 units, you could convert existing uninhabitable space into one ADU
- From 8 to 11 units up to 2 ADUs
- From 12 to 15 units up to 3 ADUs
- And so on.
No more than two detached ADUs may be constructed on a multifamily lot in Altadena.
ADU Altadena – Size & Building Constraints
No, stacked ADUs (one ADU on each floor of a two-story structure) are not allowed in Altadena.
In Altadena an attached ADU can be up to 1,200 square feet or 50% of the size of the main dwelling unit, whichever is smaller.
In Altadena, detached ADUs are limited in height to 25 feet. An attached ADU can be up to 25 feet or the height of the main dwelling unit, whichever is smaller.
Yes, you can build a two-story ADU in Altadena.
No, stacked ADUs (one ADU on each floor of a two-story structure) are not allowed in Altadena.
No, two ADUs attached together are not allowed in Altadena.
No, a JADU is defined as an ADU converted from existing space within the main structure of the primary dwelling unit. As such, it cannot be attached to an ADU.
Roof decks are not prohibited for ADUs in Altadena. However, California building safety codes about roof deck safety railings apply, and the height of the railing applies to the total height of the building, so the whole structure including the railings must comply with ADU height requirements. California requires that deck rails be 36 inches high on residential dwellings, 42 inches high on multifamily dwellings.
The exterior of the ADU must align with the architectural and aesthetic style of the rest of the neighborhood, using similar materials and design elements to maintain consistency with the main dwelling unit and surrounding dwellings similar to garage conversions.
ADU Setbacks in Altadena
The City of Altadena requires 4-foot rear and side setbacks for new-construction ADUs. ADUs converted from existing structures may be exempt.
The City of Altadena requires a minimum of 6 feet of separation between the new ADU and any other structures. ADUs converted from existing structures may be exempt.
No, ADUs are not allowed in the front yard of residential property in Altadena.
Additional Requirements in Altadena
No, you can also submit plans online to the LA County Department of Regional Planning using the EPIC-LA online submission system.
Altadena may require a soils report for ADU permitting to make sure the soil can support the new structure, especially if the ADU construction is located on a hillside, an area with significant grading, or if there are special foundation requirements.
While not setting a percentage, Altadena requires that property maintain “sufficient” open and permeable space on the lot.
The plan check requires a review by the Department of Regional Planning and the Department of Building & Safety for zoning, building standards, and safety compliance. The process takes approximately 4-6 weeks.
Parking for Altadena ADU
Altadena typically requires one additional parking spot per ADU be added to the property. However, if the property is located within a half-mile of a public transit stop, it may be exempt from this requirement. ADUs converted from an existing garage or carport are also exempt from this parking requirement.
If a parking space must be added, it can be included on the driveway or configured as tandem parking.
Additional City of Altadena ADU Regulation Resources
Per Title 24 of the California Energy Code, solar panels of appropriate size and capacity are mandatory on new-construction ADUs. ADUs built from converted existing structures are exempt, as may be ADUs located under significant shading. Smaller ADUs may be subject to reduced or modified solar panel requirements.
An ADU is not required to have a separate water or electrical meter. You can opt for the ADU to share service with the main dwelling unit. However, you can request separate water or electrical meters from the City of Altadena.
Altadena does not automatically assign a new ADU its own address. However, it may assign the ADU a new address if it is intended for rental or independent occupancy. The ADU will usually get a “½” designation — for example, an ADU located behind 1234 Pine Street would have the address “1234½ Pine Street.”
Low Impact Development (LID) is an approach to land development or redevelopment that seeks to preserve and conserve onsite water quality and natural features, with minimal detrimental effect to local waterways and ecosystems. Altadena may require LID fees for larger ADUs.
No, Altadena does not allow residents to use an ADU as an Airbnb or short-term rental (30 days or less).
Per the Costa-Hawkins Act of 1995, Altadena does not impose rent control on dwellings constructed after February 1, 1995. This means most new-build ADUs are exempt. However, if the primary dwelling unit is subject to rent control, it might apply to the ADU as well.
While encouraging the use of electric appliances for efficiency and sustainability, Altadena does not prohibit gas appliances in new ADUs.
In Altadena you are only required to install sprinklers in a new ADU if the primary dwelling unit has them as well. You may also be required to install them if the ADU is located within a special fire hazard zone.
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