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All ADU Regulations

Los Angeles County, California

The County of Los Angeles has updated its ADU ordinance to comply with new state laws. Below are the up-to-date requirements for permitting an ADU in unincorporated Los Angeles County, in places like Altadena.
Updated Sep 23, 2021

Los Angeles County - The ADU Capital of the State

As a county, Los Angeles has permitted and built more ADUs than any other. While most ADU regulations are covered by individual city regulations (such as Los Angeles City, Culver City, Santa Monica, and others), some parts of the county are considered to be "unincorporated" areas that are governed by the county regulations. Neighborhoods such as Altadena fall into this category.
Even if your property has its own city planning and building office, the county still covers your tax assessment details and holds the records for your property at the accessor's office.


All properties zoned for single family and multifamily use may be eligible to build ADUs.

Number of units

Lots zoned for single family use that have an existing dwelling are permitted to add 1 ADU and 1 Junior ADU (JADU).

Building size

Maximum square footage is determined by your ADU type:
  • 500 sq. ft. for a Junior ADU (JADU)
  • 1,200 sq. ft. for a detached ADU (unless limited by local zoning rules)
  • 50% of existing dwelling, or none if new building, up to 1,200 sq. ft.. Exception: A new construction attached or detached accessory dwelling unit shall be permitted that is eight hundred (800) square feet or less in size, provides four (4) foot minimum interior setbacks and does not exceed sixteen (16) feet in height.

Building height

Detached ADUs and Attached ADUs are limited to 25 ft. or as specified by CSD/Specific Plan, whichever is less. ADUs within 200 ft. of a Scenic Highway may not exceed 18 ft. in height.
Setbacks and height apply to new construction ADU portion only. A proposed ADU attached to the garage must comply with setbacks stated. If the garage is fully or partially converted to the ADU, existing setbacks are permitted as legally built.


Minimum setbacks of 4 feet from the side and rear lot lines, as well as 6 feet from any existing structures are required for detached ADUs.


One parking spot required unless any of the following true:
  • The accessory dwelling unit is located within one-half mile walking distance of public transit
  • The accessory dwelling unit is located within an architecturally and historically significant historic district.
  • The accessory dwelling unit is part of the proposed or existing primary residence or an accessory structure.
  • When on-street parking permits are required but not offered to the occupant of the accessory dwelling unit.
  • When there is a car share vehicle located within one block of the accessory dwelling unit.


Owner occupancy is not a requirement for permitting an ADU from January 1, 2020 until January 1, 2025. This is a great opportunity for rental property owners to increase their yields in a cost-effective way.


While long-term rentals are permitted for ADUs, short-term rentals (e.g. Airbnb for less than 30 days) are not permitted.


Do you have a utility easement over your property? Are you in a FEMA flood zone? Reach out to us and we’ll help you build a custom ADU to fit your unique property characteristics and local regulations!

Want to learn more about ADU regulations in your area?

Book a free consultation and estimate with a Cottage ADU Expert to find out what’s possible on your property!