All ADU Regulations

San Marcos, California

San Marcos has updated its ADU ordinance to comply with new state laws. Below are the up-to-date requirements for permitting an ADU in San Marcos

July 27, 2022
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San Marcos

In San Marcos, building an ADU is a great way to add extra living space to your home. An ADU can be used as an in-law unit, guest house, or rental unit, or to simply increase your home's value. If you are thinking about adding an ADU to your home in San Marcos, Cottage can help show you the way.


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

Number of units

The number of ADUs you’re able to build depends on how your lot is zoned:

  • Lots zoned for single family use may add 1 ADU and 1 Jr. ADU (JADU).
  • Lots zoned for multi-family may add 2 new detached ADUs and at least one conversion ADU (interior conversion of existing building, but no more than 25% of the original number of units. (E.g. an apartment with 4 units may add 2 detached ADUs and 1 conversion ADU.)

Building size

  • The maximum size is 500.0 sq. ft. for a JADU
  • 850 sq. ft. max for a detached ADU
  • Attached ADU Max Size: 850 sf or 50% of the main home square footage, whichever is smaller. At least 800 sf is allowed via statewide exemption rules.

Building height

  • Building height attached: Detached ADUs are limited to one story and 16 feet in height, unless located above a detached garage then the 2nd story detached ADU must conform with the applicable height limit of the zone.
  • Building height attached ADU: 1 or 2 story ADUs are subject to the limits specified under the applicable zone.


  • Backyard Existing Structure Setback: 10' eave to eaveft
  • Side Yard Existing Structure Setback: 10' eave to eaveft
  • Front Setback Min: Varies by underlying setback, no ADUs allowed in front 50% of property


Standard exemptions apply.

One parking space shall be required, which may be located within the building setbacks or in an existing driveway as tandem parking.


  • Owner occupancy not required
  • No short term rentals (<30 days)
  • No sale of the ADU separate from the main home

Design and build with confidence.