San Marcos ADU Regulations & Zoning

San Marcos allows at least 2 detached ADUs for multifamily lots with an existing multi-family building. (Meaning: In order to build 2 ADUs, the property must already have at least a duplex on it, not just a single family dwelling)

Multifamily property owners are permitted to build as many ADUs as there are existing units, with a maximum of eight detached ADUs.

Additionally, a minimum of one accessory dwelling unit, or up to 25 percent of the existing multifamily units, is permitted within an existing multifamily dwelling, as a result of the conversion of non-habitable space including, but not limited to, storage rooms, boiler rooms, passageways, attics, basements, or garages.

Read more about the rules around how many ADUs you can build on a multifamily lot.

The height of a detached ADU can be up to 16′, not to exceed one story unless the ADU is to be located above a garage, and the overall height shall remain consistent with the requirements of the applicable Zone in which the ADU is located.

Also, per recent state law change, detached ADUs within half a mile of major transit stops or high-quality transit corridors can use 4′ side & rear setbacks, up to 18 feet tall. An additional 2 feet can be added if the ADU’s roof pitch matches the primary dwelling. This 18’+2′ limit would allow for a two story unit.

Read more about two story ADUs and height limits.

The attached ADU can be no bigger than the lesser of the following:

• 50% of the primary dwelling square footage or
• 850 sqft (1BR) or 1000 sqft (2BR+)

In San Marcos, there is a requirement that all accessory structures and ADUs shall be architecturally compatible with any existing dwelling unit on the same lot. This includes incorporating the same or similar architectural features, building materials, and color, roofing material, roof design, fascia, exterior building finish, color, exterior doors and windows (including ratios of window dimensions [i.e., width to height] and window area to wall area), garage door, and architectural enhancements. Also, the design of the ADU shall relate to the design of the primary dwelling unit and shall not visually dominate it or the surrounding properties.

The minimum required distance between structures is 10′ (eave to eave).

No, San Marcos allows digital submission of applications for an ADU.

No, San Marcos no longer requires a soils report on most ADUs. As of May 2025, a City of San Marcos Plans Examiner confirmed via email, “A soils investigation report is not required unless grading is proposed, or if, during inspections, the building inspector determines that the site conditions may be unsuitable for construction.” Previously, San Marcos required a soils report for all new ADUs that exceed 500 sqft.

No areas are subject to Coastal requirements.

A CFD is a “community facility district” that residents of the city previously voted on to create the ability for the City to collect “developer impact fees” to cover the cost associated with providing services to the residents. These fees go towards building roads, fire stations, and other public services. Every new development project in San Marcos is subject to the CFD Charge, including ADUs. California state ADU laws do not prohibit the City from assessing CFD charges.

Owners can satisfy the CFD requirement in one of two ways:
1) Vote to annex into the district, which creates a annual assessment with your property taxes (usually $1K-$2K); or
2) Pay a one -time In-Lieu Fee (typically $100K-$200K)

Only the property owners of a property/development vote on this annexation – no neighbors or 3rd parties. The City Council certifies CFD votes 4 times per year. This means that typically 3-4 months will be added to the permitting process for ADUs of 750 SF or more.

No, you cannot stack ADUs on top of each other in San Marcos.

No, you cannot attach two ADUs. San Marcos has interpreted that multi-family lots are permitted two freestanding, detached accessory dwelling units which are detached from the multi-family residence as well as from one another.

Yes, existing non-livable space may be converted to ADUs (e.g. storage areas and garages). The minimum number of ADU conversions allowed is one, up to a maximum equal to 25% of the number of existing multifamily units. This is in addition to the allowed number of detached ADUs.

If a parking space is required, it may be located within the setbacks and in an existing driveway as tandem parking. Parking spaces don’t need to be replaced when a garage, carport, or covered parking structure is demolished in conjunction with the construction of an ADU or converted to an ADU.