What you can build

San Marcos ADU Regulations & Zoning


Size and Separation

Maximum size 1,000 square feet

Side/rear setbacks 4 feet

Two story Yes, above garage

Building separation 10 feet

Additional Requirements

Soils Report Yes

Building Verification Survey Yes

Coastal permit (in some areas) No

Stormwater Mitigation No

Number of ADUs Allowed

How many ADUs can you build in San Marcos?

On San Marcos single-family residential lots, you may build one ADU (either attached or detached) and one Junior ADU (JADUs have an owner occupancy requirement and must be converted from existing space within the primary residence).

How many ADUs does San Marcos allow for multifamily properties?

San Marcos allows up to 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)

Non‐livable space within existing multi‐family structures may be converted into an ADU. Examples of areas that can be converted include storage rooms and garage spaces. The maximum number of ADU allowed in a multi‐family structure is equal to 25% of the number of existing multi‐family units in the structure. Fractional units are rounded down. A minimum of one conversion ADU is allowed.

ADU Size & Building Constraints

What size detached ADU can you build in San Marcos?

San Marcos allows for a detached ADU up to 850 sqft for units with 1 bedroom, and up to 1000 sqft for 2+ bedrooms. ADUs up to 800 square feet are exempt from lot coverage requirements. (Meaning: You can build an ADU of this size even if you’re maxed out on lot coverages).

Side note if you happen to notice that we built a 1200 sqft ADU in San Marcos… that was permitted back in 2020 before San Marcos set up their own limits on ADU size, which meant the State law – which permits 1200 sqft – applied.

What is the height requirement for an ADU in San Marcos? Can you build a 2-story ADU in San Marcos?

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.

How big can an attached ADU be in San Marcos?

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+)
What are the requirements of the exterior of the ADU?

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.


What is the setback requirement for an ADU in San Marcos?

ADUs must have 4′ minimum setbacks from rear and side-yard property lines, with front yard setback per zoning standards. An ADU taking advantage of the 4′ setbacks cannot exceed a height of 16 feet. An ADU exceeding 16′ must comply with the setbacks of the underlying zoning.

Also, the ADU shall not be located within the required front property line setback and shall not be closer to the right of way than the primary dwelling (meaning: you typically cannot build an ADU in the front yard). The only exception to this rule is that, if no other feasible location on the parcel is available, an ADU of up to 800 sqft can be built without a front setback. Update as of Aug 2022: SB-897 prohibits local governments from enacting more stringent front yard setbacks on ADUs up to 800sqft, which means you may be able to build an ADU in the front yard up to the underlying zoning for the front yard setback.

What is the minimum distance the ADU must be from existing structures?

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

Additional Requirements

Does San Marcos require Building Verification Surveys for an ADU?

Yes, San Marcos requires a building verification survey as part of the plan review process for an ADU.

Additionally, San Marcos requires a setback certification (achieved through a survey) on all detached ADUs and all new habitable structures built within the City of San Marcos. The setback certification is required prior to requesting follow up inspections.

Does San Marcos require in-person submission of plans for an ADU?

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

Does San Marcos require a soils report for an ADU?

Yes, San Marcos requires a soils report for all new ADUs that exceed 500 sqft.

Does San Marcos require coastal development permits?

No areas are subject to Coastal requirements.


How many ADUs does San Marcos allow for multifamily properties?

Parcels zoned to allow or developed with existing multifamily dwelling residential use may have a maximum of two (2) detached ADUs.

In addition, portions of existing dwelling structures that are not used as livable space, including storage rooms, boiler rooms, passageways, attics, basements, and garages may be converted into attached ADUs.

Can you stack detached ADUs on top of each other?

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

Can you attach two ADUs to 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.

Can you convert existing space to an ADU?

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.


What is the parking requirement for an ADU in San Marcos?

A parking space is not required for an ADU in any of the following instances:

  • Within a one-half mile walking distance from public transit, which includes bus stops.
  • Within an established historic district.
  • Converted from the existing space of a primary residence or accessory structure, or are attached to an existing or proposed primary residence.
Where can a parking space be located?

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.