What you can build
San Marcos ADU Regulations & Zoning
ADU regulations are complex, but when you’ve done this for 100+ projects, you know the details inside and out. We publish our insights because we want people to get accurate information… not just regurgitated bullet points. We are also honored to have been chosen to design the City of San Marcos’s Permit Ready Accessory Dwelling Unit Program’s Plans. Our summary was last updated in February 2025.
Key Facts SnapShot
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 No
Building Verification Survey Yes
Coastal permit (in some areas) No
Stormwater Mitigation No
Permitting timeline
Standard Timeline: 5-7 months
Coastal Timeline: N/A
Permit Costs: $10-14/sqft
Our ADU experience in San Marcos
SnapADU is proud to have been selected by the City of San Marcos to design their pre-approved ADU plans through a competitive bid process with over two dozen applicants. This recognition highlights our expertise in delivering high-quality, efficient, and fully compliant ADU designs as well as our experience in the city. Read more about pre-approved ADU plans.
One unusual requirement in San Marcos is that ADUs generally cannot be in the front yard (though SB-897 passed Aug 2022 will limit city’s ability to enforce this). They also require a soils report for ADUs >500 sqft. San Marcos allows for digital submissions. Turnaround times at the city are moderate.
Additionally, we have seen San Marcos projects require interaction with the Community Facilities Districts (CFD), which are commonly referred to as Mello-Roos Districts, to finance local public facilities and services. This can add months to the processing time.
What is the CFD process in San Marcos for ADUs?
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.
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).
2025 UPDATE: The 2025 HCD handbook clarified that you may have one single-family converted ADU, one JADU (a conversion of existing space up to 500 sf) and one detached ADU. Prior to this clarification, there was confusion if having a converted ADU plus a JADU would prevent a homeowner from building a detached ADU. Only one detached ADU is still allowed.
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)
Effective Jan 2025 (SB1211): Multifamily property owners are permitted to build as many ADUs as there are existing units, with a maximum of eight detached ADUs. This change is due to California state law and will pre-empt existing local laws that are not brought into conformance.
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.
Setbacks
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?
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.
Does San Marcos require coastal development permits?
No areas are subject to Coastal requirements.
Multifamily
How many ADUs does San Marcos allow for multifamily properties?
Multifamily property owners are permitted to build as many ADUs as there are existing units, with a maximum of eight detached ADUs. This change is due to California state law and will pre-empt existing local laws that are not brought into conformance.
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.
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.
Parking
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.
Effective Jan 2025 (SB1211): When a garage, carport, covered parking structure, or uncovered parking space is demolished or converted for construction of an ADU, replacement parking will not be required.
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.
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About SnapADU
Since 2020, we’ve focused exclusively on designing, permitting, and building over 100 ADUs in San Diego. Our deep experience means we catch what others might miss, and our in-house team ensures every promise is delivered.
Learn MoreHow much will an ADU rent for in San Marcos?
Below are median rental prices by zip code and bedroom count. Be sure to run comps for your specific area or check with a realtor.
RENT RATES IN SAN MARCOS
| Zip | City | Neighborhood | 1BR | 2BR | 3BR |
| 92069 | San Marcos | San Marcos | 2175 | 2750 | 3575 |
| 92078 | San Marcos | San Marcos | 2425 | 3425 | 3725 |
Source: Rentometer March 2024
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San Marcos ADU Regulations & Zoning
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See the full story & ADU
We decided to build an ADU because our family's property is a big lot. That's hard to find these days, especially with the cost of housing. Plus we are closer to my mom.
Marina & Will
ADU Residents, San Marcos
