San Marcos | ADUs and SB 9
San Marcos, California ADUs and SB 9: ADU and Junior ADU Size, Height, Setbacks, Parking and Fees, SB 9 Two-Unit Projects, and Where the City Code Differs From State Law
This page covers the two ways to add homes to a San Marcos house lot without a rezoning. An accessory dwelling unit (ADU) is a second, complete home on the same lot as a main home, attached, detached or built inside an existing building. A junior ADU is a small unit of up to 500 square feet built inside the house. SB 9, the 2021 state law, allows two main homes on a lot in a single-family zone. San Marcos sets its ADU rules in Chapter 20.410, last amended by Ordinance 2024-1551 in September 2024, and its SB 9 rules in Chapter 20.470 and Chapter 19.44 of the Municipal Code. The San Marcos overview has the summary.
San Marcos ADUs and SB 9 in six facts
- San Marcos approves an ADU or junior ADU on a lot with an existing home with a building permit, ministerially, within 60 days of a complete application. Source: San Marcos Municipal Code 20.410.030
- An ADU may have up to 850 square feet with one bedroom or 1,000 square feet with more, and 50 percent of the main home if that is larger than 850. Source: San Marcos Municipal Code 20.410.060(C)(1)
- A detached ADU may be 16 feet tall, or 18 feet within one-half mile walking distance of a major transit stop, with 4-foot side and rear setbacks. Source: San Marcos Municipal Code 20.410.060(C)(3) and (E)
- San Marcos charges no public facilities fees on an ADU smaller than 750 square feet and does not assess utility fees or capacity charges on ADUs. Source: San Marcos Municipal Code 20.410.060(G)
- An SB 9 two-unit project may add one unit of up to 800 square feet to an existing house, with the new unit no taller than 16 feet. Source: San Marcos Municipal Code 20.470.030(B) and (I)
- No more than four units, counting main homes, ADUs and junior ADUs, may be created on one parcel under the SB 9 chapter. Source: San Marcos Municipal Code 20.470.030(A)
ADU Rules Beside State Law
| Rule | San Marcos Chapter 20.410 | State law (Government Code) |
|---|---|---|
| Permit | Building permit, ministerial, decided within 60 days | Ministerial; completeness in 15 business days and decision in 60 days (66317) |
| Size, one bedroom | Up to 850 sq ft, or 50% of the main home if larger, never less than 850 | Maximum may not be set below 850 sq ft (66321(b)(2)) |
| Size, more than one bedroom | Up to 1,000 sq ft | Maximum may not be set below 1,000 sq ft (66321(b)(2)) |
| Junior ADU | Up to 500 sq ft inside the house | Inside the walls of the house (66333(d)) |
| Height, detached | 16 ft; 18 ft near a major transit stop, plus 2 ft for a matching roof pitch; 18 ft on a multistory multifamily lot | The same floors (66321(b)(4)) |
| Height, attached | 25 ft or the main home limit, whichever is lower; two stories | The same (66321(b)(4)(D)) |
| Setbacks, new detached | 4 ft side and rear; rear 50% of the lot; 10 ft from other buildings | No more than 4 ft side and rear (66314(d)(7)) |
| Parking | One 9 by 18 ft space per new ADU, with six exemptions; none for a junior ADU | No more than one per ADU or per bedroom, whichever is less (66314(d)(10)); exemptions in 66322 |
| Impact fees | None on ADUs smaller than 750 sq ft | None on ADUs of 750 sq ft or less (66311.5(c)) |
| Owner occupancy, junior ADU | Required: the main home or the junior ADU | Required only with shared sanitation (66333(b)) |
Sources: San Marcos Municipal Code 20.410.020 to 20.410.060; Government Code 66314, 66317, 66321, 66322, 66333 and 66311.5.
Section 20.410.020 opens with a rule that settles many questions: "If any of the provisions noted in this chapter are in conflict with State Government Code, State Government Code shall prevail." It then says ADUs are allowed on any lot "zoned to allow single-family, mixed-use, or multifamily dwelling residential use" that has a proposed or existing dwelling, and that the number of ADUs and junior ADUs "shall be the minimum outlined by California Government Code Section 66323." On a house lot that means one ADU and one junior ADU. On a lot with an existing apartment building, Section 66323 allows ADUs in storage rooms, garages and other spaces not used as living space, at least one and up to 25 percent of the existing units, and up to eight detached ADUs, never more than the number of existing units.
Size. Section 20.410.060(C) limits an ADU to "the smallest square footage applicable to the parcel": 50 percent of the living area of the main home, "provided, however, that the maximum square footage allowed must be at least eight hundred fifty (850) square feet", 850 square feet for one bedroom, and 1,000 square feet for more than one bedroom. On lots of one acre or more a detached ADU may have 1,000 square feet. By VanPlex arithmetic, a house with 1,400 square feet of living area gives 700 square feet at 50 percent, which the 850 floor raises to 850. A house with 2,400 square feet gives 1,200, so the limit is 850 for one bedroom or 1,000 for two or more. "Living area" means interior habitable space, including basements and attics, without the garage.
Height. A detached ADU may be 16 feet tall, or 18 feet on a lot "within one-half of one mile walking distance of a major transit stop or a high-quality transit corridor", with two more feet for a roof pitch that matches the main home, for a maximum of 20. A detached ADU on a lot with a multistory multifamily building may be 18 feet. An attached ADU may reach 25 feet or the main home height limit, whichever is lower, and no more than two stories. These match the floors in Government Code 66321(b)(4).
Location. Section 20.410.060(E) places a detached ADU "within the rear fifty percent (50%) of the lot", with the setbacks of the zone except that side and rear setbacks are 4 feet, and 10 feet from any other structure "eave to eave." A converted space or a rebuild in the same place and size needs no setback. An ADU may not sit in the front setback or closer to the street than the main home. The exception in subsection (E)(4) allows an 800 square foot, 16-foot detached ADU with 4-foot setbacks anywhere on the lot "if no other feasible location on the parcel is available."
Design. Section 20.410.050 says all ADUs "shall use the same building materials as the existing primary dwelling unit", including roofing, fascia, colors, doors, windows and garage doors.
Parking, Fees and Utilities
Section 20.410.060(J) requires "a minimum of one (1) on-site paved parking space" of 9 by 18 feet for a new ADU, in addition to the parking for the main home. The space may be in a setback or in tandem on the driveway, but "No more than thirty (30) percent of a front yard shall be dedicated to vehicle parking." No parking is required when the ADU is within one-half mile walking distance of public transit, in a historic district, inside an existing home or accessory building, where on-street parking permits are required but not offered, within one block of a car share vehicle, or filed together with a new home. A junior ADU needs no added parking. When a garage or carport is converted or torn down for an ADU, the City "shall not require that those off-street parking spaces be replaced", but "any pre-existing driveway parking spaces must be maintained."
Fees follow Section 20.410.060(G). "No public facilities fees shall be imposed on ADUs smaller than 750 square feet." Larger ADUs pay fees "proportionate to the burden of the ADU", based on the size of the main home, paid "prior to the issuance of any building permit." The City "does not assess utilities fees or capacity charges", but it must get a "will serve" letter from each utility before the building permit, and the County of San Diego must approve any private sewage system. Fire sprinklers are required only if the main home needs them.
Other conditions: an ADU may be rented "for terms 30 days or longer" and may not be sold apart from the main home, except under Government Code 66341. A certificate of occupancy for the ADU may not be issued before the one for the main home. An ADU or junior ADU built without a permit before January 1, 2018 may not be denied a permit for building code or zoning reasons unless a correction is "necessary to protect the health and safety of the public or occupants of the structure."
Junior ADUs
Section 20.410.020(C) defines a junior ADU as a unit of no more than 500 square feet "contained entirely within an existing or proposed single-family residence, including all enclosed uses within the residence, such as attached garages." It needs its own entrance and an efficiency kitchen with "a cooking facility with appliances" and a food preparation counter with storage. If it shares a bathroom with the house, it also needs an interior door to the house. Before the building permit, the owner records a deed restriction that bars separate sale, caps the size at 500 square feet, keeps the outside entrance, requires rentals of 30 days or longer, and requires that "one (1) of the dwelling units (main or Junior ADU) shall be owner occupied."
SB 9 Two-Unit Projects
| Rule | San Marcos Chapter 20.470 |
|---|---|
| Units | Two new units, or one new unit added to an existing unit |
| New unit size | Not more than 800 sq ft each |
| Height and stories | 16 ft; no two-story units and no units above a detached accessory building |
| Side and rear setbacks | 4 ft or the zone setback, whichever is less; none for a rebuilt existing structure |
| Separation | 10 ft from other structures, or attached if building code safety standards are met |
| Parking | One space per unit; none within one-half mile walking distance of a high-quality transit corridor or major transit stop, or with a car share vehicle within one block |
| Total units on the parcel | Four, counting ADUs and junior ADUs |
| Owner occupancy | Recorded covenant when there is no lot split |
| Rentals | Longer than 30 days |
Source: San Marcos Municipal Code 20.470.020 and 20.470.030 (Ordinances 2021-1510 and 2021-1511).
Chapter 20.470 applies to two-unit projects that meet Government Code 65852.21(a). Such a project "shall be considered without discretionary review or hearing, and shall be approved ministerially." Section 20.470.020(D) promises that nothing in the chapter will "physically preclude" two units or either unit being "at least 800 square feet in floor area." All other zone standards apply, "including, but not limited to, height limits, stories, setback, lot coverage, landscape, and floor area ratio", except the setback relief.
Section 20.470.030(A) sets the count. "No more than four units ... may be created on a parcel." On a lot that was not split, a two-unit project may add "one accessory dwelling unit ... for each primary dwelling unit." On a lot created by an SB 9 split with two main homes, "no accessory dwelling units or junior accessory dwelling units are permitted." On a split lot with one main home, one ADU or junior ADU is allowed. These follow Government Code 65852.21(f) and 66411.7(j), which say a city need not allow more than two units on a lot created by a split.
Where the lot keeps its existing house, Section 20.470.030(B)(1) allows "one additional unit of not more than 800 square feet" and says no more than 25 percent of the existing outside walls may be torn down "unless the existing single-family home has not been occupied by a tenant in the last three years." On a vacant lot, or where the house is replaced, two units of up to 800 square feet each are allowed. Without a lot split, one unit must be "the bona fide principal residence of at least one legal owner", shown by a recorded covenant before the building permit. The lots and subdivisions page covers SB 9 splits.
Where the City Code Differs From State Law
A city rule on ADUs or SB 9 that is stricter than the statute does not apply to the extent of the conflict. San Marcos says so itself for ADUs in Section 20.410.020. The cases VanPlex found:
- Junior ADU owner occupancy. Sections 20.410.020(C) and 20.410.060(O)(5) always require an owner on site. Government Code 66333(b) says "Owner-occupancy shall not be required if the junior accessory dwelling unit has separate sanitation facilities." The statute controls.
- ADU fee line. The City exempts ADUs "smaller than 750 square feet"; Government Code 66311.5(c)(1) exempts ADUs with "750 square feet of interior livable space or less." An ADU of exactly 750 square feet pays no impact fee.
- Rental term. The City allows rentals of "30 days or longer"; Government Code 66323(e) and 66333(g) require a term "longer than 30 days."
- Location and design of a state-protected ADU. Government Code 66323(b) bars any standard "not authorized by this section" on an 800 square foot detached ADU with 4-foot setbacks. The rear-half rule, the "no other feasible location" test in Section 20.410.060(E)(4) and the matching-materials rule in Section 20.410.050 go beyond what that section allows for such a unit.
- SB 9 size and height. Section 20.470.030 caps new units at 800 square feet and 16 feet with no second story, while the R-1 zones allow two stories up to 35 feet. Government Code 65852.21(b)(3), amended by SB 1426 effective June 30, 2026, bars standards that "do not apply uniformly to development within the underlying zone."
- SB 9 owner occupancy. Section 20.470.030(H) requires a covenant without a lot split. The conditions Government Code 65852.21(c) lets a city add are parking and a percolation test; it lists no owner occupancy rule.
- SB 9 wall demolition. The 25 percent wall limit in Section 20.470.030(B)(1) does not appear in the current text of Government Code 65852.21, which bars demolishing housing "occupied by a tenant in the last three years."
- Ridgeline permits. Section 20.260.060 requires a Planning Commission ridgeline permit for "two (2) or more main structures" in the ridgeline setback area, while SB 9 projects must be approved "without discretionary review or a hearing."
Ask the Planning Division for its written position on each conflict that touches your project. The code text read here is current through Ordinance 2026-1577 of March 10, 2026; VanPlex could not read later City Council actions because the City's website refused automated requests.
Best For
- ✓ House lots where a detached ADU of 850 to 1,000 square feet fits in the rear half with 4-foot setbacks.
- ✓ Garage conversions, which need no setback and no replacement parking.
- ✓ Lots near transit, where ADUs need no parking and may be 18 feet tall.
Usually Fails When
- ✕ A plan needs a two-story SB 9 unit and the City enforces its 16-foot limit.
- ✕ An SB 9 split lot is planned for two homes plus ADUs, which Section 20.470.030 bars.
- ✕ A tenant lived in the house in the last three years, which bars SB 9 demolition.
What To Verify Before Spending Money
- → The City's written position on each code conflict listed above.
- → Whether the lot is in the Ridgeline Overlay Zone or the airport overlay.
- → The public facilities fee for an ADU of more than 750 square feet.
Where to Go Next
Related on VanPlex: the San Marcos overview, the California ADU law page and the California SB 9 page.
Frequently Asked Questions
How big can an ADU be in San Marcos, California?
How long does San Marcos take to approve an ADU?
Where can a detached ADU go on a San Marcos lot?
Does a San Marcos ADU need a parking space?
Must the owner live on the lot with a San Marcos junior ADU?
What fees does San Marcos charge on an ADU?
Can I build a second house on my San Marcos lot under SB 9?
How many homes can one San Marcos lot hold with SB 9 and ADUs?
Can San Marcos limit an SB 9 unit to 16 feet and one story?
Can I add an ADU on a San Marcos hillside lot in the ridgeline overlay?
Official sources (checked October 2026)
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