Building an ADU in San Marcos

Which rules apply in San Marcos

ADUs and junior ADUs in San Marcos are governed by Chapter 20.410 of the San Marcos Municipal Code, "Accessory Dwelling Units and Accessory Structures." The chapter was last amended by Ordinance No. 2024-1551 on September 24, 2024. It says: "If any of the provisions noted in this chapter are in conflict with State Government Code, State Government Code shall prevail."

ADUs are allowed on any lot zoned for single-family, mixed-use or multifamily housing that has an existing or proposed home. The city reviews them with a building permit, and the code calls the approval process "ministerial," meaning no hearing.

The City Council adopted a comprehensive update of the zoning code (Title 20) on December 9, 2025, Ordinance No. 2025-1568; it is now part of the online code. That ordinance amended other chapters of Title 20, not Chapter 20.410, and it refers to Chapter 20.410 for ADU rules. If you are starting a project, ask the Planning Division whether any newer changes apply. [1][4]

How many units you can build

The city code doesn't set its own count. It says the number of ADUs and JADUs on a lot "shall be the minimum outlined by California Government Code Section 66323," and their type and location follow state law.

HCD's handbook explains what that minimum is: on a single-family lot, a city "must allow at least one ADU constructed from existing space, one JADU, and one newly constructed detached ADU." On a multifamily lot it must allow at least one ADU in non-living space (and up to 25% of the existing units) and two detached ADUs, or up to eight detached ADUs (not more than the number of existing units) on a lot with existing apartments.

For any other combination, ask the Planning Division at 760-744-1050, ext. 3204. [1][3][5]

Size, height and setbacks

• Size: the code limits an ADU to "the smallest square footage applicable." An ADU with one bedroom can be up to 850 sq ft and one with more than one bedroom up to 1,000 sq ft. An ADU also can't exceed 50% of the main house's living area, but that rule can't cut the limit below 850 sq ft. On lots of one acre or more, a detached ADU can be up to 1,000 sq ft. A junior ADU is capped at 500 sq ft.
• Height of a detached ADU: 16 feet; 18 feet within a half-mile walk of a major transit stop or high-quality transit corridor, plus up to 2 more feet (20 feet total) for a roof pitch that matches the house; 18 feet on a lot with a multistory apartment building.
• Height of an attached ADU: 25 feet or the house's height limit, whichever is lower, and no more than two stories.
• Side and rear setbacks: 4 feet for any new ADU. None for existing space or an accessory structure converted to an ADU, or a new structure built in the same place and size as the old one.
• Location of a detached ADU: within the rear 50% of the lot, at least 10 feet from any other structure (eave to eave), not in the front setback and not closer to the street than the house.
• Conversions of accessory structures can add up to 150 sq ft, only for entry and exit.

The code makes an exception to the front setback and location rules for a new detached ADU of up to 800 sq ft, 16 feet tall, with 4-foot side and rear setbacks, "if no other feasible location on the parcel is available." HCD's handbook says these state-protected units "do not have to comply with lot coverage, front setbacks, and design standards," and that other local limits can't block an 800 sq ft ADU with 4-foot side and rear setbacks. If the rear-half or 10-foot rules don't fit your lot, confirm with the Planning Division. [1][3]

Design rules

The code says all ADUs "shall be architecturally compatible with any existing dwelling unit on the same lot." In practice that means the same or similar building materials, roofing, roof design, fascia, exterior finish, color, doors and windows (including window proportions), garage door and architectural details as the house. If you update the house's look, the ADU can match the updated house.

In the Ridgeline Overlay Zone, ADUs must also meet parts of Chapter 20.260 on building placement, height, architecture, colors, materials and fire fuel modification, except where state law prohibits it.

State law says the state's by-right ADUs (the 66323 units described above) don't have to meet local design standards, and that subjective standards can't be applied to any ADU. Ask Planning which design rules apply to your plan. [1][3]

Parking

A new ADU needs one paved off-street space, at least 9 by 18 feet, on top of the parking required for the house. It can be in a setback area or tandem on the driveway unless the city finds that isn't feasible, but it can't block the sidewalk or extend into the public right-of-way, and no more than 30% of a front yard can be used for parking.

If you demolish or convert a garage, carport or covered parking for the ADU, you don't have to replace those spaces, but any existing driveway parking spaces must be kept. No parking is required for a new junior ADU.

No ADU parking is required when the ADU is:

• within a half-mile walk of public transit;
• in an architecturally and historically significant historic district;
• inside an existing house or accessory structure;
• where on-street parking permits are required but not offered to the ADU occupant;
• within one block of a car-share vehicle; or
• applied for together with a new single-family or multifamily home on the lot, "provided that the ADU or the parcel satisfies the requirements of this chapter."

State law words the last exemption differently: it applies only when the ADU or the lot also meets one of the other exemptions. State law also bars parking requirements for the state's by-right ADUs. Confirm with the Planning Division how the city applies these rules to your lot. [1][3]

Fees

Building permit fees. The city's development fee schedule effective September 1, 2026 bases plan check and permit fees for a custom ADU on project valuation. For the city's standard ADU plans (up to 500, 749 or 1,000 sq ft), it lists a $1,203 plan check fee and a $2,667 permit fee.

Public facilities fees (PFF). For projects submitted after March 3, 2025, the schedule lists PFF per square foot: $6.88 in the single-family column and $7.85 in the multifamily column, covering transportation, parks, fire, planning, habitat and storm drains. The city code says "No public facilities fees shall be imposed on ADUs smaller than 750 square feet," larger ADUs pay in proportion to the size of the main house, and fees are due before the building permit is issued. The city offers a PFF deferral program that lets you pay at utility release, with a lien recorded until you pay.

Utilities and schools. The code says "The City does not assess utilities fees or capacity charges"; water and sewer come from your local agency, which must issue a "will serve" letter before the permit. The permit-ready checklist tells you to contact SDG&E, the school district and your water and sewer provider about their fees. The city's fees page links to the San Marcos Unified School District fee page.

State limits. State law exempts ADUs of 750 sq ft of interior livable space "or less" from impact fees (the city code says "smaller than 750 square feet"), and school districts can charge only ADUs over 500 sq ft. For typical costs see our ADU cost guide. [1][3][6][7][8]

Junior ADUs

A junior ADU is up to 500 sq ft and fits entirely inside an existing or proposed single-family house, including an attached garage. It needs its own entrance and an efficiency kitchen (a cooking facility with appliances, plus a prep counter and storage cabinets). It may have its own bathroom or share the house's; if it shares, it must have both a separate outside entrance and an inside entrance to the house. No extra parking is required.

Before the building permit is issued, you must record a deed restriction with the County Recorder that bans selling the JADU separately, limits it to 500 sq ft, keeps its separate exterior access, limits rentals to 30 days or longer, and requires "that one (1) of the dwelling units (main or Junior ADU) shall be owner occupied."

State law changed in 2026: owner occupancy may be required only if the JADU shares a bathroom with the house. HCD's handbook also says a JADU deed restriction includes only the ban on separate sale and the JADU's size and features. If your JADU will have its own bathroom, ask Planning how the deed restriction applies. See our junior ADU guide and garage conversion guide. [1][3]

Renting it out and older unpermitted units

Renting. An ADU or JADU may be rented for terms of 30 days or longer. You don't have to live on the property to rent out an ADU. An ADU can't be sold separately from the house except under the state's limited exception (Gov. Code 66341).

Unpermitted units. For an ADU or JADU built before January 1, 2018 without permits, the code says the city can't deny a permit just because it violates building standards or ADU rules, unless correcting the violation is needed to protect health and safety. The city doesn't have to approve a building that is substandard under state health and safety law. State law now extends this protection to ADUs built before January 1, 2020; confirm with the Building Division.

Nonconforming lots. The city can't make you fix nonconforming zoning conditions, or deny an ADU because of code violations or unpermitted structures that aren't a health and safety threat and aren't affected by the ADU.

Fire sprinklers. An ADU needs sprinklers only if the house is required to have them, and building an ADU doesn't require adding sprinklers to the existing house. [1][3]

Permit-ready ADU plans

The city's Permit Ready ADU Program offers "a selection of pre-approved, detached one-story ADU building plans." The plans are "mostly complete": you add site-specific information, including a detailed site plan, and you still need separate submittals for items such as roof trusses and solar panels.

The city won't accept changes to its plan sets. If you want a custom design, you apply for a standard ADU building permit. The city says it "will approve or deny an application within 30 days from the date the city receives a complete application," and the same 30 days applies to a resubmittal after corrections. Review covers Planning, Building, Land Development Engineering and Fire. [2][8]

Permit steps in San Marcos

1. Check your zoning and lot

   Look up your zone with the city's Zoning Portal. For setbacks, zoning or parking questions, call the Planning Division at 760-744-1050, ext. 3204. [2][5][8]

2. Choose a permit-ready plan or a custom design

   Download a city permit-ready ADU plan set and fill in the site-specific details, or have a designer prepare custom plans for a standard ADU building permit. [2]

3. Line up utilities and outside agencies

   Contact SDG&E, the school district and your water and sewer provider about their requirements and fees. The city needs a will-serve letter from the water and sewer agencies before it issues the permit, and a private septic system needs County approval. [1][8]

4. Apply for the building permit

   Submit the building permit application and plans to the Building Division (760-744-1050, ext. 3244). Planning, Building, Land Development Engineering and Fire review the plans. [8][9]

5. Record the JADU deed restriction and pay fees

   For a junior ADU, record the deed restriction with the County Recorder before the permit is issued. Pay plan check, permit and any public facilities fees; the permit is issued only to the owner or a licensed California contractor. [1][8]

6. Build, pass inspections and get your certificate of occupancy

   Schedule inspections at 760-744-1050, ext. 3260. Planning, Building, Fire and Engineering, plus the water and sewer agencies, must sign off before the final inspection, after which the Building Division issues a certificate of occupancy. [5][8][9]

Pre-approved and sample plans

• Permit Ready ADU Program (PRADU)

  Pre-approved detached one-story ADU plan sets; you add a site plan and deferred items like trusses and solar, can't alter the plans, and the city decides a complete application within 30 days. [2][8]

Who to call

• City of San Marcos Planning Division

  Zoning, setbacks, parking and ADU standards

  760-744-1050, ext. 3204

  1 Civic Center Drive, San Marcos, CA 92069

  [5]

• City of San Marcos Building Division

  Building permits, plan review and the Permit Ready ADU Program

  760-744-1050, ext. 3244

  1 Civic Center Drive, San Marcos, CA 92069

  [2][9]

• Building inspections

  Schedule ADU inspections

  760-744-1050, ext. 3260

  [9]

Questions people ask