Accessory Structures in San Marcos, CA: What Residents Actually Need to Know

If you live in San Marcos or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. San Marcos has 6 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

ADU Rules

San Marcos caps accessory dwelling units under Municipal Code § 20.410.060 at 850 square feet for a one-bedroom unit and 1,000 square feet for two or more bedrooms, or 50% of the primary home's living area, whichever is smaller. Detached ADUs top out at 16 to 20 feet in height depending on transit proximity.

Key details: One-bedroom ADU cap: 850 square feet. Multi-bedroom ADU cap: 1,000 square feet. Junior ADU cap: 500 square feet. Detached ADU height: 16 ft, up to 20 ft near transit. Minimum side/rear setback: 4 feet for new detached ADU.

Building an ADU that exceeds the applicable square-footage or height cap, or that doesn't meet the reduced 4-foot setback and 10-foot separation standards, is a zoning code violation that can block issuance of a building permit or certificate of occupancy. An ADU built without permits or in violation of these standards is subject to the same enforcement remedies as any unpermitted structure, including stop-work orders and required correction before the unit can be legally occupied.

ADU Impact Fees

San Marcos charges no utility fees or capacity charges on ADUs, and Municipal Code § 20.410.060.G exempts any ADU under 750 square feet from public facilities fees entirely. Larger units pay fees sized only to their actual burden on services, based on the primary home's size.

Key details: Utility/capacity charges on ADUs: None assessed by the City. Fee exemption threshold: ADUs under 750 sq ft: no fees. Fee basis for larger ADUs: Proportionate to burden, sized to primary home. When fees are due: Before building permit issuance. Governing section: Municipal Code § 20.410.060.G.

Charging a public facilities fee on an ADU under 750 square feet, or assessing a utility fee or capacity charge on any ADU, would conflict with Section 20.410.060.G; a fee assessed in violation of that subsection is not owed and can be challenged. On the applicant side, failing to pay a properly calculated fee before permit issuance under G.3 will hold up the building permit until the fee is paid.

The rules around adu impact fees in San Marcos lean permissive, but that does not mean anything goes.

ADU Rental Restrictions

San Marcos bans short-term rentals of accessory dwelling units outright. Municipal Code § 20.410.060.H allows an ADU to be rented only for terms of 30 days or longer, and bars selling it separately from the primary home except in narrow state-law circumstances.

Key details: Minimum rental term: 30 days. Short-term rentals: Prohibited on ADUs. Separate sale: Barred except under Gov. Code § 66341. Applies to: Standard ADUs; Junior ADUs governed by O.4. Governing section: Municipal Code § 20.410.060.H.

Renting an ADU for less than 30 days, or attempting to sell it separately from the primary dwelling outside the narrow Government Code Section 66341 exception, violates Section 20.410.060.H. Short-term rental use of an ADU is subject to code enforcement action, and a title company or lender is expected to flag an attempted sale of the ADU that isn't paired with the underlying home.

Compared to other cities, San Marcos takes a harder line on adu rental restrictions. The enforcement and penalty structure reflects that.

Shed & Carport Rules

San Marcos caps a detached shed or other accessory structure at 15 feet in height and one story, and limits total enclosed accessory floor area to 40% of the primary home's living area or 800 square feet, whichever is less, under Municipal Code § 20.410.040.

Key details: Max enclosed accessory floor area: 40% of home's living area or 800 sq ft. Max height: 15 feet, one story. Separation from other structures: 10 feet minimum if detached. Rear setback: 5 feet minimum. Kitchens/bathtubs/fireplaces: Prohibited in accessory structures.

Exceeding the 800-square-foot/40-percent floor area cap, building over 15 feet or two stories, or installing a bathtub, kitchen, or fireplace in a shed or accessory building without the required deed restriction and permit review are all violations of Section 20.410.040. The City can withhold a certificate of occupancy, require demolition or modification of a nonconforming structure, and pursue standard code enforcement against a structure converted for illegal sleeping or dwelling use.

Garage Conversions

San Marcos lets a garage be converted to an accessory dwelling unit without adding new setbacks and without replacing the lost parking under Municipal Code Section 20.410.060, but bars converting the required third garage stall on larger homes into living space under Section 20.340.050(H).

Key details: ADU garage conversion setback: None if same footprint, § 20.410.060(E)(1). Lost parking replacement: Not required for ADU conversions, § 20.410.060(J)(3). Protected required parking: 3-car garage on homes ≥3,001 sf. Protection source: Table 20.340-5, § 20.340.050(H). Interior minimum, 3-car garage: 640 square feet.

Converting the code-required garage parking on a home of 3,001 square feet or larger into living space, in violation of the no-conversion note in Section 20.340.050.H's Table 20.340-5, can be flagged during permit review or a later code compliance inspection and require restoring the required parking. An ADU garage conversion built without complying with the setback, parking, and deed-restriction terms of Section 20.410.060 is not entitled to the parking-replacement exemption and setback relief that section otherwise grants.

ADU Owner Occupancy

San Marcos requires an owner-occupancy deed restriction before any Junior ADU gets a building permit. Municipal Code § 20.410.060.O.5 mandates that either the primary home or the Junior ADU be owner-occupied, recorded against the property with the County Recorder.

Key details: Owner-occupancy rule: Main house or Junior ADU must be owner-occupied. Recorded where: Deed restriction with County Recorder. Required before: Building permit issuance for the Junior ADU. Applies to: Junior ADUs only, not standard ADUs. Governing section: Municipal Code § 20.410.060.O.5.

Building or occupying a Junior ADU without recording the required deed restriction, including the owner-occupancy commitment under O.5, means the City cannot lawfully have issued the building permit for that unit. An existing Junior ADU found out of compliance, for example where neither unit is owner-occupied, is subject to code enforcement and can jeopardize the property's ability to rent both units at all.

The Bottom Line

San Marcos's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming San Marcos is broadly strict or permissive.

This guide is based on San Marcos's current municipal code. Local rules can and do change, so check the individual ordinance pages for the latest details, penalties, and FAQs.