San Marcos Zoning and ADU Guide for Homeowners

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Last updated November 13, 2026. Reviewed by the Cash Home Buyers CA team.

San Marcos allows ADUs in residential zones, and the city’s standards, effective October 24, 2024, set size, height, setback and parking rules that follow California Government Code sections 66310 to 66342. This guide summarizes the city’s published ADU rules and what they mean if you own, build or plan to sell a home with an accessory unit.

San Marcos ADU rules at a glance

Where ADUs are allowed	Residential zones allowing single-family, mixed-use and multifamily uses, including planned residential districts and specific plan areas
Where JADUs are allowed	Single-family residential zones, inside the existing or proposed house
JADU size	No more than 500 square feet
Attached ADU, 1 bedroom	Up to 850 square feet
Detached ADU, 1 bedroom	Up to 850 square feet on lots under 1 acre; up to 1,000 on lots of 1 acre or more
ADU with 2 or more bedrooms	Up to 1,000 square feet
State-protected minimum	A detached new-construction ADU of up to 800 square feet, even if development standards cannot be met
Side and rear setbacks	4 feet from property lines; 10 feet from other structures, eave to eave
Parking	One paved on-site space for an ADU unless exempt; none for a JADU
Approval	Building permit only, processed within 60 days of a complete application

Types of ADUs the city allows

On lots zoned for single-family use, the city says state law currently allows one JADU within existing space, one attached ADU (new construction or a conversion of existing space), and one detached new-construction ADU.

On multifamily and mixed-use lots, state law allows up to two detached ADUs, conversion of non-livable attached space such as storage rooms into ADUs up to 25 percent of the existing units, and one additional ADU.

The city defines a JADU as a unit contained within an existing or proposed single-family home, which can include an attached garage, with its own exterior entrance.

Size, height and placement

An attached ADU cannot exceed 50 percent of the living area of the main home, though the allowed size must be at least 850 square feet.

Height limits for detached ADUs on single-family lots are 16 feet, or 18 feet within half a mile of a major transit stop or high-quality transit corridor, and up to 20 feet in that transit area when needed to match the roof pitch of the main house.

Multifamily detached ADUs are limited to 16 feet, with 18 feet allowed for multistory multifamily dwellings. An attached ADU may not exceed two stories and 25 feet, or the height of the main dwelling, whichever is lower.

Detached ADUs go in the side or rear yard and, unless the lot allows no other location, on the rear 50 percent of the lot. They must meet front yard setbacks for the zone and 4-foot side and rear setbacks.

No setback applies to an existing room or accessory structure converted to an ADU, or an ADU built in the same location and to the same dimensions as an existing structure. Converting an accessory structure may include an expansion of up to 150 square feet limited to ingress and egress.

The ADU needs direct exterior access and must use the same or similar materials, colors and architectural features as the main home.

The city says lot coverage and floor area ratio limits do not apply unless the underlying zone or a specific plan sets them, and there is no minimum lot size for an existing legal lot.

Parking

The city requires one paved on-site parking space for an ADU unless an exemption applies, and none for a JADU. Existing garage parking for the main home must be kept unless the garage is converted to an ADU or JADU, in which case existing driveway spaces must be maintained.

Permits, fees and timing

A building permit is required for every ADU and JADU, and plans must be prepared by a licensed design professional. The city says ADUs and JADUs need only a building permit, processed within 60 days of a complete application; the clock stops when the city sends corrections and restarts on resubmittal.

A rejected application gets a list of deficiencies, and the city cannot require correction of unrelated nonconforming zoning conditions as a condition of approval. A demolition permit for a detached garage replaced by an ADU is considered together with the ADU permit.

Fees depend on the project: plan check, permit issuance and possible outside agency fees such as utilities or school districts. Public Facilities Fees apply unless the ADU is smaller than 750 square feet. For fee details email buildingdivision@san-marcos.net.

San Marcos also runs a Permit-Ready ADU program with pre-approved detached one-story plans; the city approves or denies a complete application within 30 days and does not allow changes to its plan sets. See our San Marcos permits guide for the general process.

Renting, selling and HOA rules

An ADU or JADU may be rented for terms of 30 days or longer. An ADU generally cannot be sold separately from the main home except as state law allows under Government Code section 66341, and a JADU cannot be sold separately. Either the main home or the JADU must stay owner-occupied.

The city also notes that California law makes HOA rules that prohibit or unreasonably restrict ADUs on single-family lots void and unenforceable.

ADUs, conversions and selling a San Marcos house

A permitted ADU can be a selling point, while an unpermitted garage conversion or backyard unit can complicate financing and disclosure.

If you own a converted garage, a studio or a detached unit with no permit history, check the city’s records and ask the Building Division what legalizing it would involve.

Our statewide guide to selling as-is with unpermitted work explains disclosure, and the capital gains guide covers tax questions on the sale. A cash buyer can purchase the property as it stands, with or without permits, so you do not have to finish or legalize the unit first.

How to sell a house in San Marcos

Find out whether your ADU, garage conversion or addition was permitted.
Collect permits, plans and final inspection records.
Request a no-obligation cash offer for the property as it stands.
Choose your closing date.
Close with no repairs or permit cleanup required.

Own a San Marcos home with an unpermitted unit? Sell as-is for cash.

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Frequently asked questions

How big can an ADU be in San Marcos?

Per the city’s standards, attached and detached one-bedroom ADUs are up to 850 square feet (1,000 for detached on lots of one acre or more), units with two or more bedrooms are up to 1,000, and JADUs are up to 500.

How far must a San Marcos ADU be from the property line?

Four feet from side and rear property lines, and at least ten feet from other structures eave to eave. Conversions of existing space have no added setback.

Does a San Marcos ADU need parking?

One paved on-site space is required unless an exemption applies. A JADU needs none.

How long does ADU approval take in San Marcos?

The city processes ADU and JADU building permits within 60 days of a complete application, and permit-ready plan applications within 30 days.

Can I rent my San Marcos ADU as a short-term rental?

No. The city says ADUs and JADUs may be rented for terms of 30 days or longer.

Can I sell my San Marcos home if the ADU is unpermitted?

Yes. You can disclose it and sell as-is, including to a cash buyer who buys the home in its current condition.

More guides

Sources

This guide is general information from public sources, not legal advice. Confirm current rules with the City of San Marcos.