California ADU rules · San Diego County
Can I Build an ADU in San Marcos?
Yes — you can build an ADU in San Marcos. California's statewide ADU law requires every city, including San Marcos, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
CA state law1 ADU + 1 JADU
At least one ADU and one JADU per single-family lot (a conversion ADU may also be possible).
Units on a multifamily lot
Local ruleconversions (>=1 or 25% of units) + up to 2 detached (or up to 8, not to exceed number of existing units)
City sets a local cap of 2–8 detached ADUs on multifamily lots, not to exceed the number of existing units.
Junior ADU (JADU)
CA state lawAllowed
Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).
Size & height
Max detached ADU size
Local rule1,000 sq ft
Detached ADUs: 850 sq ft (1 bedroom), 1,000 sq ft (2+ bedrooms or lots ≥1 acre); city sets a local maximum.
Max attached ADU size
Local rule50% of primary, >=850 (1 bedroom) or >=1,000 (2+ bedrooms)
Attached ADUs: 50% of primary dwelling, but not less than 850/1,000 sq ft.
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
CA state law16 ft
16 ft baseline; cities must allow 18 ft within ½ mi of a major transit stop or where the primary dwelling is two-story, and 20 ft on a lot with a multistory multifamily building.
Setbacks
Side setback
CA state law4 ft
A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.
Rear setback
CA state law4 ft
Front setback
Local ruleADU cannot be in required front setback or closer to right-of-way than primary dwelling, except if no feasible location exists (then 4 ft side/rear, 800 sq ft max, 16 ft height)
Front setback exceptions only if no feasible location exists.
Separation from main house
Local rule10 ft
Detached ADUs must be at least 10 ft from any other structure (eave to eave).
Parking
Parking required
Local ruleUp to 1 space
1 on-site paved space per new ADU, with state-mandated exemptions.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
Local ruleNot required
No replacement parking required for garage/carport conversion.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
CA state law60 days
The agency must act within 60 days of a complete application — and the ADU is deemed approved if it does not.
Owner-occupancy
Local ruleOnly for a JADU
Owner-occupancy required for JADUs only.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
CA state lawBuilding permit
Only a ministerial building permit — no conditional use permit or discretionary design review for a qualifying ADU.
San Marcos sets a local detached ADU maximum of 1,000 sq ft (2+ bedrooms/lots ≥1 acre) and requires a 10 ft separation from other structures. Height limits and multifamily detached ADU caps are locally defined.
Frequently asked questions
Can I build an ADU in San Marcos?
Yes. California's statewide ADU law requires San Marcos to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in San Marcos?
A detached ADU of at least 1,000 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in San Marcos?
Side and rear setbacks are limited to 4 ft. ADU cannot be in required front setback or closer to right-of-way than primary dwelling, except if no feasible location exists (then 4 ft side/rear, 800 sq ft max, 16 ft height).
Is parking required for an ADU in San Marcos?
Up to 1 space. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in San Marcos?
A complete application must be acted on within 60 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in San Marcos?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- San Marcos Municipal Code Chapter 20.410 (Ord. No. 2024-1551, 9-24-2024)
- San Marcos Municipal Code § 20.410.060(D)(1)
- San Marcos Municipal Code § 20.410.060(C)(1), (C)(5)
- San Marcos Municipal Code § 20.410.060(C)(1)
- San Marcos Municipal Code § 20.410.060(E)
- San Marcos Municipal Code § 20.410.060(J)
- San Marcos Municipal Code § 20.410.060(J)(4)
- San Marcos Municipal Code § 20.410.060(H)
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated 2026-06-25. This is an AI-assisted summary of San Marcos's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with San Marcos Planning before relying on it.
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