ADU builders and rules in San Marcos

San Marcos ADU rules

San Marcos ADU rules are in Chapter 20.410 of the San Marcos Municipal Code. In September 2024 the city's Planning Division filed a notice for text amendment TA24-0003, which repeals and replaces Chapter 20.410 to bring it into full compliance with current state ADU law. We couldn't read the adopted text, so we don't list its size or height limits here.

The city's ADU FAQ dates from 2021, before that rewrite and before several state law changes, so treat it as background. Two points in it are specific to San Marcos and worth asking about:

• Spacing. It said a detached ADU must sit at least 10 ft from other structures, eave to eave, in addition to 4 ft side and rear setbacks.
• Fees beyond the permit. It said ADUs pay Public Facilities Fees unless smaller than 750 sq ft, and may be subject to Community Facilities District (CFD) fees.

San Marcos permit-ready ADU plans

San Marcos runs a Permit Ready Accessory Dwelling Unit (PRADU) Program under the state's pre-approved plan law. It offers a selection of pre-approved, detached, one-story ADU plan sets that you download from the city's page and complete with your site details. The city's 2025 fee schedule lists these “City Standard” ADUs in three size classes: up to 500, up to 749 and up to 1,000 sq ft.

• No changes allowed. The city won't consider alterations to its plan sets. If you want a custom design, you apply for a standard ADU building permit instead.
• You fill in the site work. According to the PRADU checklist, you add a detailed site plan, a pervious and impervious area table, grading quantities, a stormwater (SWQMP) form and BMP plan.
• Deferred submittals. Roof trusses and solar panels are separate submittals, and fire sprinklers may be required after first review. The city recommends hiring licensed professionals for these early to avoid delays.
• 30-day decisions. The city approves or denies a complete PRADU application within 30 days, and again within 30 days of each resubmittal.

The city also invites designers to submit their own plans for the program through the Planning Division. Compare the regional options in our pre-approved ADU plans guide, and see ADU floor plans for layout ideas.

San Marcos ADU fees

The city offers a Public Facilities Fee deferral program that lets you pay at utility release instead, with a lien recorded until you pay. The four CFDs the city administers fund police, fire and paramedic services, lighting and landscaping maintenance, and congestion management; the special tax shows up on the next year's property tax bill.

As a builder's observation, Snap ADU says every new San Marcos project it sees, including ADUs, is subject to the CFD requirement. It estimates annexing adds about $1,000 to $2,000 a year in taxes, while the one-time in-lieu fee typically runs $100,000 to $200,000, and that CFD votes add 3 to 4 months for ADUs of 750 sq ft or more. Snap ADU also puts San Marcos permit costs at about $10 to $14 per sq ft and says the city dropped its soils report requirement for most ADUs in May 2025 unless grading is proposed. Confirm these with the city. For build costs, see what an ADU costs in San Diego or the ADU cost calculator.

Getting a San Marcos ADU permit

Every ADU needs a building permit. The city's PRADU fact sheet lays out the steps: download the permit materials, complete the plan package, consult staff if needed (buildingdivision@san-marcos.net), submit, and go through review by the Planning, Building, Land Development Engineering and Fire Prevention divisions. It also tells you to contact SDG&E, the school district and your water and sewer provider directly for their fees.

A San Marcos building permit can be issued only to the property owner or a California-licensed contractor. After final inspection, the Building Division issues a Certificate of Occupancy and the ADU can be occupied. Snap ADU reports a standard San Marcos ADU permit takes about 5 to 7 months. If the ADU is filed alongside a permit for a new main house, the city may wait to act on the ADU until it decides the house permit.

Before you hire anyone, read how to hire an ADU contractor, check a contractor's license, and compare verified ADU builders in San Diego County.

Common questions