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Permit guides
ADU Permits in Millbrae: a Homeowner's Guide
Millbrae offers pre-approved ADU plans — but pairs them with some of the Peninsula's most detailed site requirements.
Last updated October 2026. Rules change; always confirm current requirements with the city before you build.
Who handles your permit
ADU permits in Millbrae run through the Community Development Department, 621 Magnolia Ave., Millbrae, CA 94030 (Planning: 650-259-2341). The city publishes a Pre-Approved ADU Checklist that governs the whole submittal — read it before you draw anything.
What you can build
1,200 sq ftMax for a detached ADU under state law
Pre-approvedVendor plan sets available — vendor authorization required to use them
10 ftA surveyor map is required if the ADU sits within 10 feet of any property line
50 ftTree inventory required for all trees over 5 inches within 50 feet of construction
5% slopeA geotechnical report may be required where the site slope exceeds 5%
4 ftMinimum side and rear setbacks for detached ADUs
Pre-approved plans, Millbrae style
Millbrae's program works through vendor plan sets: your submittal must include the vendor's written authorization to use the pre-approved plans, plus the city's planning pre-approved ADU approval stamp on the floor plans and elevations. The building design is pre-cleared — but everything about your site is reviewed from scratch.
What goes into the submittal
- Grant deed with legal description, plus the vendor authorization for pre-approved plans
- Site plan with property lines, dimensions, lot size, roadways, and easements called out; dimensioned ADU setbacks measured at the closest point to property lines and other structures
- Water and sewer line locations; existing and proposed driveways and parking (standard space is 9' × 19')
- Tree inventory: location, species, common name, diameter at 54 inches above grade, and driplines for every tree over 5 inches within 50 feet of construction
- Grading and drainage plan showing existing and finish grades, piping with size, slope, and alignment, and the method of discharge
- Foundation plan and details — engineer-stamped depending on soil conditions
Trees and slopes drive the extras. Any protected tree within 10 feet of the proposed ADU triggers an arborist report, and slopes over 5% can trigger a geotechnical investigation. On Millbrae's hillside lots, budget for both early.
Where applications usually get stuck
- Tree requirements. The 50-foot inventory and 10-foot arborist trigger catch almost every applicant with mature landscaping.
- Surveyor maps. ADUs tucked close to property lines need a licensed survey — plan for the cost and the lead time.
- Geotechnical surprises. Millbrae's hills mean soil reports surface late if nobody checks the slope first.
Not sure where to start? That's the part we do. We check your property against the requirements, coordinate the plans and specialists, and manage the submittal and corrections — one point of contact.