Home Additions in San Mateo, CA
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When SFDDR Applies
Single-Family Dwelling Design Review (SFDDR) is required for a new two-story home, a replacement home, any second-story addition to a one-story home, and second-story additions over 50 sq ft on a two-story home. Additions to a two-story home of up to 200 sq ft or 20% of floor area, whichever is less, can go straight to a building permit if they follow the guidelines. The guidelines set heights of 24 feet to the plate and 32 feet to the peak, with a daylight plane starting 12 feet up at the side lot line and sloping in at 45 degrees. Applicants reach out to neighbors before filing, and the Zoning Administrator decides or refers the project to the Planning Commission.
Addition Work
Trees, Sprinklers and Site Rules
Heritage trees: oaks 10 inches or more in diameter and other species 15 inches or more at 54 inches, plus all street trees. An arborist protection plan is required when construction comes within 10 times the trunk diameter, and every submittal includes a Tree Disclosure Statement.
Sprinklers: San Mateo Consolidated Fire, 2121 S. El Camino Real, (650) 522-7900, requires a sprinkler retrofit when work affects more than 50% of the existing floor area.
- A stormwater (STOPPP) permit for more than 20 cubic yards of soil disturbance.
- On slopes of 25% or more, grading is limited to the driveway and foundations, generally April 15 to October 15.
- A 2024 FEMA map revision removed about 1,600 addresses in North Shoreview and North Central from the flood zone; we check your parcel’s current status.
330 W. 20th Avenue, San Mateo, CA 94403 · Community Development (650) 522-7200
Planning (650) 522-7212 · Permit and inspection help (650) 522-7172
Building counter Monday, Wednesday and Thursday 8 a.m. to noon and 1 to 4:30 p.m.; Tuesday and Friday mornings (Friday afternoons by appointment). Permits run through the Customer Self-Service (CSS) portal. Inspections are requested there by 3 p.m. for the next day; the inspector calls before 9 a.m. with a two-hour window.
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Other San Mateo services: Bathrooms · Kitchens · New Homes · San Mateo ADU builder · San Mateo overview
Same service nearby: Burlingame · Belmont · Hillsborough
San Mateo Home Additions FAQ
Does a second story on a one-story home need design review?
Yes, Single-Family Dwelling Design Review.
Can a small addition skip design review?
Additions to a two-story home of up to 200 sq ft or 20% can go straight to building permit.
What are the height guidelines?
24 feet to the plate and 32 feet to the peak.
When are sprinklers required?
When work affects more than 50% of the existing floor area.