The ADU Permit Process in San Mateo County (2026)

Getting a permit for an ADU in San Mateo County is more navigable than it was five years ago — but it still requires understanding how state law interacts with local rules across more than a dozen cities on the Peninsula. Whether you're planning a garage conversion in Redwood City, a detached backyard unit in Menlo Park, or an attached addition in Burlingame, the ADU permit process in San Mateo County follows a common framework with important city-by-city variations. This guide walks through that process step by step.

Why Permits Matter — and What Happens Without One

Skipping the permit process is a significant risk that homeowners sometimes underestimate. An unpermitted ADU cannot be legally rented, cannot be disclosed as a legitimate dwelling unit when you sell, and may trigger code enforcement proceedings that require costly demolition or retrofitting. Lenders and buyers flag unpermitted structures, which can kill a sale or force a price reduction that far exceeds what permits would have cost.

Beyond the financial risk, unpermitted construction often skips critical inspections — electrical, framing, and waterproofing checks that exist to protect occupants. The permit process is not bureaucratic friction; it is the mechanism that ensures the unit is safe and legally habitable.

How State Law Changed the Landscape

California's ADU reform legislation — AB 68, SB 9, and a series of follow-on bills updated through 2025 — fundamentally shifted the balance between state and local authority. Under current law:

• Cities and counties cannot deny an ADU application that meets the state's objective standards, regardless of neighborhood opposition.
• Owner-occupancy requirements were eliminated for ADUs built between 2020 and 2025; confirm current status for your timeline with your local building department.
• Setbacks for detached ADUs are capped at 4 feet from rear and side property lines for structures up to 16 feet in height — cities cannot impose stricter setbacks.
• Development impact fees are waived or capped for ADUs under 750 sq ft, and many Peninsula cities have reduced fees across the board.
• Cities must approve or deny ADU applications within 60 days of receiving a complete application; incomplete applications must be noticed within 30 days.

The practical effect: most single-family residential lots in San Mateo County can support at least a Junior ADU or garage conversion. Lot coverage limits, existing structure placement, and utility capacity still shape what is feasible, but outright denial is rare for compliant applications.

Who Issues the Permit: County vs. City

San Mateo County's unincorporated areas — portions of the coast and inland foothills — are governed by the San Mateo County Planning and Building Division. Every incorporated city in the county has its own building and planning department with its own forms, submittal portals, and review staff.

Cities in San Mateo County where we work include:

• Redwood City — Permits through the Community Development Department; online submittal available
• Menlo Park — Planning and Building Department handles ADU applications
• San Mateo — Building and Safety Division; one of the higher-volume ADU markets on the Peninsula
• Foster City — Community Development Department; smaller city with shorter review queues historically
• Daly City — Permit Center; southern gateway of the Peninsula
• South San Francisco — Building Division; benefits from proximity to SFO-corridor rental demand
• Burlingame — Community Development Department; premium rental market with strong ADU ROI
• San Bruno — Building Division; mid-Peninsula access to Caltrain corridor

Each city uses the state's 60-day review clock, but actual timelines depend on staff capacity and the completeness of your submittal package.

The ADU Permit Process in San Mateo County: Step by Step

The table below outlines the typical sequence for a detached or attached ADU in an incorporated Peninsula city. Garage conversions move through the same stages but generally faster, since structural engineering requirements are lighter.

Step	What Happens	Typical Duration
1. Site Assessment	Parcel review for setbacks, lot coverage, utilities, easements	1 – 2 weeks
2. Schematic Design	Floor plan, site plan, elevation drawings	2 – 4 weeks
3. Construction Documents	Engineered drawings, Title 24 energy calcs, structural calcs	3 – 6 weeks
4. Application Submittal	Submit to city building dept with fees	1 – 3 days
5. Plan Check (1st Review)	City reviews for zoning and building code compliance	3 – 8 weeks
6. Plan Check Response	Respond to correction comments, resubmit revised plans	1 – 3 weeks
7. Permit Issuance	City issues building permit; construction can begin	1 – 5 days
8. Construction + Inspections	Framing, rough MEP, insulation, drywall, final inspections	10 – 26 weeks
9. Certificate of Occupancy	Final inspection passed; unit is legally habitable	1 – 2 weeks

Total typical range: 7 – 16 months from site assessment to certificate of occupancy, depending on ADU type and city workload. This range reflects real Peninsula conditions; our team cannot guarantee permit timelines, as review queues are controlled by each city.

What Goes Into the Permit Package

A complete ADU submittal in San Mateo County cities typically requires:

• Site plan showing property lines, existing structures, setbacks, and proposed ADU footprint
• Floor plans at 1/4" scale with room dimensions, egress windows, door swings
• Elevations showing all four sides of the ADU with exterior materials noted
• Foundation and structural drawings (stamped by a licensed engineer for new construction)
• Title 24 energy compliance documentation — California's mandatory energy code
• Mechanical, electrical, and plumbing (MEP) plans
• Grading and drainage plan if site grading is involved

Missing or incomplete documents are the single most common cause of delays. Cities issue a "plan check correction notice" listing deficiencies, and each round of corrections adds weeks to the schedule. Submitting a complete, code-compliant package the first time is the most reliable way to keep the process on track.

ADU Costs and What Permits Add

Permitting fees and engineering costs are part of the total project budget. Bay Area 2026 cost ranges by type — all figures vary by scope; get a fixed quote before committing:

ADU Type	Typical Cost Range
Garage Conversion	$120,000 – $200,000
Attached ADU	$200,000 – $350,000
Detached New Construction	$250,000 – $450,000+ (~$400–$650/sq ft)

Permit fees, engineering drawings, and energy compliance documentation typically add $15,000–$35,000 to these ranges depending on city fee schedules and project complexity. These costs are embedded in our fixed-price proposals.

The Design-Build Advantage for Permits

Fragmented projects — where the homeowner hires a designer separately, then a permit expediter, then a general contractor — create handoff points where errors multiply and accountability diffuses. When design and construction are under one roof, the team that produces the drawings also builds from them, which means the plans are buildable and the permit package reflects how the project will actually be constructed.

Our ADU design-build service manages the entire process: site assessment, design, structural engineering, permit submittal, construction, and final inspections. One contract, one point of contact.

Frequently Asked Questions

How long does the ADU permit process take in San Mateo County?

It depends on ADU type and the specific city. Garage conversions in cities with shorter review queues can move from application to permit in 2–4 months. Detached new construction in higher-volume departments can take 4–8 months just for the permit, before construction begins. Total project timelines — design through certificate of occupancy — typically range from 7 to 16 months. Our team tracks every open permit application and responds to city comments quickly to avoid unnecessary extensions.

Can a city in San Mateo County deny my ADU application?

Under California state ADU law, cities have very limited grounds to deny an ADU that meets objective standards on setbacks, height, lot coverage, and utility connections. Political opposition, neighborhood aesthetics, and general opposition to density are not valid bases for denial. If your ADU application meets the state's objective standards and the city rejects it anyway, you have legal recourse. In practice, the vast majority of compliant applications in San Mateo County cities are approved.

Do I need an architect or engineer for an ADU permit?

Architectural drawings are required for any permitted ADU. A licensed engineer's stamp is required for structural drawings on new construction and for any work that modifies the structural system of an existing building (including some garage conversions with shear wall upgrades). Title 24 energy compliance documentation must be prepared by a certified energy analyst or someone with equivalent credentials. Our team includes licensed architects, structural engineers, and energy consultants — all handled internally as part of the project.

Plan Your ADU With Our Team

The ADU permit process in San Mateo County is manageable with the right preparation and a team that knows how each city's department operates. We've been serving Peninsula homeowners since 2012 — CSLB #975092, rated 4.8 across Google, Houzz & Thumbtack — and we back every project with a 1-year workmanship warranty.

If you're considering an ADU in Redwood City, Menlo Park, San Mateo, Burlingame, or anywhere on the Peninsula, the first step is a site assessment and honest feasibility conversation. No commitment required.

Schedule a free ADU consultation — or browse our free planning guides to go into that conversation well-prepared.

Related reading:

• How Much Does an ADU Cost to Build in San Jose? (2026)
• How Much Does an ADU Cost in Sunnyvale? (2026)