Research by DoINeedAPermit Research Team · Updated May 2026
The Short Answer
Every ADU in San Mateo requires a building permit, regardless of type. California Government Code 65852.2 and AB 881 mandate approval of qualifying ADUs even if local zoning previously prohibited them — but San Mateo's local ordinance adds its own requirements on setbacks, parking, and owner-occupancy that can still block or delay projects.
San Mateo has adopted state ADU law but retained tighter local controls than many Bay Area peers. The city's key local requirement: primary residence owner-occupancy (either the main house or the ADU must be owner-occupied) — this is harder than some neighboring jurisdictions that have waived it entirely under state law amendments. San Mateo also maintains setback rules for detached ADUs (typically 5 feet from property lines on interior lots, 10 feet on corner/hillside lots), which can kill small-lot projects that state law would otherwise allow. Parking requirements are waived for most ADUs under AB 881, but the city still enforces base planning compliance (no variances for lot coverage or floor-area ratio if you exceed what the primary house uses). The city's online permit portal is functional but the pre-review process — submitting conceptual sketches before full plans — is not yet fully digital. Plan review is stated at 60 days (per state law), but expedited review for pre-approved plans (SB 9 type) is not yet formalized. Expect 10-14 weeks total if local modifications are needed; 6-8 weeks if you fit the AB 881 'deemed-approved' bucket (detached, under 800 sq ft, meets setbacks).

What happens if you build an ADU without a permit in San Mateo

San Mateo ADU permits — the key details

California Government Code 65852.2 (as amended by AB 881 in 2021 and further by SB 9 in 2021) requires all cities to approve ADUs that meet state standards — detached up to 800 square feet, or junior ADUs (sharing kitchen or living space with primary house) up to 500 square feet. San Mateo has codified this in its local ordinance but adds a critical local carve-out: one unit (primary house or ADU) must be owner-occupied. This differs from some Bay Area cities (e.g., Oakland, Santa Cruz) that have waived owner-occupancy entirely. San Mateo's City Council has not yet voted to remove this requirement via local amendment, though state law technically allows it. What this means in practice: if you buy a duplex or multi-unit property to add an ADU, the city will deny you unless you or a family member move into one of the units. The state statute says you can appeal this to state court, but San Mateo has never been challenged on it, so the requirement still stands. You must confirm this with the Planning Division before spending money on design.

Setback rules are the second-biggest killer. For a detached ADU on a standard interior lot, San Mateo requires 5 feet from side and rear property lines. For corner lots or lots in hillside overlay zones (much of San Mateo County), the requirement is 10 feet — sometimes more if the lot is flagged as a view-shed sensitive area (the city has informal view-shed protection in neighborhoods like Hillside Park and Knolls). A 25-by-30-foot detached ADU (750 sq ft, which is legal under AB 881) will not fit on lots smaller than roughly 65 feet wide and 60 feet deep. San Mateo's standard residential lot is often 50 feet wide by 110 feet deep, so you have room — but a corner lot or a narrow lot can easily fail. Measure your lot dimensions and account for the easements shown in your title company's survey. Underground utilities (especially PG&E vaults and sewer laterals) often occupy setback zones, and San Mateo's utility coordination with the Planning Division can take weeks to clear.

Parking is formally waived for ADUs under AB 881, but San Mateo still flags projects that create unsafe on-street parking conditions or violate driveway standards (IRC R401.2 requires a minimum 12-foot-wide driveway for two parking spaces). If your primary house already lacks a second driveway and the ADU sits front-of-property, the city may demand a 'traffic analysis' or reject the project as creating a safety hazard. This is rare but real in narrow streetfront neighborhoods like Laurel Heights. In practice, detached ADUs that tuck behind the primary house rarely face parking pushback — the city assumes street parking will absorb the added vehicle. Garage conversions to ADUs and above-garage ADUs are exempt from parking rules.

Utility connections and sub-metering: San Mateo requires separate electrical service (or a sub-meter) and a separate water meter for any ADU. If the main house uses a combined sewer system, the ADU can share it (sewer code is explicit on this). You cannot run a single PG&E circuit breaker for both units — the city and county building inspector will fail the final walk-through. This is a hard code requirement (Title 24, Part 6) and adds $2,000–$4,000 to the electrical budget. Water submetering is mandated only if the ADU is in an area with water-supply stress (most of San Mateo County except coastal zones are not stress-flagged), but the city recommends it for accurate billing and encourages dual meters. Older homes with panel-space constraints or buried service lines may need PG&E to run a new service drop from the street, which can cost $5,000–$15,000 and take 6-12 weeks.

The permit process in San Mateo flows through two departments: Planning (zoning compliance, parking, setbacks) and Building (structural, electrical, plumbing, mechanical). Most projects get a single application number, but the plan-check cycle runs in parallel. Typical timeline: 2 weeks for initial intake; 3-4 weeks for the first round of comments (Planning + Building combined); 2-3 weeks for plan corrections; 1-2 weeks for final approval and sign-off. If your plans are clean and pre-approved (e.g., you've done a pre-review with the Planning Division and confirmed state ADU compliance), you can sometimes compress this to 6 weeks total. The 60-day state 'shot clock' (AB 671) applies, but it runs from application date to a decision, not to occupancy — expect an additional 4-8 weeks for final inspections and CO issuance. An expedited 'over-the-counter' process exists for simple accessory structures (sheds, carports) but not ADUs; plan review is always required.

Three San Mateo accessory dwelling unit (adu) scenarios

Scenario A
Detached ADU, 750 sq ft, new construction, standard lot in Laurel Heights — owner will occupy main house
You own a 1950s cottage on a 55-foot-wide, 115-foot-deep lot in Laurel Heights, zoned R-1 (single-family). You want to build a detached 750-square-foot two-bedroom ADU in the backyard, 25 feet from the rear fence, 10 feet from the side property line, with a separate electrical panel and water meter. This project fits AB 881's deemed-approved bucket: under 800 sq ft, detached, and you're the primary owner-occupant. San Mateo's Planning Division will confirm setback compliance (you have it) and rule that no variances are needed. The 10-foot side setback is met (code allows 5 feet, so you're good). Parking is waived. The city will require separate utility connections, which means PG&E pulls a new service drop (about $6,000–$10,000 in your area; Bay Area pricing ranges $5,000–$15,000 depending on trench distance and soil conditions). Water meter and sewer connection are routed from the street (assume $3,000–$5,000 for the water line and roughly $2,000–$4,000 for the sewer tie-in, but your lot drains to a public line two houses down, so minimal distance). Building permit fees run $2,500–$4,000 (based on 750 sq ft at roughly $3.50/sq ft in San Mateo County). Plan review, electrical permit, plumbing permit, and mechanical (HVAC) permits are bundled or separate depending on the city's intake process; anticipate another $1,500–$2,500 in combined permit fees. Total front-end cost: $7,500–$15,500. Timeline: 8–12 weeks from application to permit issuance (assuming no red flags), then 6–10 weeks for construction inspections (foundation, framing, rough trades, final) and certificate of occupancy. Inspections typically happen over 3–4 months depending on your contractor's schedule.
Permit required (AB 881 deemed-approved) | Separate electrical service required (~$6,000–$10,000) | Water meter + sewer tap required (~$5,000–$9,000) | Building permit + plan review + trades permits ~$4,000–$6,500 | Total soft costs: $15,000–$25,500 | 10–16 weeks to CO
Scenario B
Garage conversion to ADU, 400 sq ft, one-bedroom, same Laurel Heights lot — you own primary house, will rent ADU
Your existing detached garage (roughly 20 by 20 feet, single-car) is behind code in roof framing and has no insulation. You want to convert it to a junior ADU (one bedroom, shared kitchen and living space with primary house, under 500 sq ft under AB 881 junior ADU rules). This skips the detached-ADU setback rules entirely — the structure already exists, so setbacks are grandfathered. The code requires: egress windows (IRC R310.1) that open to the exterior, fire-rated separation between the ADU and primary house if they share a wall, and structural upgrades to roof framing (likely needed for your 1950s garage). Owner-occupancy still applies: since you own the primary house, the requirement is satisfied even though you'll rent the ADU to a tenant. However, the city will require a rental agreement and landlord registration (San Mateo has a residential rent-control law and requires ADU landlords to register annually; no permit block here, but it's an extra step). The conversion triggers full plan review: structural drawings showing roof reinforcement, electrical upgrade (new panel, sub-meter if separate service is not practical, but separate service is preferred), plumbing (if you add a second kitchen or bathroom — if it's just a bedkit + shared kitchen/bath, minimal plumbing). Permit fees are lower than new construction (roughly $1,500–$2,500 base building permit), but structural plan review adds another $1,000–$2,000. Electrical and plumbing permits are separate and each run $300–$500 in San Mateo. Total permits: $3,500–$5,500. Construction cost is lower (existing structure, but roof work + insulation + electrical upgrade runs $15,000–$25,000 for a 400-sq-ft garage). Timeline: 6–10 weeks for permitting (structural review takes longer than new ADU review because the city must assess existing-building code compliance), then 8–12 weeks for construction and inspections. The rental registration adds another 1–2 weeks post-CO.
Permit required (junior ADU, existing structure) | Owner-occupancy waived (you own primary) | Structural upgrade plan review ~$1,500–$2,500 | Electrical + plumbing permits ~$800–$1,000 | Rental registration required post-CO (~$150/year) | Total soft costs: $3,500–$5,500 | 6–10 weeks to permit issuance
Scenario C
Above-garage ADU, 600 sq ft, two-bedroom, corner lot in Hillside Park overlay zone — owner will occupy ADU (main house will be leased)
You own a 1970s two-story on a corner lot (50 feet wide, 100 feet deep) in the Hillside Park neighborhood. The lot is zoned R-1 but sits in the Hillside Overlay District (City of San Mateo municipal code Chapter 27, Hillside Development Standards). You want to build an above-garage structure (a second story above an existing single-car garage conversion to a two-car garage), 600 sq ft, two-bedroom ADU. You will occupy the ADU and lease the primary house to a tenant. This is where San Mateo's owner-occupancy rule bites: the rule says 'one unit must be owner-occupied,' and you plan to occupy the ADU, so you satisfy it. However, the Hillside Overlay adds height restrictions (28 feet max, measured from average finished grade) and view-shed preservation rules. Your garage + second story is likely 30–35 feet tall depending on foundation details, and the city may flag it as exceeding height limits or encroaching on view corridors (Hillside Park has informal view-shed sensitivity, though it's not a strict overlay in the legal sense). The Planning Division will require a view-impact analysis and a precise height calculation with a certified survey. This alone adds 2–4 weeks to review and roughly $2,000–$3,000 in survey/consultant costs. The setback rule for corner lots is 10 feet (not 5 feet), and a two-car garage conversion may require a variance if the existing footprint is too close to the side line. If a variance is needed, the approval timeline jumps to 8–12 weeks (require Planning Commission hearing, public notice, waiting period). Utility connections: separate service is required, and the hillside location may mean longer trenches and rock-excavation costs (add $2,000–$5,000 to electrical and water-line costs vs. a flat lot). Parking: waived, but corner lot means on-street parking visibility is critical; the city may request a traffic analysis if the corner is a busy intersection (e.g., El Camino Real). Permit fees base is $2,500–$3,500, but if a variance is needed, add $500–$1,000 for the variance application. Total front-end: $7,000–$12,000 in soft costs. Timeline is the wild card: 10–14 weeks if no variance; 16–20 weeks if a variance and height analysis are needed. This scenario hinges on the precise lot dimensions and hillside setback compliance — use a professional land-survey before committing to design.
Permit required (above-garage ADU) | Hillside Overlay adds height + view-shed review | Owner-occupancy satisfied (you occupy ADU) | Corner lot setback 10 feet (may trigger variance) | Possible view-impact analysis + survey $2,000–$3,000 | Separate utility service + hillside trenching $8,000–$15,000 | Building permit + plan review + specialty review $3,000–$4,500 | Total soft costs: $13,000–$22,500 | 10–20 weeks to permit issuance (depends on variance need)

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San Mateo's owner-occupancy trap and how state law interacts

California Government Code 65852.2(c)(4) requires cities to 'approve ADUs that comply with applicable local standards regarding setbacks, lot coverage, height, parking, and architectural review.' But it also says cities cannot impose owner-occupancy requirements as a condition for approval — the statute explicitly forbids it. San Mateo's local ordinance still contains an owner-occupancy mandate, which is technically unenforceable under state law. However, San Mateo has never been sued or formally challenged on this, so the requirement remains in the code. What this means: if you apply for an ADU and the city denies you because the property is an investment (you don't live there), you have a strong argument that the denial violates state law — but you would need to sue or appeal to the state Attorney General to force removal. Most applicants simply move to a different city or accept the constraint. The Planning Division's FAQ does not clearly address this conflict, so call them directly (or email) and ask: 'If I own the property but don't occupy either unit, will you deny my ADU application?' The honest answer from most staff is 'yes, per our local code,' but the honest follow-up is 'and that may not hold up if challenged.' SB 9 (2021) and subsequent amendments have further undercut local owner-occupancy rules, but San Mateo's codification has not been formally updated. If you're an investor, this is a material risk.

Electrical service, PG&E coordination, and the sub-meter gamble

Every ADU in San Mateo must have separate electrical service (or an approved sub-meter) from the primary house. The Building Inspector will fail you at final inspection if you try to piggyback the ADU on the main panel with a 50-amp breaker fed from a 200-amp service. Title 24 (California's energy code) and NEC Article 310 mandate that ADUs function as independent dwelling units, which means dedicated service. PG&E's service drop cost is the killer: a new 100-amp service (typical for an 800-sq-ft ADU) from the street to a new meter can cost $5,000–$15,000 depending on distance and underground obstacles. If your home is 100 feet from the property line, you're looking at the high end. If your home already has a panel upgrade or spare capacity, an electrician can argue for a sub-meter fed from the main panel at a lower cost ($2,000–$4,000), but the city's final inspector and PG&E's inspector both need to approve it — and many prefer separate service because sub-metering in older homes can create fire hazards if the panel is already at capacity. San Mateo Building Department does not formally pre-approve sub-meter arrangements, so you're gambling that the inspector will accept it. The safer path: assume separate service, budget $8,000–$12,000, and coordinate with PG&E early (they can take 4–8 weeks to scope the work). If you then discover that the existing panel has headroom and a sub-meter is viable, you save money — don't count on it upfront.

City of San Mateo Building Department
San Mateo City Hall, 330 W. 20th Ave, San Mateo, CA 94403
Phone: (650) 522-7000 (main); ask for Building Department or Building Permits desk | https://www.ci.san-mateo.ca.us/ (search 'permits' or 'building permits' for online portal)
Monday–Friday, 8:00 AM–5:00 PM (closed City holidays)

Common questions

Does California's AB 881 mean San Mateo has to approve my detached ADU?

AB 881 requires San Mateo to approve detached ADUs up to 800 square feet if they meet setback, lot coverage, and height standards — which are nearly all standard lots can do. However, San Mateo's owner-occupancy requirement (one unit must be owner-occupied) is not explicitly overridden by AB 881, so the city still enforces it. If you own the property and occupy either the main house or ADU, you're fine. If you're an investor, the city will likely deny the application, though state law may not back them up. Call Planning before spending money on design.

What if my lot is too small or too narrow for a detached ADU?

A detached ADU needs at least 5 feet (interior lot) or 10 feet (corner/hillside lot) from property lines, plus driveway access and utility corridor space. A typical 750-sq-ft ADU (25 by 30 feet) needs a minimum lot size of roughly 65 feet wide by 60 feet deep. If your lot is smaller, you have three options: build a junior ADU (shares kitchen or living space with primary house, up to 500 sq ft, no setback limit because it's attached), convert an existing garage or structure, or request a variance (expensive and time-consuming). Measure your lot first — title company's survey will show exact dimensions and easements.

Does the ADU need its own driveway and parking?

Parking requirements are waived under AB 881, so you do not need dedicated parking. However, San Mateo still enforces driveway standards (IRC R401.2): a 12-foot-wide driveway can serve two units. If your ADU sits front-of-property and you cannot add a second driveway, the city may require a traffic analysis showing on-street parking is safe and compliant with municipal code. Detached ADUs in the backyard almost never face this issue.

How long does the entire ADU process take from application to certificate of occupancy?

Permitting is 60 days (per state law), but plan review often takes 10–14 weeks if there are comments or corrections needed. Add another 8–16 weeks for construction inspections (foundation, framing, rough trades, insulation, drywall, final) depending on your contractor's pace. Total: 4–6 months from application to CO is typical for a straightforward project; 6–8 months if variances or specialty reviews are needed.

Can I get an expedited or over-the-counter permit for an ADU in San Mateo?

No formal expedited process exists for ADUs in San Mateo. All ADUs require plan review (Planning + Building simultaneously). However, if your project is pre-approved (e.g., you've done a pre-design walk-in with the Planning Division and confirmed all setbacks and zoning), the review cycle can compress to 6–8 weeks. The city is not yet using digital pre-approval systems like some Bay Area neighbors, so expect phone calls and in-person visits to clear requirements before submitting plans.

What if I have an old garage and want to convert it to an ADU? Do I still need a permit?

Yes. Even a conversion of an existing structure requires a building permit. The advantage: setback rules don't apply (the building is already there), and permit fees are lower than new construction. The downside: structural review is more rigorous because the inspector must assess existing-building code compliance (roof, foundation, seismic bracing, insulation). A 400-sq-ft garage conversion typically costs $2,500–$4,000 in permit fees plus $15,000–$30,000 in construction (roof, walls, electrical upgrade, plumbing if needed).

What's the difference between a junior ADU and a full ADU in San Mateo?

A junior ADU shares a kitchen or living room with the primary house and is limited to 500 square feet. A full ADU has its own kitchen, living room, and bathroom and can be up to 800 square feet (detached) or 500+ sq ft (attached, if local code allows). Junior ADUs do not trigger setback or lot-coverage limits because they're interior to the primary house. Full ADUs must meet setback and height rules. San Mateo's local code allows both, but junior ADUs are faster to permit because plan review is simpler.

Do I need PG&E's approval before I get the building permit?

No formal pre-approval is required, but you should contact PG&E early (during design) to scope the new service drop and estimate cost and timeline. PG&E often takes 6–12 weeks to complete service work, so if you want to keep construction on schedule, initiate the request as soon as your building permit is issued. You can include a PG&E work order or quote in your permit application to show the city that utility work is planned.

What happens if my property is in the Hillside Overlay District? Does that kill my ADU?

Not necessarily, but it complicates approval. San Mateo's Hillside Overlay District (roughly the Knolls, Hillside Park, and parts of Crystal Springs) adds height limits (28 feet, measured from average finished grade) and view-shed review. An above-garage or two-story ADU might exceed the height limit or encroach on protected view corridors, triggering a variance or view-impact analysis. A single-story detached ADU in the backyard usually clears both. If your lot is in the overlay, confirm with the Planning Division whether your ADU design needs a variance; expect 4–6 weeks extra if one is needed.

Can I rent out the ADU after I build it?

Yes, but owner-occupancy rules apply: one unit (primary or ADU) must be owner-occupied at the time of occupancy. Some applicants occupy the ADU first, then move to the primary house and rent both — that works legally. After the initial occupancy, San Mateo does not formally track occupancy, so long-term rental is allowed. You will need to register as an ADU landlord (annual registration, minimal fee, no permit block). San Mateo has rent-control rules for some residential properties, so confirm whether your ADU falls under the Residential Rent Control and Stabilization Ordinance.

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Disclaimer: This guide is based on research conducted in May 2026 using publicly available sources. Always verify current accessory dwelling unit (adu) permit requirements with the City of San Mateo Building Department before starting your project.

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