What happens if you skip the permit (and you needed one)
- Stop-work order plus $500–$2,000 fine from City of Millbrae Building Department; you must pull a permit and pay double-fee retroactive permit ($2,000–$4,000) to legalize the work.
- Home sale complications: title insurance may flag unpermitted ADU; buyer's lender will refuse financing, and you'll be forced to disclose the unpermitted unit in your Transfer Disclosure Statement, tanking the sale or killing the deal entirely.
- Tenant liability trap: if an unpermitted ADU tenant is injured (fall, fire, electrical shock), your homeowner's insurance will deny the claim and you face personal liability up to $1 million+ depending on injury severity.
- Code enforcement lien: San Mateo County can place a non-monetary lien on your property to force removal or legalization; even if you eventually permit it, the lien stays on title until the city signs off.
Millbrae ADU permits — the key details
California Government Code 65852.2 (amended by AB 881 effective 2022) mandates that Millbrae allow one ADU per single-family residential lot, plus one junior ADU (smaller, attached unit with shared kitchen or laundry), regardless of local zoning. This is a state-override — Millbrae cannot say no based on land-use policy. However, the city absolutely requires a building permit for every ADU type: detached new construction, garage conversion, second-story above-garage, or junior ADU all trigger full plan review. The permit includes building, electrical, mechanical, plumbing, and planning sign-offs. No exemptions exist for ADUs in Millbrae, even if the unit is tiny (e.g., junior ADU under 500 sq ft). The city's online portal (e-permitting system) accepts ADU applications 24/7; you upload floor plans, site plans, utility diagrams, and proof of lot ownership or authorization. Most applications are deemed complete within 3–5 days and enter plan review immediately.
Millbrae's most important local rule is the parking requirement: the city requires one off-street parking space per ADU, even if you live car-free or the tenant does. This is NOT waived by state law (unlike in Oakland, San Jose, or San Francisco where parking waivers are common). However, the city does allow tandem parking (driveway stacking), shared driveway frontage, or a good-faith waiver application if you can argue hardship (e.g., lot too small, pre-existing non-compliant parking, easement blocking). The parking requirement is a major cost driver in Millbrae because most lots are small (5,000–7,500 sq ft in the older neighborhoods near El Camino Real). A detached ADU on a narrow 50-foot-wide lot may not have room for a new space; in that case, you either reduce ADU scope, apply for a waiver, or pay $40,000–$60,000 to construct a separate driveway and pad. Parking waivers take 2–4 weeks of additional back-and-forth with the planning department.
Setbacks and lot coverage are the second major hurdle for detached ADUs in Millbrae. The city's municipal code (adopted from the state model ordinance) requires detached ADUs to comply with standard setbacks: typically 5 feet from side and rear property lines, and 20–25 feet from the front (varies by street width). Millbrae's lots are often narrow and deep (e.g., 50 feet wide by 150 feet deep), which means a detached ADU can only fit in the very back corner, and that placement may violate rear-setback if you're already 5–10 feet from the property line with fencing or easements. Lot-coverage limits (typically 50–65% of lot area) are also tight in Millbrae; if your main house is 2,000 sq ft on a 7,000 sq ft lot, you've used 28.5% coverage — but add a 1,000 sq ft ADU and you're at 42.8%, which may trigger design review or require variance. Before paying for plans, measure your lot and run the math with the City of Millbrae Planning Department (they offer a free 30-minute pre-application consultation, highly recommended).
Utility connections are mandatory separate utility infrastructure for all detached ADUs and required-kitchen junior ADUs in Millbrae. This means: (1) separate water service line from the street or sub-metering if you share the main line; (2) separate electrical panel and service (or sub-metering per NEC 705.12); (3) separate gas if applicable; (4) separate sewer and storm connections or dedicated grease trap if ADU is rental or commercial kitchen. The electrical sub-metering must meet NEC 705.12(D)(1) and be accessible to the utility company for reading. Water sub-metering must be installed by the city's water department or an approved contractor. These utilities typically add $8,000–$15,000 to project cost (water main line: $5,000–$8,000; electrical sub-panel: $2,500–$4,000; sewer connection: $3,000–$6,000, depending on ADU distance from main house). Millbrae requires utility plans to be stamped by a licensed engineer or plumber and reviewed by PG&E (electrical) and Millbrae Water Department before building permit issuance.
The city's 60-day shot clock (AB 671 effective 2018) applies to ADU permits, BUT the clock is suspended if the application is deemed incomplete, if you request changes, or if BCDC or airport consultants need to review (both apply in Millbrae). Realistic timeline: submit application → 3–5 days for completeness review → 2–3 weeks plan review → 1 week for minor revisions → permit issuance. If BCDC or airport review is triggered (wetlands, flight path, noise), add 3–6 weeks. Most Millbrae ADUs are issued within 8–12 weeks. Once issued, inspections follow standard building sequence: foundation/footings → framing → rough trades (electrical, plumbing, HVAC) → insulation → drywall → final + planning verification. Owner-builders can pull permits if they live in the ADU; all electrical work must be done by a California-licensed electrician or self-inspected via the state's owner-builder electrical permit program (adds $300–$500 and 2–3 weeks). Plumbing can be owner-installed but must pass city inspection.
Three Millbrae accessory dwelling unit (adu) scenarios
Why California state law overrides Millbrae zoning — and what it means for your ADU
California Government Code 65852.2 (AB 68, effective 2017; amended AB 881 effective 2022) mandates that every city and county in California allow at least one ADU per single-family residential lot, even in neighborhoods zoned exclusively single-family. Millbrae cannot reject your ADU application based on zoning alone. However, this state preemption does NOT exempt you from local building codes, design review, parking rules, setback rules, or environmental review. Millbrae still has full authority over building safety (IRC/IBC), fire/seismic, utility capacity, and parking — and the city WILL enforce those standards. The quirk: Millbrae's parking requirement (one space per ADU) is not overridden by state law because parking is a development standard (not a land-use/zoning rule). Cities like San Jose and San Francisco have waived parking for ADUs; Millbrae has not. This means your ADU is allowed by state law, but you still need to solve the parking puzzle locally.
AB 881 (effective 2022) added second-unit flexibility: California now allows one ADU plus one junior ADU (shared kitchen, <500 sq ft) per single-family lot. This is huge for small lots — you can add a 400 sq ft junior ADU attached to the main house (minimal construction) plus a separate detached ADU (800 sq ft). Millbrae has adopted this dual-unit framework. However, both units still require permits, separate utilities (or sub-metering), and compliance with local standards. The state law does NOT waive parking, impact fees, or architectural review in Millbrae — those are local costs you must absorb.
Millbrae's position on pre-approved ADU plans: California allows cities to streamline approval for pre-approved plans (SB 9 plans, ADU model designs). Millbrae has not yet published an official pre-approved ADU design library, unlike San Jose or Oakland. However, you can request expedited review if your project matches a standard floor plan (e.g., 800 sq ft detached, 400 sq ft garage conversion) and your site is straightforward (no wetlands, no airport, no historic overlay). Expedited review can shorten timeline to 6–8 weeks. Contact the Millbrae Planning Department for a list of any accepted standard plans.
Millbrae utilities, BCDC, and airport overlays — what makes this Bay Peninsula location unique
Millbrae sits in a constrained utility and environmental zone that affects almost every ADU project. The city's water supply comes from San Francisco's Hetch Hetchy system (imported) with local recycled water available for irrigation. Separately, the city is governed by two powerful regional overlays: (1) Bay Conservation and Development Commission (BCDC), which has jurisdiction over any new structure within 100 feet of bay shoreline or significant wetlands, and (2) San Francisco International Airport (SFO) Airport Land Use Compatibility Plan (ALUCP), which applies noise, overflight, and safety regulations to properties in the airport influence zone (covers most of Millbrae). If your ADU lot is within either zone — which includes much of the south side of Millbrae and the entire eastern border near the Bay — you trigger additional review that suspends Millbrae's 60-day shot clock. BCDC exemptions for minor ADU structures take 4–6 weeks. Airport compatibility reviews take 2–3 weeks. These are not errors or delays; they are mandatory consultations that add 3–6 weeks to the timeline.
Water capacity is another Millbrae constraint: the city is in a water-shortage region subject to California State Water Resources Control Board restrictions. Millbrae must verify that adding an ADU does not exceed water-system capacity. For detached ADUs, the city requires a separate water meter or sub-meter; for junior ADUs, you may share the main house meter if water demand stays below a local threshold (typically 150 gpd total household + ADU). A studio ADU is usually low-demand (~75 gpd); a 2-bed ADU (~120 gpd) may exceed the threshold and force you to sub-meter. This is not a deal-breaker, but it adds cost ($2,000–$3,000) and requires Millbrae Water Department sign-off before building permit issuance.
Sewer capacity is tighter than water in Millbrae. The city's sewer system serves a dense corridor (El Camino Real, Millbrae Ave) and many pipes are from the 1920s–1950s. Millbrae Public Works does a sewer-capacity review for every ADU, especially if the unit is rental (higher occupancy assumption). If the main sewer line to your property is undersized or already saturated, you may be required to pay a sewer-capacity fee ($3,000–$8,000, depending on unit size and local conditions) or install a separate grease trap/treatment system. This is reviewed during plan-check, not at the counter; expect 1–2 weeks for sewer review, and if they flag an issue, you'll need a sewer engineer ($1,500–$2,500) to study options.
Millbrae City Hall, 621 Magnolia Ave, Millbrae, CA 94030
Phone: (650) 558-7600 — ask for Building & Planning Division | https://www.millbraeCA.gov/departments/community-development (building permits and e-permitting portal link)
Monday–Friday, 8:00 AM – 5:00 PM; closed city holidays. Pre-application meetings by appointment.
Common questions
Does California state law mean Millbrae can't charge impact fees for my ADU?
No. State law overrides zoning (Millbrae must allow the ADU) but NOT impact fees. California Government Code 65852.2(d) allows cities to charge reasonable development and utility connections fees. Millbrae charges building permit, plan review, and utility-connection fees for ADUs just like any other project. Expect $5,000–$12,000 in permits + fees depending on ADU type and utility work. Impact fees (traffic, school, park) are waived for ADUs under 500 sq ft per some state rules, but Millbrae may still charge connection fees (sewer, water).
Can I do an ADU without a separate electrical panel / sub-meter, or share utilities with the main house?
For a junior ADU (shared kitchen with main house), you can usually share water and electrical via sub-metering on the main panel, per NEC 705.12(D)(1). Millbrae requires sub-meter accessibility for utility billing and safety. For a full detached ADU with its own kitchen, separate electrical service is mandatory per NEC 690 and Millbrae code; you cannot share. Water and sewer can be sub-metered if you have a single connection line, but Millbrae requires separate meter boxes accessible to the city for reading. Plan on $3,000–$5,000 for electrical sub-metering; $2,000–$3,000 for water; $3,000–$6,000 for sewer (if a new tap is needed).
What happens if my lot is in the airport noise zone — does that block my ADU?
No, it does not block the ADU. However, you must comply with SFO Airport Land Use Compatibility Plan noise-insulation standards: 45 dB interior noise level for habitable rooms. This triggers design requirements (triple-pane windows, heavy insulation, HVAC noise-reduction) that add $8,000–$12,000 to construction cost. The airport compatibility review also adds 2–3 weeks to the permitting timeline. You will need an acoustic engineer or architect familiar with airport-adjacent design to prepare noise-mitigation specs. This is not a showstopper, but it is a real cost and timeline factor in Millbrae.
Does Millbrae require owner-occupancy — can I build an ADU just for rental income?
California state law (Gov Code 65852.2) eliminated owner-occupancy requirements as of 2022. Millbrae does NOT require you to live in the main house or ADU. You can own a home in Millbrae and rent both the main house and ADU, or rent only the ADU to a tenant. However, the city still requires the ADU to be a legal dwelling unit (full kitchen, bath, egress, utilities, etc.) and comply with all building codes. Rental ADUs may trigger higher sewer/water-demand assumptions and may require additional insurance or rental licensing (check with Millbrae Planning on any rental registration rules).
I have a lot near the Bay — does BCDC approval take a long time and cost money?
If your lot is within 100 feet of bay shoreline or wetlands, you need BCDC (San Francisco Bay Conservation and Development Commission) clearance. Most minor ADU conversions (garage conversions, modest detached units not requiring fill or bay disturbance) are eligible for BCDC exemption, which means no BCDC permit needed, just an exemption letter (4–6 weeks, no cost if you file yourself). Some projects require a minor permit ($500–$1,500 and 6–8 weeks). Contact BCDC directly at (415) 352-3600 or check their online application portal. In parallel with BCDC, file your Millbrae building permit; the city will accept your BCDC exemption letter as part of environmental clearance. Total timeline impact: add 4–6 weeks if BCDC is triggered.
My lot is very small (4,500 sq ft) — can I still build an ADU if it violates setbacks?
Yes, but you likely need a variance or design waiver. Millbrae's detached ADU setbacks are 5 feet from side/rear (and 20–25 feet from front). On a very small lot, these constraints may make a detached ADU impossible. Your options: (1) pursue a parking waiver + reduced setback waiver (apply to Planning, 4–6 week process, no guarantee of approval); (2) build a garage conversion or junior ADU instead (attached units have less stringent setback rules); (3) work with an architect to propose a compliant footprint (e.g., smaller ADU, different location on lot). Most Millbrae Planning staff will advise you in a free 30-minute pre-application meeting whether a waiver is realistic before you pay for design.
How much does a Millbrae ADU permit actually cost, all-in?
Permit and plan-review fees typically run $4,500–$6,000 for a detached new-construction ADU or second-story addition; $2,500–$3,500 for a garage conversion. Add design costs: architect or engineer ($2,000–$5,000 for simple projects, more if BCDC or structural work is needed); utility engineer ($1,500–$2,500 if sub-metering or new connections are required). Total soft costs (permits + design): $8,000–$13,000. Construction cost depends on type: detached new ADU (800 sq ft): $150,000–$200,000 (labor + materials, Bay Area rates); garage conversion (400 sq ft): $40,000–$60,000; second-story addition: $90,000–$130,000. Grand total for a typical detached ADU: $158,000–$213,000.
Can I use an owner-builder permit to avoid hiring a contractor?
Yes, partially. California Business & Professions Code § 7044 allows owner-builders (you must live in the ADU you're building, or be an owner-occupant of the lot). You can pull the building permit yourself and do framing, carpentry, and finishing work. However, electrical and plumbing must be performed by licensed contractors OR you must obtain a separate owner-builder electrical/plumbing permit from the state (adds $300–$500 per trade and requires state inspection). In practice, most ADU owners hire a licensed contractor for the whole project to avoid complexity. If you go owner-builder, expect closer scrutiny from Millbrae inspectors and longer inspection schedules.
What is the real timeline from idea to move-in for a Millbrae ADU?
Detached new construction: 16–20 weeks total (4–6 weeks design + permitting; 10–14 weeks construction + inspections). Garage conversion: 14–18 weeks (2–4 weeks design; 12–16 weeks construction; add 4–6 weeks if BCDC review is needed). Second-story addition: 18–24 weeks (4–6 weeks design + parking waiver; 12–18 weeks construction). If your lot is in airport or BCDC zone, add 3–6 weeks to every scenario. If you hit a plan-review comment or revision, add 1–2 weeks. Weather, inspection scheduling, or construction delays can push timelines longer. Most Millbrae ADUs see occupancy 16–24 months after the initial project idea.
Is there a Millbrae pre-application or design-review meeting I should take before spending money on plans?
Absolutely, yes. Millbrae Planning offers a free 30-minute pre-application meeting (call (650) 558-7600 to book). Bring a survey, your lot size, and a rough sketch of your ADU idea. The planner will confirm: (1) ADU is allowed on your lot (almost always yes due to state law); (2) setbacks are feasible or if variance/waiver is needed; (3) parking requirements and your options (dedicated space vs. waiver); (4) any environmental/overlay triggers (BCDC, airport, historic district); (5) utility capacity and required connections. This 30-minute meeting saves you $1,500–$2,500 in wasted design work. Schedule it before hiring an architect.