What happens if you skip the permit (and you needed one)
- Stop-work order plus $500–$1,500 fine if the city discovers unpermitted construction; you'll owe double permit fees and must remove the unit or legalize it retroactively at 3x cost.
- Home insurance denial and lender refinance block — title companies flag unpermitted ADUs on preliminary reports, freezing equity access.
- Seller's disclosure liability: California Real Estate Transfer Disclosure (TDS) requires you to disclose unpermitted work; buyer can sue for hidden defects or rescind.
- Neighbor code complaint triggers city inspection; if you're running a rental without permit, city can issue use-violation citations ($100–$500 per day) and demand tenant relocation.
San Carlos ADU permits — the key details
San Carlos Building Department requires a building permit for every ADU — detached new construction, garage conversion, junior ADU (JADU), or above-garage unit. This is non-discretionary under California Government Code 65852.22 and 65852.26. State law now prohibits San Carlos from imposing owner-occupancy, maximum size (up to 1,200 SF detached, 500 SF JADU), parking, or design review as barriers. However, San Carlos still enforces baseline code compliance: IRC R310 egress windows (minimum 5.7 SF operable, sill ≤44 inches above floor), separate electrical meter or approved sub-metering, plumbing stub or separate connection, and setbacks (typically 5 feet front, 5 feet side, 15 feet rear — confirm with city as they can vary). The permit application requires a site plan showing lot dimensions, existing structures, ADU footprint, setbacks, parking (if required), utility locations, and egress windows. For detached ADUs, IRC R403/R404 foundation design is required; San Carlos has no frost-depth issue on most of the city (coastal), but the mountainous east side (Coyote Hill area) may require deeper footings — the city's geotechnical review will catch this.
The 60-day clock under AB 671 applies to ADU applications in San Carlos. This means from the date you submit a complete application, the city must approve or deny within 60 calendar days (or application is deemed approved). In practice, San Carlos Building Department processes ADU permits in 4–8 weeks if your design is clean and site plan clear. The city does NOT charge a separate ADU application fee, design review fee, or architectural fee — only standard plan review ($350–$800 depending on square footage), building permit ($400–$1,200 based on valuation), and electrical/plumbing permits ($200–$400 each). Total: $3,000–$5,000 for a garage conversion; $6,000–$12,000 for new detached construction. If you hire a licensed architect or engineer (recommended for detached new), add $2,000–$5,000. The city's online permit portal (accessible via San Carlos website) lets you upload documents, track status, and pay fees without a site visit — a significant advantage over in-person-only jurisdictions like Gilroy or Morgan Hill.
Parking in San Carlos is addressed by state law but with local enforcement. Governor Code 65852.26(d) waives parking for ADUs within a half-mile of public transit OR within a transit-rich area. San Carlos qualifies: the Caltrain downtown station (local bus hub) is within half-mile of most residential lots, and the city has designated Transit-Oriented Development zones. However, if your lot is NOT within the 0.5-mile radius or TDP zone, you must provide 1 parking space (or pay a fine). This is checked ministerially — no negotiation. Setbacks are straightforward: detached ADUs must be at least 5 feet from side/front property lines and 15 feet from rear (IRC R306.1, but San Carlos's local ordinance may differ slightly — verify during pre-check). Junior ADUs (ADUs built inside the primary home) have no setback requirement since they occupy the existing structure. Garage conversions (most common ADU type in San Carlos's 1950s–1970s neighborhoods) inherit the primary home's setback but must retain tandem parking or comply with the parking rules above.
Utility connections are mandatory and must be shown on plans. For a detached ADU, you need either: (a) a separate electrical meter from the main service (cost $500–$1,500 for electrician + meter installation), or (b) a sub-metering kit approved by local utility (PG&E in San Carlos) at $800–$2,000. For water, separate service or sub-metering is required; a separate sewer connection is required unless the city approves a shared lateral (rare). All utility plans must be stamped by a licensed contractor (electrician for power, plumber for water/sewer). San Carlos Building Department requires these utility plans on file before they issue the building permit — this is a hard stop, not optional. Plan review typically takes 2–3 weeks if utilities are clearly shown; missing or vague utility details trigger a resubmittal (adds 1–2 weeks). The city's online portal lets you upload revised utility plans immediately; in-person jurisdictions add another week of delay.
Inspections for ADUs in San Carlos follow the standard building sequence: foundation/concrete (if detached), framing, rough electrical/plumbing/mechanical (mandatory for egress and safety), insulation/drywall, interior finishes, and final building inspection. A separate rough electrical and rough plumbing inspection is non-waivable for ADUs (this is state requirement under Title 24). The final inspection includes a utility sign-off (PG&E/water district confirm separate metering is live), a planning sign-off (confirms parking compliance), and building sign-off (egress windows, kitchen, bathroom, ventilation). Total inspection timeline is 8–12 weeks from permit issuance to final sign-off. Owner-builders may pull their own permit (no licensed contractor required), BUT electrical and plumbing work must be performed or supervised by the property owner with a B&P Code § 7044 exemption, or a licensed electrician/plumber must sign off. Most owner-builders hire a licensed electrician ($1,500–$3,000) and plumber ($1,500–$3,000) for rough and finish; this is cheaper than a general contractor markup but requires you to coordinate trades and schedule inspections yourself.
Three San Carlos accessory dwelling unit (adu) scenarios
San Carlos's state-law ADU override: why local zoning no longer matters
Before 2017, San Carlos zoning code restricted ADUs to single-family zones only and required conditional-use permits, design review, and owner-occupancy. SB 1069 (2017), AB 68 (2019), AB 881 (2020), and SB 9 (2021) progressively stripped away these barriers statewide. California Government Code 65852.22 now says any local government must allow at least one ADU per single-family lot, ministerially (no discretionary approval), regardless of local zoning. San Carlos updated its ordinance in 2018 to comply, then again in 2022 to implement SB 9 (allows two detached ADUs + one JADU per lot, with some conditions). The result: your application for an ADU hits a 60-day shot clock (AB 671) with no design review hearing. The city cannot deny you for reasons like neighborhood character, parking impact, or compatibility — those are off the table.
However, 'ministerial' does not mean 'free pass.' San Carlos still enforces code: setbacks, egress, utility capacity, geotechnical safety, parking (in non-transit zones), and consistency with local general plan housing policies. If your ADU meets these objective standards, the city must approve within 60 days. If it doesn't (e.g., your lot is only 3 feet wide and can't fit the ADU), the city denies on objective grounds — not discretion. This is why pre-checking with the city's online portal is crucial: you can verify setback, lot-size, and utility feasibility in 1–2 days before investing in plans.
San Carlos has no separate ADU fee (unlike some Bay Area cities that imposed $2,000–$5,000 ADU application fees, now mostly struck down as preempted by state law). You pay only standard building permit fees based on valuation. This is an advantage: total permitting cost is $1,500–$3,000, not $5,000–$8,000 like in Sunnyvale or Los Altos. The city's 2022 update also clarified that ADUs do not need to meet parking minimums if located within 0.5 miles of public transit or in a 'transit-rich area.' San Carlos designated most of downtown and Laurel Heights as transit-rich (near Caltrain and local bus lines), so many ADUs are parking-exempt. This further cuts project cost and timeline.
One caveat: state law allows local governments to impose parking for ADUs if 'necessary to meet local parking demand.' San Carlos has not imposed this, so assume parking waived in transit zones. But read your project area's specific zoning map or call the city to confirm before designing your project. The city's online portal has a 'parking requirement checker' tool — use it.
Utility connections and sub-metering: why separate service is mandatory, not optional
California's ADU laws (Gov. Code 65852.26(e)) require 'separate utility connections' or approved sub-metering for detached ADUs and garage conversions. San Carlos Building Department enforces this strictly: you cannot share a main electrical panel, water meter, or sewer lateral with the primary home. Why? Utility companies and local governments need separate billing, liability, and disconnection ability if the ADU becomes a problem (code violation, unpaid bills, etc.). PG&E (the local utility) has a standard sub-metering kit (~$1,200–$1,500 installed) that allows one electrical service to feed a sub-meter, then split to the ADU. For water, the water district (local utility or San Carlos Public Works) requires either a separate service line (cost $2,000–$4,000 for new meter, trenching, installation) or an approved sub-meter. For sewer, a separate lateral connection is required unless the city approves a shared line (rare, requires written waiver). This is non-negotiable.
Plan review in San Carlos requires utility documentation on file before permit issuance: (1) a signed PG&E Letter of Availability showing the electrical sub-meter location and capacity; (2) a water district form confirming separate or sub-metered service; (3) a plumber's sewer lateral diagram showing connection point. If these are missing, the city will not issue the permit, period. This is a hard requirement, not a suggestion. Many DIY applicants skip utility coordination and then hit a dead stop during plan review. Budget 2–4 weeks for utility coordination before you even submit plans. Call PG&E San Carlos service center, San Mateo County Water District (if applicable), and the city to confirm requirements for your specific lot and ADU type.
Junior ADUs (interior) are exempt from separate utility requirements — they can share the primary home's electrical panel, water, and sewer. This is a huge cost and timeline advantage: no sub-meter ($1,200), no separate water service ($2,000–$4,000), no sewer lateral ($2,500–$5,000). Total utility savings: $5,700–$12,000. This is why JADUs are the most affordable ADU option and fastest to permit in San Carlos. If your primary motivation is cost and speed, JADU is your answer.
One more note: if you're doing a garage conversion (detached from primary home but built on same lot), PG&E may allow a sub-meter off the main service panel rather than a full separate service. This saves ~$1,500 vs. a second meter. But confirm with PG&E and the city before design — some conversions need full service, especially if the garage is >15 feet from the main panel.
600 Elm Street, San Carlos, CA 94070 (City Hall main line; confirm building department address via city website)
Phone: (650) 802-4200 (main city number; ask for Building Department or Planning) | https://www.san-carlos.org (check 'Services' or 'Building & Planning' for online permit portal and pre-application checker)
Monday–Friday, 8:00 AM – 5:00 PM (verify current hours on city website, subject to holiday closures)
Common questions
Can I build an ADU even if my neighborhood's zoning says no?
Yes. California state law (Gov. Code 65852.22) overrides local zoning restrictions for ADUs. San Carlos cannot deny you based on neighborhood designation, land-use compatibility, or discretionary design review. However, your ADU must still meet objective code standards: setbacks, egress windows, utility capacity, and parking (if required by transit distance). If your project meets these, the city must approve within 60 days.
What's the difference between a detached ADU, garage conversion, and Junior ADU, and which is cheapest/fastest?
Detached ADU: new building on lot (full foundation, framing, utilities). Cost $120,000–$200,000+; timeline 20–24 weeks. Garage conversion: existing garage converted (shares utilities or adds sub-meter). Cost $35,000–$60,000; timeline 16–20 weeks. Junior ADU (JADU): second bedroom inside primary home (shares utilities, interior entry). Cost $18,000–$35,000; timeline 10–14 weeks. JADU is cheapest and fastest because no separate utilities, foundation, or new framing. If cost/speed are priorities, JADU is the answer.
Do I need a separate electrical meter for my ADU?
For detached ADUs and garage conversions, yes — either a separate meter from the main panel (full service, $3,000–$4,000) or a sub-meter kit approved by PG&E ($1,200–$1,500 installed). For JADUs (interior), no — you can share the primary home's panel and service. This is a major cost difference. Verify meter requirements with PG&E San Carlos and the city during pre-application.
Do I lose my garage parking if I convert it to an ADU?
Yes, you lose garage parking by converting. However, parking may be waived if your lot is within 0.5 miles of public transit (Caltrain station, bus line) or in a transit-rich area. San Carlos designates downtown, Laurel Heights, and parts of Heather Park as transit-rich. If you're in these zones, no parking required. If outside, you must provide 1 parking space on your lot (detached parking spot or driveway). Confirm your lot's parking requirement via the city's online portal or a call to Building Department.
Can I do the framing and construction myself as an owner-builder, or do I need to hire a contractor?
California law allows owner-builders to pull permits and perform their own work on their own property (B&P Code § 7044). However, electrical and plumbing must be done by a licensed electrician or plumber, or you must obtain an owner-builder C-10 (general building) license. Most owner-builders hire licensed subs for rough electrical, plumbing, and HVAC (mandatory inspections), then do framing/drywall themselves. This saves 10–20% vs. general contractor overhead but requires you to coordinate trades and manage timeline.
How long does the permit process take in San Carlos?
San Carlos operates under California's 60-day shot clock (AB 671) for ADU applications. From the date you submit a complete application, the city has 60 calendar days to approve or deny. In practice, plan review takes 3–6 weeks if your application is clear and utilities are pre-coordinated. Resubmittals (missing utility documents, unclear setbacks) add 1–2 weeks each. Once permitted, construction and inspections take 8–16 weeks depending on ADU type (JADU ~8 weeks, garage conversion ~10 weeks, new detached ~16 weeks). Total timeline: 4–8 weeks permit + 8–16 weeks construction = 12–24 weeks.
What if my lot is too small or oddly shaped for an ADU?
San Carlos has no minimum lot size for ADUs under state law, but setbacks and utility feasibility are hard limits. A detached ADU needs 5 feet from front/side lines and 15 feet from rear (IRC R306.1 + local code). If your lot is smaller than ~4,000 SF or has odd geometry, a detached ADU may not fit. In that case, a JADU (interior) or garage conversion is more feasible. Pre-check your lot dimensions and setbacks via the city's online portal or a quick call to Building Department — this takes 10 minutes and saves weeks of wasted design work.
Do I need to own and occupy the primary home to have an ADU, or can I be an investor?
Under California state law, no owner-occupancy requirement applies (Gov. Code 65852.22 prohibits local governments from imposing it). You can own the property as an investor, own the primary home and rent the ADU, own both as rentals, or any combination. However, some ADU programs or financing (e.g., CalHFA loans) may require owner-occupancy of the primary home — check your lender. San Carlos's local code has no owner-occupancy rule, so from the city's perspective, you're free to rent both units.
What inspections does my ADU need to pass?
ADUs require the full building inspection sequence: foundation/concrete (if detached), framing, rough electrical/plumbing/mechanical (non-waivable for ADUs), insulation/drywall, interior finishes, and final building inspection. A separate rough electrical and rough plumbing inspection is mandatory for safety and Title 24 energy compliance. The final inspection includes a utility sign-off (PG&E/water district confirms separate metering is live), planning sign-off (confirms parking, ADU use, no zoning violations), and building sign-off (egress windows, kitchen, bathroom, ventilation, smoke detectors). Budget 8–12 weeks for the full inspection sequence after framing starts.
What happens if I build an ADU without a permit and the city finds out?
Stop-work order plus a $500–$1,500 fine and mandatory removal or retroactive legalization at 3x cost. Your homeowner's insurance may deny claims for unpermitted work. Title companies flag unpermitted ADUs on preliminary reports, blocking refinance and resale. Neighbor complaints trigger city inspection; if you're renting the unpermitted ADU, the city can issue use-violation citations ($100–$500 per day) and demand tenant relocation. California Real Estate Transfer Disclosure (TDS) requires you to disclose unpermitted work to any future buyer. Do the permit — it costs $1,500–$3,000 and saves legal/financial catastrophe.