California state law requires every city to approve a code-compliant ADU within 60 days of a complete application. "Complete application" is the key phrase — most projects spend 2–6 months preparing one. This is the realistic timeline from idea to certificate of occupancy.
Since AB 2221 (2022) and reinforced by AB 976 (2023) and AB 1033 (2024), California cities must process ADU applications under "ministerial review" — meaning no public hearings, no design-review board, no discretionary approval if the application meets adopted standards.
The clock: 60 days from when the city deems your application complete to when they must issue the permit. If they don't act in 60 days, the permit is deemed approved.
What state law doesn't prevent: cities setting design standards (height, materials, setbacks within state minimums), requiring fire-sprinkler plans, requiring electrical-load calculations, charging permit fees, requiring soils reports for sloped lots. Those are all legitimate. Cities just can't refuse the ADU itself.
Months 1–2: Design. You hire an architect or designer (or use a prefab vendor's catalog). Schematic design + design development to the point you can submit. Many homeowners underestimate this — a custom detached ADU design is 4–8 weeks of architect time even when nothing's complicated.
Months 2–4: Engineering + permit prep. Structural engineer, Title 24 energy compliance, MEP (mechanical/electrical/plumbing) plans. Soils report if required by your city. Survey if your lot lines aren't already on file. This is where projects stall — every missing document delays the application from being deemed "complete".
Months 4–6: Plan check. Submit to the city. They have 60 days under state law. In practice they often request 2–3 rounds of corrections ("plan check comments"), each adding 2–4 weeks. Real-world: 2–4 months from submission to permit issuance.
Months 6–14: Construction. 6–12 months for new detached, 4–8 months for garage conversion or attached. Weather, contractor backlog, and material lead times all affect this.
Months 14–16: Inspections + certificate of occupancy. Foundation, framing, MEP rough-in, insulation, drywall, final. Each must be scheduled and pass before the next stage. Then final inspection issues the certificate of occupancy.
Months 16–18: Buffer. Almost every project hits at least one delay. Plan for it.
The 60 days starts when the city deems your application complete. They have 30 days to make that determination after you submit. Then 60 days to issue the permit if everything's fine.
If your submittal is missing a single required document (calc sheet, energy doc, soils report), the clock doesn't start. They send you a completeness letter, you respond, the 30-day clock restarts. This is where projects can sit for months without realising it.
Practical advice: confirm with the city's planning counter — in person if possible — exactly which documents they require for ADU submittal in your zoning district. Get the list in writing. Submit nothing until you have all of them.
Some cities are genuinely streamlined. Los Angeles, Encinitas, Berkeley, San Jose, and Santa Monica all run dedicated ADU desks with sub-60-day average turnarounds for compliant submittals.
Others have a reputation for slowness. San Francisco averages 4–6 months despite the state law (multiple departments must sign off; SFFD has its own queue). Bay-area suburbs like Atherton and Hillsborough have a history of using design-standard challenges to drag timelines.
If your city's reputation is slow, two tactics help: (1) hire a designer who has recently permitted in that city — they know the desk reviewers' preferences; (2) consider a prefab vendor with HCD-approved models, which short-circuits much of plan check.
Zoning compliance: setbacks, height, lot coverage, FAR (floor-area-ratio), parking exemptions, and any local design standards (materials, roof pitch, façade rules).
Building code compliance: California Building Code 2022 (or 2025 once adopted) — egress, structural, fire-rated assemblies if attached, smoke alarms, CO detectors.
Energy code (Title 24): insulation R-values, window U-values, HVAC efficiency, mandatory solar (if 250+ sqft and meeting state criteria as of 2026).
MEP review: electrical load calculations confirming the existing service can handle the ADU (or upgrade plans if not), plumbing fixture-unit calculations, gas line sizing.
Fire department review: sprinkler requirements (state law: ADU not required IF main house doesn't have them), defensible-space inspection if in fire-hazard severity zones, hydrant flow tests in some jurisdictions.
Public works review: sewer/water connection capacity, sometimes encroachment permits if work touches the public right-of-way.
Building permit: $1,500–$8,000 depending on city and ADU size.
Plan check fee: $1,000–$5,000.
Impact fees: historically $5,000–$30,000 — but state law since 2020 exempts ADUs under 750 sqft from impact fees in most cases. Larger ADUs pay proportional impact fees.
School impact fees: waived for ADUs under 750 sqft per state law; larger ADUs pay $4–6 per sqft.
Sewer connection / water meter: if your ADU triggers a new connection (separate meter), $5,000–$25,000 depending on city.
Soils / engineering reports: $2,000–$8,000 if required.
Total realistic permit-stage cash: $8,000–$35,000 for a typical project.
Foundation inspection, framing inspection, rough MEP, insulation, drywall, gas line pressure test, and final. Each requires a 24-48 hour notice scheduled with the building department.
If an inspection fails, you fix the issue and call for re-inspection. Each failure = a few days delay (longer in cities with backlogged inspectors).
A good general contractor will schedule and manage all inspections. Owner-builders need to be on top of this themselves — missed inspections can require demolition of work to expose what should have been inspected.
Soils report finds expansive clay or seismic concerns → engineering redesign → 4–8 weeks.
Plan-check reviewer requests changes you didn't anticipate (e.g., separate utility meter requirement) → redesign + resubmit → 4–6 weeks.
Tree-protection issue: arborist required, may require alternative siting → 3–6 weeks.
Contractor's licence lapsed or insurance expired during build → city stops work → days to weeks to resolve.
Subcontractor no-show on a critical-path day (foundation, electrical inspection day) → cascading delays.
Material shortage or lead time (windows, custom doors, specialty fixtures) → 2–10 weeks.
No. Building without a permit triggers stop-work orders, double permit fees, demolition orders for non-compliant work, and complications when you eventually try to sell or refinance the property. Always wait for the issued permit.
These are typically rolled into the main building permit as MEP sub-permits. Some cities issue them separately; either way, they're processed concurrently and don't add to the timeline.
If your application meets all adopted standards, denial is illegal under state law. The remedy is an appeal — first internal at the city, then potentially to the California HCD or to the courts. Most denials we see are actually "corrections requested" framed as denials by frustrated planning desk staff. Resubmit with corrections.
Most CA cities issue building permits valid for 12 months from issuance, with one 6-month extension typically granted on request. If construction doesn't start within 180 days, the permit can lapse. Don't pull the permit until your contractor is ready to start.
Yes — turning a garage into habitable space changes its occupancy classification under building code. Even if you don't change the exterior, you need permits for the change of use, electrical, plumbing (if added), insulation, egress windows, and HVAC. Unpermitted garage conversions are a common issue at sale time.