Getting an ADU permitted in California involves navigating multiple agencies, document types, review cycles, and inspection milestones. While state law has significantly streamlined the process—requiring ministerial approval and capping review times at 60 days for complete applications—the practical experience of permitting an ADU varies considerably by city. Understanding the full process before you start prevents the most common sources of delay: incomplete applications, missing documents, and correction notices that could have been avoided.
Phase 1: Pre-Application Research
Before submitting any drawings, the most valuable investment of time is understanding your specific city's ADU standards. Every California city has either adopted a local ADU ordinance or is subject to state default standards. The local ordinance may impose height limits, setback requirements, and architectural standards that go beyond state minimums. Reviewing the local ordinance before design begins prevents the need to revise drawings mid-permit to meet a requirement you didn't know about. Many cities offer a pre-application conference to discuss your project concept before submitting formal drawings.
Phase 2: Assembling the Permit Package
A complete ADU permit application requires: a site plan showing property boundaries, existing structures, proposed ADU location, setback dimensions, and utilities; floor plans with all rooms, door and window locations, dimensions, and square footage; elevations showing all four exterior sides; sections and details showing construction type and insulation locations; structural plans with foundation and framing (engineered for SDC D, which covers most of California); a Title 24 energy compliance report signed by a certified energy consultant; and mechanical, electrical, and plumbing (MEP) plans.
Phase 3: Application Submittal and Completeness Review
Once drawings are complete, the application is submitted to the local building and planning departments—most jurisdictions now accept electronic submittals. Within 5 business days of submittal, the city must determine whether the application is complete. An incomplete application is returned with a list of missing items; the 60-day review clock does not start until the application is deemed complete.
Phase 4: Plan Check Review
During plan check, city staff review the submitted drawings for compliance with zoning, building code, fire code, Title 24 energy standards, and special conditions. If the plans comply with all applicable standards, the city issues plan check approval and the permit is ready for issuance. If there are deficiencies, the city issues a correction notice listing each non-compliant item. The applicant revises the drawings and resubmits; the second review cycle focuses only on corrected items.
Phase 5: Permit Issuance
Once plan check is approved and permit fees are paid, the building permit is issued. Permit fees vary by city and by ADU size—state law limits impact fees on ADUs under 750 sq ft to zero, but other building permit fees are typically still assessed. The permit is valid for 12–18 months from issuance; construction must begin within that period.
Phase 6: Construction Inspections
Building construction requires inspections at specific milestones before work can be covered up. The required sequence for a new detached ADU typically includes: foundation/footing inspection before concrete is poured; underground plumbing/electrical inspection before backfill; framing inspection after all framing, sheathing, and rough MEP are complete but before insulation; insulation inspection after insulation is installed; and final inspection when all work is complete.
Phase 7: Certificate of Occupancy
The Certificate of Occupancy (C of O) is issued by the building department after a successful final inspection. It certifies that the ADU has been built in substantial conformance with the approved plans and is safe for occupancy. No tenant can legally occupy the ADU until the C of O has been issued. If HERS verification was required by the Title 24 compliance report, the HERS rater's verification certificates must be submitted and accepted before the C of O will be issued.
Timeline Benchmarks by City Type
ADU permit timelines vary dramatically by jurisdiction. Smaller cities with over-the-counter review: 2–4 weeks for a simple garage conversion. Mid-size California cities: 6–10 weeks for a detached ADU. Large cities like Los Angeles, San Jose, and San Diego: 8–16 weeks due to high application volume. Each correction cycle adds 4–8 weeks. Coastal Zone projects requiring a CDP add 2–6 months before the building permit can be issued. Submitting a complete, code-compliant permit package on the first submittal is the single most effective way to compress this timeline.