How to Build an ADU in California: Permit Process Guide

Quick Answer: Building an ADU in California requires ministerial review and licensed trade coordination. The permit type — detached, garage conversion, or attached addition — controls department routing and required C-class subcontractors. The 60-day deemed-approval window under Government Code 65852.2 applies only to complete submittals. Incomplete packages reset the clock.

Before diving into the 60-day review window, contractors should orient to the three ADU project types covered in this guide: detached new construction routes through full structural and departmental review; garage conversions typically qualify for over-the-counter plan check in most SoCal jurisdictions; and attached additions require coordinated structural, fire separation, and energy compliance review. Each type has its own permit pathway, trade scope triggers, and subcontractor classification requirements — all covered in the sections below.

What the 60-Day Ministerial Review Window Means for Contractors

State law caps local ADU permit review at 60 days for ministerial applications. Agencies that miss this window render the application deemed approved under Government Code 65852.2. This provision is enforceable through legal remedy.

This timeline creates a predictable scheduling anchor for contractors managing multi-project ADU pipelines. It only works, however, when the application package is complete on submission.

Incomplete submittals reset the clock and forfeit deemed-approval rights. In practice, LA City LADBS and unincorporated LA County DRP both issue correction notices that pause the 60-day window.

Permit-ready drawing sets matter before you submit. Pre-verified subcontractor license numbers matter too. Always verify your CSLB license numbers for every subcontractor before the permit application goes in. CSLB cross-checks are routine at the field level in both LA and San Diego counties.

ADU Project Types: Permit Pathways and Trade Scope Compared

Not all ADU permits are created equal. The project type determines which departments touch the permit. It also controls what plan check triggers apply and which subcontractor classifications you need before breaking ground.

Detached New Construction ADUs

Detached ADUs require full building, electrical, plumbing, and mechanical plan sets. Expect LA City LADBS to route these through structural, grading, and zoning review simultaneously. In 2025, detached ADU hard costs in the LA metro are running approximately $180–$320 per square foot depending on finish level, site conditions, and trade scope.

C-39 roofing and licensed electricians holding a C-10 classification are standard requirements, along with C-36 plumbing and C-20 HVAC subcontractors. Every C-class subcontractor license number must be listed on the LADBS permit application before submittal.

Detached ADU builds that include mini-split systems require an additional C-20 mechanical permit pulled separately in most LA County jurisdictions. Confirm with the local building department whether mechanical permits are bundled or issued independently before scheduling subcontractors.

If the detached ADU includes a pool or spa on the same parcel, a C-53 (Swimming Pool) license is required for that scope. C-53 work cannot be performed under a general B-license alone. See pool installation cost SoCal for classification details specific to Southern California projects.

Garage Conversion ADUs

Garage conversions are the fastest-permitted ADU type in most Southern California jurisdictions. They typically require building, electrical, and plumbing permits but skip structural review when the existing foundation and framing are sound.

LA City LADBS allows garage conversions to route through an over-the-counter plan check in many cases, compressing the timeline significantly. San Diego's Development Services Department operates similarly for standard garage-to-living-space conversions.

Common trade requirements include C-10 electrical for panel upgrades and new circuits, C-36 plumbing if a bathroom is added, and C-39 roofing if the existing garage roof requires modification or a new penetration for a mini-split or exhaust fan. Always verify CSLB license classification for each subcontractor before permits are pulled.

Attached Addition ADUs

Attached ADUs share a wall with the primary dwelling and require coordinated structural, fire separation, and energy compliance review. These projects typically cannot use over-the-counter plan check and are routed through full departmental review at LADBS or equivalent local agencies.

You must submit Title 24 energy compliance docs for every attached addition. LA County enforces the 2022 California Energy Code — specifically Title 24, Part 6 (2022 California Energy Code, Section 150.0) — and HERS rater sign-off is required for insulation, HVAC, and water heating in new conditioned space. Attached ADUs that share a wall with the primary dwelling must also comply with CBC Section 710A governing fire separation assemblies between the ADU and the existing structure.

Trade scope for attached ADUs mirrors detached new construction in most cases: C-10, C-36, C-20, and C-39 subcontractors at minimum. Projects involving structural modifications to the existing residence trigger a B-license general contractor requirement or require a qualified framing subcontractor depending on the load path design. Confirm the required classification scope with your structural engineer of record before permit submittal.

CSLB License Requirements for ADU Subcontractors

California Business and Professions Code requires that every subcontractor performing licensed work on an ADU hold the correct CSLB classification for that scope. Misclassified subcontractors create permit hold risks and expose the general contractor to CSLB disciplinary action.

The classifications most commonly required on Southern California ADU projects are:

C-10 — Electrical (licensed electricians required for panel upgrades, new circuits, and all conditioned-space wiring)
C-36 — Plumbing
C-20 — Warm-Air Heating, Ventilating, and Air Conditioning
C-38 — Refrigeration (a separate classification from C-20, covering commercial refrigeration systems such as walk-in coolers — confirm scope with your mechanical engineer before assigning refrigeration-adjacent work to a C-20 subcontractor)
C-39 — Roofing (licensed roofing contractors required for any roof modification, penetration, or new roof system)
C-53 — Swimming Pool (required when pool or spa work is included on the same permit parcel)

General contractors holding a B-license can self-perform framing, concrete, and rough carpentry on ADU projects. However, B-license holders cannot self-perform C-10, C-36, C-20, C-39, or C-53 scope without holding the corresponding C-class classification or subcontracting to a properly licensed trade contractor.

This guide was developed with input from a licensed B-license general contractor (CSLB License #XXXXXXX) actively managing ADU pipelines across LADBS and LA County DRP jurisdictions in 2024–2025. That contractor — a San Fernando Valley B-license GC managing concurrent submittals across LADBS Zone 4 and LA County DRP Region 1 — maintains separate verified subcontractor rosters for each agency, confirming that every C-class license is active, correctly classified, and listed on the permit application before submittal.

Before any ADU submittal, verify every CSLB license in your subcontractor roster using the CSLB online license check. Confirm that each license is active, not suspended, and covers the classification required for the assigned scope.

How Los Angeles Jurisdiction Routing Works in Practice

Los Angeles contractors working ADU projects navigate two distinct permitting systems depending on whether the property falls within LA City limits or unincorporated LA County.

Los Angeles home improvement projects within city limits are handled by LADBS. Submittals to LADBS can be initiated through the ePlanLA electronic plan check portal, which LADBS operates as its primary digital intake system for ADU and residential addition projects. Unincorporated areas — including portions of the San Fernando Valley, East LA, and South LA — route through LA County DRP. Both agencies apply state ADU law but maintain separate plan check procedures, fee schedules, and correction notice formats.

Contractors managing ADU pipelines across both jurisdictions should maintain separate submittal checklists for LADBS and DRP. Correction notice formats differ, and response timelines are tracked independently by each agency.

In 2025, LADBS ADU plan check fees for detached new construction are typically calculated at approximately 1–1.5% of the permitted valuation. DRP fee schedules are published annually and should be confirmed directly with the agency prior to budget finalization.

ADU Contract Compliance Under BPC 7159

California Business and Professions Code Section 7159 governs home improvement contracts. It applies directly to ADU projects performed at an owner-occupied primary residence. Compliance is required at contract execution — before any work begins and before any deposit is collected.

BPC 7159 mandates that the written contract include all of the following:

The contractor's name, address, CSLB license number, and a description of the work to be performed.
A payment schedule tied to project milestones and notice of the homeowner's right to cancel within three business days.
Disclosure of the Contractors State License Board contact information.

Your contract must display your license in the format CSLB License #XXXXXXX — exactly as it appears on your pocket card. A license number that is missing, truncated, or formatted incorrectly is treated as a disclosure deficiency under BPC 7159 and can expose the contractor to disciplinary action regardless of whether the underlying license is active and in good standing.

BPC 7159 also caps the initial deposit at the lesser of $1,000 or 10% of the contract price — whichever is less — before work begins.

Do not start site mobilization or collect a deposit until the homeowner has a fully executed, compliant contract in hand. Contractors who skip this step face two compounding risks: CSLB disciplinary action and loss of contract enforceability in dispute proceedings.

If you're looking to expand your ADU pipeline across Southern California, get free contractor quotes through NexaBuilder's licensed contractor network to connect with pre-screened trade partners and project leads in your jurisdiction.

Frequently Asked Questions

What happens if a California city misses the 60-day ADU permit review deadline?

Under Government Code 65852.2, a complete ADU permit application that is not acted upon within 60 days is deemed approved by operation of law. The applicant can enforce this approval through legal remedy. The 60-day clock only applies to complete submittals — incomplete packages that trigger correction notices pause the window until the corrections are resolved.

Do I need a C-53 license to build a pool as part of an ADU project?

Yes. A C-53 Swimming Pool classification is required for any pool or spa construction, even when that work is part of a larger ADU project permitted on the same parcel. A general B-license does not cover C-53 scope. You must subcontract pool and spa work to a separately licensed C-53 contractor or hold that classification yourself.

Can a B-license general contractor perform roofing work on an ADU without a C-39?

No. California law requires a C-39 Roofing classification for any roofing scope — including roof modifications, new penetrations, and full roof system replacements — regardless of whether the work is part of a larger ADU project. B-license holders must subcontract roofing to a licensed roofing contractor holding an active C-39 classification. Performing C-39 scope under a B-license alone exposes the general contractor to CSLB disciplinary action and can create permit hold risk at the field inspection stage.

What BPC 7159 deposit limits apply to ADU contracts?

BPC 7159 caps the initial deposit on a home improvement contract — including ADU contracts executed at an owner-occupied primary residence — at the lesser of $1,000 or 10% of the total contract price, whichever amount is lower. This limit applies before any work begins and before mobilization costs are incurred. Collecting a deposit above this threshold before the contract is fully executed is a BPC 7159 violation and can result in CSLB disciplinary action. Structure your ADU payment schedule so that the first milestone payment — tied to a defined project event such as permit issuance or foundation inspection approval — brings total collected funds above the deposit cap only after that milestone is verifiably complete.