ADU Permit Requirements in California: 2026 Contractor Guide

Accessory dwelling units are one of the highest-value project types a California contractor can take on right now. State law keeps making them easier to entitle, and demand is strong in every Bay Area city. The part that still trips up even experienced builders is the permitting stack.

An ADU is never “just one permit.” You are coordinating a building permit that bundles structural, electrical, plumbing, and mechanical work, plus Title 24 energy compliance, utility service upgrades, and sometimes grading or coastal overlays. Get the coordination wrong and a project that should have taken four months sits in plan check or utility queues for nine.

This is the practical 2026 picture: what the state actually requires, how the major Bay Area jurisdictions handle the bundle, and the specific mistakes that turn profitable ADU jobs into cash-flow disasters.

Yes, You Always Need a Building Permit — And Usually Several Others

Under California State ADU Law (Government Code sections 66310–66335), every accessory dwelling unit requires a building permit. There is no de minimis exemption for small units or garage conversions. The permit is ministerial: if the plans meet code, the city must approve them without discretionary hearings or neighbor notification.

The building permit covers the core structure, all electrical, plumbing, mechanical, and energy compliance. In most jurisdictions you will also need to address:

• Mechanical permit (or scope inside the building permit) for HVAC — usually a heat pump or multi-head mini-split system on new ADUs.
• Electrical permit for new circuits, subpanels, or service upgrades.
• Plumbing permit for water heater, kitchen, and bath rough-in (often rolled into the building permit but still reviewed).
• Utility service applications (PG&E or municipal utility) for new electric, gas, water, and sewer laterals.
• Grading permit on sloped lots or when you are moving significant dirt.
• Coastal development permit only in the coastal zone, and recent reforms have narrowed when it applies to ADUs.

Garage conversions add a demolition scope that used to be a separate fight. SB 897 now requires cities to process the demolition as part of the ADU building permit when the drawings clearly show what is being removed and rebuilt.

The 60-Day Rule and the 15-Day Completeness Check

This is the part most contractors still under-appreciate. Once you submit a complete ADU application, the city has 15 business days to notify you in writing if it is incomplete and exactly what is missing. After that, they have 60 calendar days total to approve or deny.

If they miss the 60-day deadline on a complete application, the permit is deemed approved by state law. You still have to pay the fees, post the permit, and pass inspections, but the leverage is real when a jurisdiction is slow-walking.

The practical problem is “complete.” Many ADU applications get kicked back in week two because the HVAC sizing, Title 24 forms, or utility coordination letters were not included on day one. Every incomplete notice effectively restarts the clock for that scope. The contractors winning on ADUs are the ones who submit a package that survives the 15-day completeness review the first time.

Mechanical, Electrical, and Plumbing Scope on a Typical ADU

Most new detached ADUs and garage conversions in the Bay Area are being built with heat pumps or multi-head mini-split systems. That means you are looking at a mechanical permit (or mechanical scope inside the building permit), an electrical permit for the dedicated circuit and any panel work, and Title 24 documentation that is stricter than a simple equipment changeout.

We have detailed guides on heat pump permit requirements and mini-split and ductless HVAC permits. The short version for ADUs: the energy code treats the ADU as an addition or new construction, so the prescriptive path is available but you still need a proper Manual J, correct climate zone on the CF-1R, and HERS verification on the HVAC system in almost every case.

Water heater and kitchen/bath plumbing are usually included in the building permit review, but the inspector will still want to see proper strapping, expansion tanks, and condensate drainage. If you are running new gas lines for backup heat, that can trigger a separate gas permit in some cities.

Title 24 and Energy Compliance for ADUs

ADUs must comply with the current Title 24 energy code. Because they are new habitable space, you generally cannot use the simpler “like-for-like” alteration rules that apply to equipment replacements in existing homes. You will need a CF-1R generated through approved software, and the HVAC portion will almost always require HERS testing for refrigerant charge, airflow, and any duct leakage.

The good news is that heat pumps and mini-splits get favorable treatment under the 2025 code cycle now in force. The prescriptive path is easier to clear when you go all-electric or high-efficiency heat pump. The bad news is that plan reviewers are starting to ask for the actual Manual J load calculation on ADU projects instead of just accepting the CF-1R summary.

Full details are in our guide to Title 24 HVAC requirements in 2026.

Utility Coordination Is the Real Timeline Killer

This is the coordination step that destroys ADU schedules. PG&E (and some municipal utilities) new service requests for an ADU can easily take four to eight months in the current Bay Area environment. If you wait until the building permit is approved to start the utility application, your client is looking at a nine-month delay between permit issuance and being able to turn the lights on.

The winning move is to submit the utility new service or upgrade application the same week you submit the city building permit package. The one-line diagrams and load calculations need to match what the city sees. Mismatches between the city-approved set and the utility-approved set are the most common cause of “we passed inspection but still can’t get PTO” stories on ADU jobs.

We see the same pattern on solar-plus-storage projects — see our battery storage permit guide for the exact workflow.

Bay Area City Variations (2026 Snapshot)

Every city is different in how they bundle the scopes and how long the queue actually is:

• San Francisco DBI: Separate mechanical and electrical permits, slower intake, strict historic and coastal overlays on many lots. Plan on 8–12 weeks even for clean submissions.
• San Jose / Santa Clara County: High volume, generally bundles scopes well, faster on complete packages. 4–7 weeks typical for straightforward ADUs.
• Oakland: Better organized than it used to be, but still slower than the peninsula on anything with utility work.
• Peninsula cities (San Mateo, Redwood City, Belmont, San Carlos, etc.): Often the fastest for complete submissions. Many are SolarAPP+ cities for the PV portion if the ADU includes solar.

Fees are usually based on valuation or square footage on top of the base building permit. We broke down the HVAC-related pieces in HVAC permit costs in the Bay Area. ADU projects simply multiply the number of scopes.

The Most Common ADU Permit Mistakes

The failures we see over and over on ADU jobs:

• Submitting the building permit package without the mechanical and electrical details, then getting a completeness rejection at day 14.
• Treating the ADU HVAC as a simple changeout for Title 24 purposes instead of an addition.
• Starting the PG&E new service application after the building permit is approved instead of in parallel.
• Using a generic structural letter for the main house when the ADU addition changes the load path or roof framing.
• Assuming “mini-split = no HERS testing” on the ADU.
• Filing the mechanical permit on a different day than the building permit in cities that expect everything together.
• Not pulling the WUI or coastal status before finalizing the site plan.

Most of these are the same coordination failures we document in 7 permit filing mistakes that cost contractors weeks, just with higher dollar amounts at stake.

Unpermitted or Delayed ADU Work at Sale Time

Lenders, title companies, and buyers are now extremely sensitive to unpermitted or partially permitted ADUs. If the unit was built without permits, or if the permit is still open because utility work was never completed, you are looking at disclosure fights, appraisal problems, and potential escrow holdbacks that can exceed the cost of the original construction.

We covered the exact mechanics in unpermitted HVAC work and home sale disclosure in California. The same principles apply to the entire ADU.