Getting it permitted

The ADU permit process, step by step

Checked against state laws and official agency sources · Last reviewed October 9, 2026

Step 1: Check what your lot allows

Start with your zoning and, if your state has one, your state's ADU law. Where a state ADU law applies, it sets a floor, and your city adds its own rules on top. See what an ADU is for examples of states whose laws require many cities to allow ADUs.

Some limits depend on the lot itself. In California, a city may limit where ADUs go, but only based on water and sewer capacity, traffic flow and public safety. Washington lets cities restrict ADUs near septic systems, critical areas or other "unsuitable physical characteristics of a property," and lets them ban ADUs on lots not served by public sewers. In Montana, a city may require will-serve letters from the city water and sewer systems. In Arizona, an ADU may not be built on a public utility easement without written consent from the utilities that use or may use it.

Many cities have online map tools. For example, Phoenix points homeowners to its My Community Map to find a lot's zoning and lot coverage limits. [1][3][4][5][6]

Step 2: Ask questions early and look for pre-approved plans

Talk to the permit office before you pay for full drawings. Portland, for example, offers free 15-minute appointments for permit, code and zoning questions.

Pre-approved plans can save time. California law required each city and county to set up a program to pre-approve ADU plans by January 1, 2025, and to post the approved plans online. Seattle says that with one of its pre-approved backyard ADU plans, "in most cases, you can get your permit in just 2-6 weeks." Phoenix offers a free ADU Standard Plan Library for city residents, but you must submit those plans without changes.

For help choosing a designer or builder, see hiring ADU help. For factory-built options, see prefab ADUs. [1][6][7][8]

Step 3: Prepare your plans

Expect to submit a full set of drawings. Los Angeles requires "a complete set of plans" for plan check. Federal Way, Washington, asks garage conversion applicants for floor plans, a cross-section of the floor, walls and roof, framing details, energy code forms, and engineered drawings if the work changes the structure.

Expect your plans to be checked against the building code. California HCD says "all ADUs must comply with building code and health and safety requirements for dwellings," even ADUs that are exempt from some local zoning standards.

In California, fire sprinklers are required in an ADU only if the main home is required to have them, and building an ADU can't trigger a sprinkler requirement for the main home. [1][2][9]

Step 4: Apply, completeness check and plan review

In California, state law sets the clock:

• The permitting agency must decide whether your application is complete and tell you in writing within 15 business days. If it is incomplete, it must list what is missing and how to fix it.
• If the agency misses the 15-business-day deadline, the application is "deemed complete."
• The agency must approve or deny a complete application within 60 days if there is an existing home on the lot.
• If it denies the application, it must give you "a full set of comments" from every reviewing agency, with a description of how to fix the problems.
• Agencies must offer an appeal of an incompleteness finding or a denial, and must give a final written decision within 60 business days of your written appeal.

California ADUs are approved "ministerially," meaning without discretionary review or a public hearing. Reviewers check the plans against objective standards, often using checklists.

In Washington, state law sets default deadlines for "project permit" decisions in cities and counties that plan under the state's growth management law. For permits that need no public notice, the default is a final decision within 65 days after the application is found complete. The clock stops while the city waits for you to send more information. Cities may modify these timelines or exclude some permit types, so ask your city which deadline applies. [1][10]

Step 5: Other agencies' sign-offs and fees

The building department is often not the only reviewer. California defines a "permitting agency" to include planning and building departments, utilities and special districts. In Los Angeles, an ADU conversion may need clearances from the water and power department (for work near a utility easement), the Bureau of Engineering (for example, sewer availability), the Fire Department (hydrants and access) and City Planning before the permit is issued.

Fees depend on your state and city. Some state limits:

• California: no impact fees on an ADU of 750 square feet of interior livable space or less; larger ADUs pay in proportion to the main home's size. School districts may charge fees on ADUs over 500 square feet.
• Washington: impact fees can't exceed 50 percent of what the main home would pay.
• Montana: no impact fees, and the ADU application fee is capped at $250 per unit, though the city can still charge its usual building fees.

Seattle reports every ADU permit to King County for sewer treatment capacity charges. See our ADU cost guide. [1][2][4][8][11]

Step 6: Permit, inspections and certificate of occupancy

Los Angeles's ADU conversion bulletin describes the last steps this way. Permits are issued after all plan corrections and other departments' clearances are done. Once you have a permit, you or your builder request inspections. The inspector may require covered work to be opened up for a look. The Certificate of Occupancy is issued "after the final inspection is approved" and all clearances are signed off.

In California, you must get a Certificate of Occupancy before anyone lives in an ADU or junior ADU. HCD says a city can issue it before the main home's certificate "only under specific circumstances." Its example: an ADU in a county under a governor's state of emergency proclamation made on or after February 1, 2025, where the main home was substantially damaged or destroyed by the event and other requirements are met. [1][2]

What slows ADU permits down

Common causes of delay:

• Incomplete applications. In California, an incomplete application gets a written list of missing items. A denial and new application starts a new 60-day period, and the agency may charge a fee to review a resubmitted application.
• Many reviewers. Planning, building, fire, utilities and special districts may all review the same project.
• Waiting on you. In California, if you ask for a delay, the 60-day period is paused. In Washington, the review clock stops while the city waits for information you need to send.
• The coast. In California's coastal zone, ADUs may also need a coastal development permit. The Coastal Commission may hold an ADU permit until it decides on a new main home submitted at the same time.
• Permit difficulty in general. UC Berkeley's 2021 statewide survey of California ADU owners named "difficulty obtaining permits" as a significant barrier.

[1][10][12]

If the city misses a deadline or says no

First use your city's appeal process. In California, the permitting agency must provide one for denials and incompleteness findings.

In California you can also ask the state for help. HCD takes questions through its online ADU portal on the HCD ADU page. HCD says it may give technical assistance once an application has been submitted, but not when no application is pending. Since January 1, 2024, HCD has had authority to enforce all of state ADU law. [1]

Questions people ask