Getting it permitted

How long it takes to build an ADU

Checked against state laws and official agency sources · Last reviewed October 10, 2026

The stages, and what cities say each one takes

Most ADUs go through the same stages. Each city sets its own process, and only some publish time estimates. These are the estimates from city ADU guides we checked:

• Research and design. Boston expects a "Feasibility and Design phase" of up to 3 months. Bakersfield, California, allows about a month for research and 1 to 3 months to design and plan. Escondido, California, in the guidebook for its pre-approved ADU program, lists 1 to 3 months for planning and 1 to 6 months for design.
• Permitting (review and corrections). Boston: up to 3 months; its building department review "could take about 5 weeks" if your documents are complete and no code problems are flagged. Bakersfield: 1 to 6 months. Escondido: 1 to 4 months. Newport Beach: "typically" 1 to 6 months.
• Construction, with inspections. Boston: 6 to 12 months, depending on the type of ADU and the scope of work. Bakersfield: 2 to 12 months. Escondido: 6 to 14 months.
• The whole project. Newport Beach says "Most ADU projects take 12-18 months to complete, but some extend to 24 months or more."

These are rough planning figures from each city, not promises, and they cover only these cities. Ask your own building department what to expect. Our ADU permit process guide walks through each step. [1][2][4][5]

Permit review and corrections

Review usually happens in rounds. The city checks your plans, sends comments, and you revise and resubmit. Newport Beach says it returns comments within 10 to 20 days and that "Most projects require 2-3 rounds of review." Escondido says ADU applications are reviewed in cycles, with each cycle in its pre-approved ADU program "lasting up to 30 days," and "Plan review can take multiple cycles." Boston says review takes longer if you need to update plans, send more documents, or get zoning relief.

That's why a complete, code-compliant first submittal saves the most time. How fast you and your designer answer comments counts too. [1][2][5]

State deadlines for permit review

Some state laws set how long a city has to act. These are the deadlines in the laws on our state pages. If your state isn't listed, ask your city or county how long review takes.

• California: completeness answer within 15 business days; approve or deny within 60 days of a complete application if there is an existing home on the lot; 30 days for a detached ADU using a pre-approved plan. If the 60 days pass with no decision, the application is "deemed approved."
• Connecticut: the as-of-right process must produce a decision within 65 days after the zoning commission receives the application. You can agree to extensions of up to 65 more days each.
• Minnesota: an agency must approve or deny a written request relating to zoning within 60 days of a complete request. If it doesn't deny it in time, the request is approved. The law lists zoning, septic system and a few other kinds of requests; it doesn't list building permits.
• North Carolina: if your city or county reviews residential building plans, the first review is due within 15 business days, with partial fee refunds after 20 business days.
• Washington: default deadlines of 65, 100 or 170 days apply to land use "project permits," but not to permits issued based on building code compliance.

Details and conditions for each state are below. [3][6][7][8][9][10][11]

California: 15 business days, then 60 days

California's ADU law (Government Code 66317) sets two clocks:

• Completeness, 15 business days. The permitting agency must tell you in writing whether your application is complete "not later than 15 business days" after receiving it. If it's incomplete, the notice must list the missing items and how to fix them. When you resubmit, the city can't ask for items that weren't on that list, and the 15 business days apply again. If the agency misses this deadline, the application "shall be deemed to be complete."
• Decision, 60 days. The agency must approve or deny the application "within 60 days from the date the permitting agency receives a completed application if there is an existing single-family or multifamily dwelling on the lot."

Conditions to know:

• If you apply for the ADU together with a new house, the agency may wait to decide the ADU until it decides the house.
• If you ask for a delay, the 60 days are paused for the length of the delay.
• If the agency denies the application, it must give you, within the same time period, "a full set of comments" listing what is deficient and how to fix it.
• You can appeal an incompleteness finding or a denial in writing. The agency must give a final written decision within 60 business days of your appeal.

[3]

If a California city misses the deadline

The statute answers this directly: "If the local agency has not approved or denied the completed application within 60 days, the application shall be deemed approved." For the completeness check, a missed 15-business-day deadline means the application is deemed complete.

The 60-day rule depends on its conditions: a completed application and an existing home on the lot. Any delay you asked for pauses the clock. Ask your city how it counts the days on your application, and use the appeal process if you disagree. The California page explains how to contact HCD, the state housing agency that enforces ADU law. [3]

Connecticut, Minnesota, North Carolina and Washington

Connecticut. State law (General Statutes 8-2o) says a town's as-of-right review of an accessory apartment "shall require that a decision on any such application be rendered not later than sixty-five days after receipt of such application by the applicable zoning commission." You may consent to "one or more extensions of not more than an additional sixty-five days," or withdraw the application. Towns could opt out of the accessory apartment rules, this deadline included, only before January 1, 2023, so ask your town whether it opted out.

Minnesota. Minnesota Statutes 15.99 says an agency "must approve or deny within 60 days a written request relating to zoning" for a permit or other approval, and "Failure of an agency to deny a request within 60 days is approval of the request." The clock starts when the agency receives a request with all required information, including the fee. If something is missing, the 60 days start over only if the agency tells you in writing within 15 business days what is missing. The law covers requests relating to zoning, septic systems and a few other reviews; building permits aren't on that list. Ask your city whether your ADU application is a zoning request under this law.

North Carolina. If a city or county chooses to review residential building plans, G.S. 160D-1110 says "all initial reviews for the building permit shall be performed within 15 business days of submission of the plans." If the first review takes more than 20 business days, the local government must refund 10% of the total permit application fee for each extra business day, for up to 10 business days.

Washington. RCW 36.70B.080 sets default decision times for "project permits" in cities and counties planning under the Growth Management Act: 65 days after the application is found complete when no public notice is needed, 100 days with public notice, and 170 days with notice and a hearing, unless the local government modifies them. Days spent waiting for information you were asked to provide don't count. But a "project permit" doesn't include "permits issued based on compliance with state and local building codes," so ask your city which timeline, if any, applies to your ADU. [7][8][9][10][11]

Pre-approved and permit-ready plans

Using a plan the city has already reviewed can shorten the permit stage.

California (AB 1332). Every city and county had to set up a program to pre-approve ADU plans by January 1, 2025, and post the approved plans online. Under Government Code 65852.27, the agency must approve or deny a detached ADU application "within 30 days from the date the local agency receives a completed application" if it uses a plan pre-approved by the agency within the current building code cycle, or a plan identical to one in a detached ADU application the agency already approved in that cycle.

City examples:

• Rocklin, California: the city code says applications using a pre-approved ADU plan are processed "within thirty calendar days of receipt of a complete application," compared with 60 calendar days for other ADU applications.
• Seattle: with one of the city's pre-approved backyard cottage plans, "In most cases, you can get your permit in just 2-6 weeks."

Arizona. State law (ARS 9-461.20) requires cities to create pre-approved housing plans, including at least three ADU plans from July 1, 2026, and to approve or deny plans submitted to its pre-approved plan program administratively, without discretionary review. See the Arizona page.

For factory-built options, see prefab ADUs. [6][12][13][14][15]

Construction, inspections and move-in

You can't start building until the permit is issued. Bakersfield's toolkit says: "You must have your approved construction document before you can start construction." During construction you schedule inspections with the city, and the city finalizes the permit once the work is done "in accordance with the approved plans." In California, you need a certificate of occupancy before anyone lives in the ADU.

City estimates for construction range from 2 to 12 months (Bakersfield) to 6 to 14 months (Escondido). The type of ADU matters: Boston says the construction timeline "greatly depends on the type of ADU you've opted to build and the scope of work on your property." Your contractor can give you a schedule for your project. See hiring ADU help and our ADU cost guide. [2][4][5][12]

Questions people ask