Building an accessory dwelling unit (ADU) in Flagstaff

Does Arizona's ADU law cover Flagstaff?

Arizona's ADU law, ARS 9-461.18 (HB 2720, 2024), says: "This section applies to a municipality with a population of more than seventy-five thousand persons." The statute doesn't say which population count to use.

By the Census Bureau's counts, Flagstaff is just over that number. The U.S. Census Bureau's QuickFacts lists 76,831 people in the April 1, 2020 census and an estimate of 77,260 for July 1, 2025. The city treats the law as applying: in 2024 it amended its ADU code (Ordinance 2024-24, adopted October 15, 2024) "in response to House Bill 2720," noting the law covers "municipalities with populations greater than 75,000." The ADU rules are in Zoning Code Section 10-40.60.030, last amended December 2, 2025. See our Arizona ADU law page.

Before 2024, Flagstaff's code allowed only one ADU, required an extra parking space, set architectural compatibility rules and required a restrictive covenant on renting. The 2024 amendment, which city staff said was intended to comply with HB 2720, removed those. [1][4][5][6]

What you can build in Flagstaff

Flagstaff has three ADU types:

• Attached ADU: shares a wall or roof structure with the main home.
• Detached ADU: a separate building with its own roof.
• Interior ADU: a separate area inside the main home.

A lot developed with one single-family home can have one detached ADU and one attached or interior ADU. A lot of at least one acre can have one more detached ADU if at least one ADU is a Category 1 permanently affordable unit under Zoning Code Section 10-30.20.030. ADUs aren't allowed on a lot with a duplex or multifamily development.

An ADU must have its own living, sleeping and sanitation facilities and may include a kitchen. A manufactured or modular unit on a permanent foundation can be an ADU where the zone allows these units, but a mobile home, RV or other movable habitable space can't. A home occupation is allowed in the ADU or the main home, but not both. See our backyard tiny house guide. [1][2]

Size, height, setbacks and lot coverage

Size. An ADU can't exceed 75% of the main home's gross floor area (interior habitable area) or 1,000 square feet, whichever is less. But the code also says an ADU "may have a gross floor area of up to 800 square feet regardless of the size of the Single-Family Dwelling," as long as it meets the other rules.

Setbacks. An ADU can sit in the rear and interior side setbacks if it stays at least 5 feet from a rear line that borders another lot and 5 feet from interior side lines. On a corner lot, the street-side setbacks come from your zone. A detached accessory building built before February 16, 2016 that already sits inside the 5-foot setback can be converted to an ADU, or get an ADU on a second floor, as long as nothing is added or raised within the setback.

Height. An ADU can be as tall as your zone allows. In the Townsite historic overlay, the city says detached ADUs can be up to 25 feet.

Lot coverage. The zone's lot coverage limits don't apply to attached and detached ADUs. They do apply to a home that contains an interior ADU. [1][2]

Parking and occupancy

ADUs in Flagstaff don't need any parking on the property. The city removed its old one-space requirement in 2024 because HB 2720 bars cities from requiring added parking for an ADU.

No more than two people may live in an ADU. Arizona's ADU law doesn't set an occupant limit either way; if you're planning for a larger household, ask the Planner of the Day. [1][2][4][6]

Historic districts

ADUs are allowed in historic districts, and the city says most only need a building permit. In the Townsite Historic District, ADUs must meet the Townsite design standards and need a Certificate of Appropriateness before you apply for a building permit. The city says staff will review with the Heritage Preservation Commission which model plans already meet the Townsite standards.

Arizona law says a covered city must allow ADUs as a "permitted use," which it defines as approval "without requiring a public hearing, variance, conditional use permit, special permit or special exception," other than a site plan check. The city's own 2024 summary of HB 2720 noted "There are no exceptions for historic districts." If your lot is in Townsite, ask the city what the Certificate of Appropriateness review involves for an ADU. [2][4][6]

Utilities, septic and fees

An attached or interior ADU must connect to the same utilities as the main home (except internet, phone and TV). The code doesn't say whether a detached ADU may have its own meters; ask Water Services at 928-213-2400. Your site plan must show the electric, gas, water and sewer tap locations.

If your lot is on a septic system, the plan review doesn't need a septic design, but you must show Coconino County's approval of the septic system design before the city issues a certificate of occupancy.

Fees. Flagstaff's ADU sources don't list ADU-specific fees. A building permit needs a plan review deposit, which is non-refundable. The city's water and sewer capacity fee schedule (effective October 2, 2025) has no ADU line; for a single-family home with a 3/4-inch meter it lists $13,162.87 in total fees (meter, water capacity, sewer capacity and service fees plus tax). Ask Water Services which fees apply to your ADU. A model plan also has a reuse/licensing fee, set by its designer. See our ADU cost guide. [1][2][7][8][9]

Timeline

Flagstaff's residential permit timeframes don't list ADUs separately. They give an overall time of 35 working days for small residential plans and 45 working days for a new single-family home, made up of a 10-day completeness review and a 25- or 35-day substantive review. The clock stops while you answer the city's comments. All applications must be complete when you submit; documents added mid-review restart the review. The city says model plan reviews "should be shorter, easier to navigate, and less expensive" than custom plans. See our ADU timeline guide. [2][3][8]

Renting your ADU

The main home or any ADU may be "separately leased as long-term rental housing" (a lease of 90 days or more, or month to month). An ADU can't be sold separately from the main home. A lot with an ADU can be split only if each new lot meets the Zoning Code and Subdivision Ordinance, with separate utilities for each unit.

Short-term rentals. Flagstaff's code says that when a property has an ADU, "the owner of the property must reside on the property if any ADU is used as either a vacation or short-term rental." Arizona law (ARS 9-500.39) lets cities require this when the ADU's certificate of occupancy or final approval was issued on or after September 14, 2024, and says the rule "does not apply" when the ADU's final approval came on or before September 13, 2024. Flagstaff's code doesn't make that distinction. If your ADU was approved before then, confirm with the city.

A rental of fewer than 30 consecutive days needs a Flagstaff Short Term Rental License, renewed every year for $250. Before you apply, get an Arizona transaction privilege tax (TPT) license and notify your neighbors in writing. The city's 2026 updates also require at least $500,000 in liability insurance (or a booking platform that provides it). [1][10][11][12][13]

Model plans and state pre-approved plan law

Flagstaff's ADU Model Plans Library has 12 pre-approved detached ADU plans, from 384 to 1,000 square feet, ranging from studios to two-bedroom, two-bath units with a garage. They are designed to the 2024 energy code, all-electric with cold-climate heat pumps. You pay the designer's reuse/licensing fee for a single lot (you don't own the plan), choose only from the plan's pre-approved options, and still need a building permit. Any other change turns it into a custom plan review. The plans can also be built as a single-family home on a lot with no other home.

Arizona's pre-approved plan law, ARS 9-461.20 (SB 1529, 2025), says a city "shall establish standard preapproved housing design plans or a preapproved housing design plan program." Beginning July 1, 2026, in single-family areas that allow ADUs, it "shall create at least three standard preapproved housing design plans for accessory dwelling units," at 200, 600 or 1,000 square feet, and a plan program must accept submissions from anyone. Flagstaff's library includes a 1,000 sq ft plan; its other plans are between 384 and 805 square feet. We didn't find a city statement on how the library lines up with the state's size classes. [2][14]

Permit steps in Flagstaff

1. Check your lot

   Your lot must have a single-family home and no duplex or multifamily development. Look up your zone on the Flagstaff Zoning Map and check any HOA rules. Zoning questions go to the Planner of the Day at planneroftheday@flagstaffaz.gov. [1][2][15]

2. Historic districts: get approval first

   In the Townsite Historic District, ADUs must meet the Townsite design standards and you need a Certificate of Appropriateness before you apply for a building permit. The city says that in most other historic districts, ADUs only need a building permit, though its permit guide notes that any project in a historic district or overlay may need Heritage Preservation Commission approval or a cultural resource study before you apply. [2][8]

3. Pick a model plan or hire a designer

   To use a model plan, contact its designer and pay the reuse/licensing fee; the designer prepares a site plan for your lot and helps with pre-approved options. Otherwise, have custom construction documents drawn. [2]

4. Apply for a building permit

   Submit a complete Residential Building Permit Application and plans through the city's Citizen Access Portal, linked from the Building Permits page, with a plan review deposit. For a model plan, list its plan number on the "Same-As" line and include the designer's permission letter. [3][7][8]

5. Plan review

   Review has two stages: an administrative completeness review (10 working days) and a substantive code review. Model plans get a shorter "same-as" review that checks the ADU fits your site and meets height, access and utility standards. [2][7][8]

6. Build, inspect and get a certificate of occupancy

   The city assigns the ADU its own address during permit review. If the ADU uses a septic system, you need Coconino County's approval of the septic design before the city issues the certificate of occupancy. [2][7]

Pre-approved and sample plans

• Flagstaff ADU Model Plans Library

  Twelve pre-approved detached ADU plans, from a 384 sq ft studio to a 1,000 sq ft two-bedroom with a garage, that you license from the designer and submit for a shorter "same-as" building permit review. [2]

Who to call

• Planner of the Day, Planning and Zoning

  Zoning Code questions about ADUs: planneroftheday@flagstaffaz.gov

  [2]

• Building Safety and Code Compliance

  Building permits and model plan permits: amy.egan@flagstaffaz.gov; large residential files to BuildingPermits@flagstaffaz.gov

  928-213-2627

  211 West Aspen Avenue, Flagstaff, AZ 86001

  [2][3][7]

• Zoning Code Manager

  Zoning Code text and interpretations: TAntol@flagstaffaz.gov

  928-213-2605

  [15]

• Water Services

  Water and sewer capacity fees, meters and taps

  928-213-2400

  [9]

• Short Term Rental licensing

  STR licenses: ShortTermRental@flagstaffaz.gov

  (928) 213-2231

  [11]

Questions people ask