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Zoning Guide

ADU Rules in Flagstaff, Arizona

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ADU Rules in Flagstaff, Arizona

Arizona's casita statute is HB 2720 (2024), codified at A.R.S. §9-461.18, applying to cities with 75,000+ population. HB 2673 is unrelated.

Permitted ADU Types

  • Detached ADU: Standalone structure on the lot
  • Attached ADU: Addition to primary home with own entrance
  • Garage Conversion: Converting existing garage to dwelling unit

Size Limits

ADU Type Maximum Size
Detached ADU 1,000 sq ft or 50% of primary dwelling
Attached ADU 50% of primary dwelling floor area
Garage Conversion Existing footprint

Setbacks

  • Front: Base zone standard (typically 20–25 ft)
  • Side: 5 feet minimum
  • Rear: 5–10 feet minimum

Height

  • Detached ADU: Maximum 18–20 feet
  • Cannot exceed primary dwelling height

Mountain Climate Requirements

Flagstaff's elevation and climate create unique construction needs:

  • Snow load: 100+ inches average annual snowfall — roof structures must meet heavy snow load requirements
  • Frost depth: 36+ inches — footings must extend below frost line
  • Cold climate insulation: High altitude and cold winters require superior insulation values
  • Seismic: Arizona has some seismic activity; standard design required
  • Wildfire: Parts of Flagstaff are in Wildland-Urban Interface (WUI) zones requiring fire-resistant construction

Wildfire Considerations

Flagstaff is surrounded by the Coconino National Forest. Properties in WUI zones may require:

  • Class A fire-rated roofing
  • Fire-resistant siding materials
  • Defensible space clearance
  • Review by Flagstaff Fire Department

Parking

One off-street parking space required per ADU. Parking areas must accommodate snow storage.

Permit Process

  1. Check WUI zone: Verify wildfire zone status
  2. Verify zoning: Contact Flagstaff Development Services
  3. Submit plans: Including snow load and cold climate design
  4. Review: Zoning, building, and WUI compliance
  5. Building permit: Issued after approval
  6. Inspections: All required phases
  7. Certificate of Occupancy: Before occupancy

Contact

Flagstaff Development Services 211 W. Aspen Ave, Flagstaff, AZ 86001 Phone: (928) 779-7632 Website: flagstaff.az.gov/Development-Services

Disclaimer: Flagstaff's mountain climate and wildfire risk create unique requirements. Verify all current standards with Development Services before starting any project.

More about Flagstaff Zoning

Sources

  1. Flagstaff Zoning Code — Residential Districts·library.municode.com·Accessed 2026-04-07·Direct link
  2. City of Flagstaff Development Services·flagstaff.az.gov·Accessed 2026-04-07·Direct link

FAQ

Are ADUs allowed in Flagstaff, Arizona?
Arizona's casita statute is HB 2720 (2024), codified at A.R.S. §9-461.18, applying to cities with 75,000+ population. HB 2673 is unrelated.
What is the maximum ADU size in Flagstaff?
Detached ADUs in Flagstaff are generally limited to 1,000 square feet or 50% of the primary dwelling, whichever is less.
How does Flagstaff's mountain climate affect ADU construction?
Flagstaff sits at 7,000 feet elevation and receives 100+ inches of snow annually. ADU construction must meet cold climate building requirements including snow load design, frost depth requirements (36+ inches), and energy efficiency standards for high altitude.