Flagstaff, AZ Accessory Structures: ADU Rules (2026)

Section 10-40.60.030 governs every accessory dwelling unit, attached, detached, or interior, in Flagstaff's residential zones. An ADU cannot be sold separately from the primary structure, and Table 10-40.60.030.A sets the design and development standards. An ADU structure may sit in the rear or interior side setback area, provided it keeps a minimum 5-foot setback from the rear property line and a minimum 5-foot interior side setback; a detached accessory structure built before February 16, 2016, that already sits in that 5-foot zone can be converted into an ADU or gain an ADU as a second floor without exterior expansion.

Building height follows the same maximum the underlying zone allows for any structure, and lot coverage rules do not apply to attached or detached ADUs, only to a dwelling that contains an interior ADU. Density is limited to one detached ADU plus one attached or interior ADU per lot with a single-family dwelling, with a second detached ADU allowed on lots of at least one acre if one of the units is a Category 1 Permanently Affordable unit; ADUs are barred entirely on lots already holding a duplex or multi-family development.

Occupancy is capped at two persons per ADU, and a mobile home, recreational vehicle, or other movable habitable space cannot serve as an ADU, though a properly foundation-mounted manufactured or modular unit can. Size is capped at 75 percent of the primary dwelling's gross floor area or 1,000 square feet, whichever is less, but any ADU may reach 800 square feet regardless of the primary home's size.