How big can an ADU be in Arizona?
Arizona law requires cities and towns of more than 75,000 people, and counties, to allow an ADU of 75% of the main house's interior habitable area or 1,000 sq ft, whichever is less, and the county statute says outright that larger units may be allowed.
The figure is the size a city or county must be willing to permit. The statutes set it as a minimum for local rules. They set no statewide maximum, apart from the ADU staying smaller than the house.12
Working the numbers
Both statutes define "gross floor area" as the interior habitable area of the house, not its footprint.12 The lesser of the two limits applies:
| Main house, interior habitable area | 75% of it | Size that must be allowed |
|---|---|---|
| 1,000 sq ft | 750 sq ft | 750 sq ft |
| 1,200 sq ft | 900 sq ft | 900 sq ft |
| 1,333 sq ft | 1,000 sq ft | 1,000 sq ft |
| 2,000 sq ft | 1,500 sq ft | 1,000 sq ft |
Above about 1,333 sq ft of house, the 1,000 sq ft figure is the one that applies.
Cities and counties are worded differently
The county statute says a county must allow each ADU to be "at least" that size, and that it "does not prohibit a county from allowing" a larger one.2 The city statute requires cities to allow an ADU of that size but has no sentence about larger units.1 In a city, ask the planning department what maximum its own ordinance sets.
The one hard limit
Both statutes define an ADU as a self-contained unit on the same lot as a single-family home "of greater square footage than the accessory dwelling unit".12 Whatever the local rules allow, the ADU has to stay smaller than the main house to count as an ADU under these laws. It needs its own sleeping and sanitation facilities, and it may have its own kitchen.
Who publishes this
Kyrax Developments is a BC Housing-licensed builder based in British Columbia. It publishes this page as a reference to Arizona state law. The page is not legal advice and not an offer of construction services in Arizona. The city or county planning department has the final word on any specific lot.
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