A.R.S. 11-810.01: Arizona's County ADU Mandate

If your lot sits in unincorporated Maricopa or Pinal County instead of inside a city, the city ADU law does not touch you. The statute that does is A.R.S. 11-810.01. It is the county twin of the municipal rule, and it carries a hard date: every Arizona county had to adopt regulations allowing accessory dwelling units (a smaller second home on a single-family lot, often a detached casita) by January 1, 2026. Miss that date and the county loses its grip entirely.

What counties have to allow

A county must let you build at least one attached and one detached ADU as a permitted use on any parcel zoned for a single-family home. On larger rural lots the law goes further: on a parcel of one acre or more, the county must allow a second detached ADU. That matters in places like Rio Verde and the desert fringes of Cave Creek, where parcels routinely run an acre or larger.

A county shall adopt development regulations that allow at least one attached and one detached accessory dwelling unit as a permitted use on lots or parcels zoned for residential use that include a single-family dwelling.

As with the city rule, the size floor protects a livable unit: the county must allow an ADU of at least 75 percent of the main home's floor area or 1,000 square feet, whichever is less. Rear and side setbacks for the ADU cannot be forced past five feet from the property line.

What this means for you

The county cannot pile on the conditions that historically blocked a guest house on rural land. It cannot bar you from renting either home on a long-term lease, cannot require the occupants to be related, cannot make you add parking, cannot demand the casita's design match the main house, and cannot apply height or setback rules stricter than what your single-family zoning already allows. It also cannot charge a special ADU fee or force street improvements, except to repair damage your own construction causes.

A concrete example. You own a 1.5-acre parcel in unincorporated Pinal County with a 2,400-square-foot house. Under this section the county must permit one detached casita of up to 1,000 square feet, and because your parcel tops one acre, it must allow a second detached ADU as well. No relative requirement, no extra parking demand, no design-match rule.

The deadline is the lever. If a county failed to adopt the required regulations on or before January 1, 2026, the statute says ADUs are then allowed on every residentially zoned lot in that county with no county limits at all, which removes the county's discretion until it complies. The mandate does not reach tribal land or parcels inside certain military and FAA airport noise zones above 65 decibels.

How this connects to the rest of the law

This is the county half of Arizona's ADU pair. The city version is A.R.S. 9-461.18 , which binds municipalities over 75,000 people with a January 1, 2025 deadline, so the first question on any project is whether your lot is inside city limits or in the unincorporated county. A backyard detached unit is a casita ; a smaller unit built inside the existing house is a junior accessory dwelling unit . For practical specs see Arizona ADU requirements and how many ADUs per property in Arizona . Rural builders should also read what you need to build on rural land in Rio Verde , since water and septic rules stack on top of the zoning allowance here.

Full text and source

Read the current version, including the affordable-rate provision and the full list of prohibited county conditions, on the legislature's site: View A.R.S. 11-810.01 on azleg.gov .

This section settles zoning, not utilities, so on a rural ADU confirm separately that your well and septic can serve a second home before you rely on the right this statute grants.