Phoenix ADU requirements: what the law guarantees, and what it doesn't
In Phoenix, an accessory dwelling unit — a casita, in local usage — is permitted on any lot where a single-family home is allowed. State law caps its size at 75% of the primary home's gross floor area or 1,000 square feet, whichever is smaller, and bars the city from requiring owner-occupancy, added off-street parking, or a rear or side setback greater than five feet. That is the floor. Phoenix's own ordinance fills in the rest, and in places is more generous.
Which law governs your lot — the question most articles skip
Arizona does not have one ADU law. It has two, plus a gap, and which one applies depends entirely on who has zoning authority over your parcel.
A city over 75,000 people — Phoenix, Mesa, Chandler, Gilbert, Scottsdale, Tempe — is bound by A.R.S. § 9-461.18, the 2024 statute that created the guarantees above.
Unincorporated county land is bound instead by A.R.S. § 11-810.01, added in 2025 and effective 1 January 2026. Similar in shape, different statute, different text.
A town under 75,000 people is bound by neither. Its own zoning code is the whole answer.
If your property is in Paradise Valley, none of this applies
Paradise Valley's population is roughly 12,250 — far below the 75,000-person threshold written into the statute. Arizona's statewide ADU guarantees do not reach it. Every setback, size limit, parking question and owner-occupancy rule has to come from the Town's own zoning ordinance instead. If you own land there and you have been reading articles about "Arizona's new ADU law," none of what you read governs your project. Start with the Town.
The same caution applies to any parcel with a Phoenix, Mesa or Scottsdale mailing address that sits outside those city limits. Mailing addresses do not follow jurisdiction boundaries. Confirm the parcel, not the envelope.
How many, and how big, in Phoenix
A qualifying lot must be allowed at least two ADUs — one attached and one detached — and lots over one acre must be allowed a third if one unit is deed-restricted as affordable housing. On size, Phoenix layers two caps and applies whichever is smaller: 75% of the primary home's floor area, and then a lot-based cap of 1,000 square feet on lots up to 10,000 square feet, or the lesser of 3,000 square feet and 10% of net lot area on larger lots.
The practical consequence is that most Phoenix lots will only ever support one detached ADU at a workable size, and the binding constraint is usually the 75% figure rather than the square-foot cap.
Custom plans or the standard plan library
Phoenix maintains a library of pre-reviewed standard ADU plans, and building from one unmodified generally moves faster through review because the design itself was already vetted by the City. The trade-off is that the moment you modify a standard plan — even slightly — it moves into the custom review path, and you are back to a full review anyway.
The library is worth looking at seriously if your lot is straightforward and you can live within a published design. It is worth skipping if your lot has a slope, an unusual shape, an easement, or a layout you actually care about — a half-modified standard plan gives you the constraints of both paths and the advantages of neither.
What still applies
The state law removed zoning barriers. It did not remove building code, and it did not remove impact or utility connection fees. Your ADU still needs a full permit set, engineering where the scope requires it, its own address and utility coordination, and it still passes the same inspection sequence as any other dwelling.
Get the full guide — $29
The 33-page Phoenix ADU Owner's Guide covers the jurisdiction question in detail, Phoenix's size and setback rules with a lot diagram, the six-step permitting roadmap, the standard plan library, utilities and addressing, long- and short-term rental rules, cost ranges by ADU type, and a readiness self-assessment.
See also: As-built drawings (free) · Garage conversion permits · Home addition permits · Services