Residential Services
Accessory Dwelling Units (ADUs) — casitas, guest houses, garage conversions and attached suites — for rental income or multi-generational living. E-Project designs them from the zoning up: a custom ADU drawn to your lot, or a pre-designed, pre-engineered standard plan that is ready to adapt.
On most single-family lots in Phoenix and Scottsdale, yes — you can build an ADU. Arizona law requires cities with more than 75,000 residents to allow at least one attached and one detached ADU wherever a single-family home is allowed. How large it can be, where it can sit and how tall it can go are set by each city, by your lot size, and by how much lot coverage the main house already uses.
Arizona ADU Law
HB 2720 (2024), codified at A.R.S. § 9-461.18, required municipalities with more than 75,000 residents to adopt ADU rules by January 1, 2025. On any lot where a single-family home is allowed, they must permit at least one attached and one detached ADU, plus an additional detached ADU on lots of one acre or more when one of the units is restricted-affordable housing.
HB 2928 (2025) extended ADU requirements to Arizona counties, which had to adopt regulations for unincorporated areas by January 1, 2026. Cities and towns with 75,000 or fewer residents — Paradise Valley, Fountain Hills and Cave Creek among them — are not covered by these mandates and set their own rules.
Private HOA covenants and CC&Rs are a separate matter — if your lot is in an HOA, check the recorded covenants before design.
Sources: A.R.S. § 9-461.18 · HB 2928 Senate Fact Sheet
City Rules
| Rule | City of Phoenix | City of Scottsdale |
|---|---|---|
| How many ADUs | Two per single-family lot; a third on lots of one acre or more when one unit is affordable housing. Lots with a duplex or triplex may not add an ADU. | One attached and one detached; a third detached ADU on lots of one acre or more when one unit is affordable housing. |
| Maximum size | Lots up to 10,000 sq ft: 1,000 sq ft or 75% of the main house, whichever is less. Lots over 10,000 sq ft: 3,000 sq ft, 10% of net lot area, or 75% of the main house, whichever is less. | First ADU: 1,000 sq ft or 75% of the main house, whichever is less. Second and third ADU: 500 sq ft. |
| Height | Detached ADU in the required rear yard: 15 ft, unless a use permit is obtained. | Maximum height of the underlying zoning district. |
| Placement | Detached: 5 ft from a street-side property line, 3 ft from an interior property line, 0 ft where the lot abuts a fully dedicated alley. Attached ADU in the rear yard: 3 ft from the rear and 5 ft from the side property line. | Attached: the zoning district's setbacks. Detached: not in a required front or side yard, and as close as 5 ft to a side or rear yard. At least 10 ft from the main house and any other ADU. |
| Lot coverage | The ADU must comply with the lot coverage requirements for the lot. | — |
| Parking | Garage or carport conversions need replacement off-street parking, not in the front setback. | Parking for the main house must be maintained. |
| Renting | ADUs may be used as short-term rentals. | Owner must live on the property if an ADU is rented, leased or used as a vacation or short-term rental. |
| Utilities & sale | — | Separately metered electric and gas; the ADU cannot be sold separately from the main house. |
Figures are taken from each city's published ADU guidance as of September 2026 and show the kind of limit that governs. They are not a zoning determination for your parcel — E-Project confirms the zoning district, lot size, existing coverage and any overlay for every project before design.
Sources: City of Phoenix ADU Guide · Phoenix PDD — Accessory Dwelling Units · City of Scottsdale — Accessory Dwelling Units
Two Ways to Start
Drawn to your parcel from the start: an attached suite, a detached casita, a garage conversion or a second-story unit, placed where the setbacks, coverage and height limits actually allow, and designed to work with the house that is already there.
884 sq ft, two bedrooms, one bath, one story, 29'-6" wide by 40'-2" deep, with a deep covered patio. Sealed by a Registered Architect and engineered for construction. It sits under the 1,000 sq ft ADU cap; because both cities also cap an ADU at 75% of the main house, it suits a main house of at least 1,179 sq ft, and in Scottsdale it must be the first ADU on the lot.
Design Only $663 · Plan Review Ready $2,652 · Permit Ready (City of Phoenix) $3,536. Seven more casita plans from 603 to 1,508 sq ft are in preparation.
Before You Draw Anything
Your zoning district and lot size decide how many ADUs, how large and how tall. We look these up from your address or parcel number before the first meeting.
Every roofed structure counts — the house, garage, covered patios, sheds and ramadas. Coverage, more than the ADU size cap, is often what limits a back-yard unit.
Setbacks from property lines and alleys, and in Scottsdale the 10-foot separation from the main house, decide where a detached casita can actually go.
State ADU law does not override private CC&Rs. If your lot is in an HOA, the recorded covenants are checked before design starts.
Scottsdale requires separately metered electric and gas for an ADU, with water and sewer branching on private property — details that shape the site plan.
In Phoenix, an ADU outside a Hillside Development Area or FEMA flood hazard area is eligible for Self-Certification, which the City says can issue permits in one to five business days. See Residential Permitting.
Source: Phoenix Self-Certification Eligibility Chart (Rev. 08/26)
On most single-family lots, yes. Arizona law (A.R.S. 9-461.18) requires cities with more than 75,000 residents, including Phoenix and Scottsdale, to allow at least one attached and one detached accessory dwelling unit wherever a single-family home is allowed. Phoenix allows two ADUs per single-family lot and Scottsdale allows one attached plus one detached. What fits on your lot depends on its size, zoning district, existing lot coverage and any HOA covenants, which state ADU law does not override.
On lots up to 10,000 square feet, an ADU can be up to 1,000 square feet or 75% of the gross floor area of the main house, whichever is less. On lots over 10,000 square feet, the limit is 3,000 square feet, 10% of the net lot area, or 75% of the main house, whichever is less. A detached ADU in the required rear yard is limited to 15 feet in height unless a use permit is obtained, and every ADU must stay within the lot coverage allowed for the lot.
The first ADU on a lot can be up to 1,000 square feet or 75% of the main house, whichever is less. A second (and, where allowed, third) ADU is limited to 500 square feet. ADUs must be at least 10 feet from the main house and from any other ADU, and height follows the underlying zoning district. Scottsdale adopted these rules in Ordinance 4687 on September 30, 2025.
The City of Phoenix ADU Guide states that ADUs are allowed to be used as short-term rentals. The City of Scottsdale requires the property owner to live on the property if an ADU is rented, leased, offered for rent or used as a vacation or short-term rental, and a Scottsdale ADU cannot be sold separately from the main house. Check the current city rules for your parcel before you plan the finances.
Arizona law bars cities with more than 75,000 residents from requiring additional parking, or a fee in lieu of parking, for an ADU. There are two practical exceptions to plan for: in Phoenix, converting a garage or carport into an ADU requires replacement off-street parking outside the front setback, and in Scottsdale the parking for the main house must be maintained.
Generally yes. The City of Phoenix Self-Certification Program eligibility chart (revised August 2026) makes all new building construction and alterations eligible except hazardous occupancies, new high-rises, extra-large assembly occupancies, alternative materials, and projects in a Hillside Development Area or FEMA Special Flood Hazard Area. A registered architect certifies code compliance in place of City plan review, and the City states permits can be issued within one to five business days depending on scope.
That is a two-hour question, not a two-month one. Bring your address or parcel number — we will tell you how many units, how large, and where they can go.
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