How much does it cost to build an ADU in Arizona?

A simpler garage or interior conversion can start near $80,000, since it reuses an existing slab, walls, and roof. A common 600 to 1,000 square foot detached ADU lands at roughly $150,000 to $350,000, and high-end Scottsdale casitas with premium finishes and difficult lots run $450 per square foot or more. Averaged across those tiers, an ADU (accessory dwelling unit) in the Phoenix and Scottsdale metro costs about $200 to $400 per square foot to build in 2026. These are planning ranges, not quotes, because prices move with material and labor costs.

Where your project lands depends on whether it is detached or attached, how large the state and city let you build, your finish level, and how much utility and site work the lot needs. Below is how the dollars break down for a real Arizona ADU.

What an ADU costs per square foot in Arizona

Plan on $200 to $400 per square foot for a new detached ADU in metro Phoenix, with luxury Scottsdale builds climbing past $450. Per square foot means the total build cost divided by the finished living area. A small ADU often costs more per foot than a full house, because a kitchen, bathroom, HVAC system, and full set of utility connections get packed into a tiny footprint. You pay for those systems whether the unit is 500 square feet or 1,500.

The pricing splits into three rough tiers:

• Conversion (lowest cost). Turning an existing garage or part of the main house into an attached ADU. The shell already exists, so you mainly pay for plumbing, electrical, insulation, and finishes. These often run $80,000 to $180,000.
• Standard detached casita. A new free-standing unit with everyday finishes. Expect $200 to $325 per square foot, or about $150,000 to $300,000 for 600 to 1,000 square feet.
• High-end detached. Custom design, premium finishes, large glass, and a hard lot. These reach $350 to $450+ per square foot.

A conversion is the cheapest path because it skips the most expensive early stages of a build: there is no new foundation, no new framing, and no new roof. You are mostly paying to add plumbing, wiring, insulation, a kitchen, and a bathroom inside a shell that already stands. The trade-off is that you give up a garage or interior space, and older walls sometimes need upgrades to meet current code. A new detached casita costs more, but you keep the garage and get a purpose-built unit you can rent on its own.

Size is capped by law, which also caps your spend. Under A.R.S. 9-461.18, the state requires a city to allow an ADU that is "seventy-five percent of the gross floor area of the single-family dwelling on the same lot or parcel or one thousand square feet, whichever is less." Scottsdale, for example, limits a first ADU to 1,000 square feet and any second unit to 500 square feet. That ceiling keeps most ADU budgets well below a full custom home, since you are building a few hundred square feet, not a few thousand.

Where the money goes in an ADU build

A detached ADU carries the same cost categories as a full house, just compressed, and interior finishes plus systems eat the largest share. The NAHB national construction survey found that across all new homes, construction costs made up 64.4% of the sale price, the finished lot 13.7%, and builder profit about 11% in 2024. An ADU follows the same logic, with a few items that hit harder per square foot:

• Kitchen and bath. The most expensive rooms in any home, and an ADU is mostly kitchen and bath. Cabinets, counters, a range, and plumbing fixtures are a fixed cost that a small unit cannot spread out.
• HVAC. Arizona ADUs need cooling sized for 110-degree summers. A separate mini-split or small package unit adds several thousand dollars.
• Utility connections. Running water, sewer or septic, and a separate electric line to a backyard unit is a real line item, often $10,000 to $30,000 depending on distance and whether you add a separate meter.
• Finishes. Flooring, tile, paint, and trim. This is where budgets swing most, the same way they do on a full home.

The order holds: systems and finishes dominate, and the smaller the unit, the more those fixed costs show up in the per-square-foot number.

Soft costs are real too. Plan for design and engineering (architect or designer fees, structural and energy calculations), city permit and plan-review fees, and any impact or development fees the city charges. On an ADU these typically add a few thousand to low five figures, depending on the city. Phoenix and Scottsdale both require a permit and inspections for an ADU, so this is not optional. Build these into the budget from day one rather than treating them as a surprise at permit time.

What site work and utilities add

Site work and utility hookups can add $15,000 to $60,000 or more on top of the build, and they are the biggest reason two ADUs of the same size cost different amounts. The per-square-foot figures above cover the structure. Getting power, water, and sewer to it is separate.

An ADU behind an existing Phoenix or Scottsdale home with utilities already at the house is the cheaper case. A short trench to extend water, sewer, and power may run $10,000 to $25,000. A unit on a large or rural lot in Rio Verde Foothills, Cave Creek, or Carefree is the expensive case. There you may need:

• A septic system or expansion ($15,000 to $40,000+) where there is no sewer.
• Well capacity or a hauled-water setup if the existing well cannot serve a second unit.
• A long power and water run across the property.

State law helps your budget here in one specific way. A city cannot require extra parking for an ADU and cannot force setbacks greater than five feet from the lot line, so you are not paying for a new parking pad or losing usable yard. Confirm utility capacity before you design the unit, because a panel upgrade or septic expansion can change the whole budget.

Confirm current ADU pricing before you build

Treat every number here as a 2026 planning range, not a quote, because construction prices move with material costs, labor, and interest rates. Lumber, concrete, and skilled-trade labor have all swung sharply in recent years, and a per-square-foot figure that was right last quarter can be off by the time you sign a contract. Always confirm current pricing with a builder on your actual plan and lot.

The real number comes down to four answers: detached or conversion, how large, what finish level, and what shape the lot and its utilities are in. Once you have those, a builder can turn the broad $200 to $400 per square foot range into a firm, itemized bid. For a backyard rental, also weigh the income side: a legal, separately rentable ADU can offset much of the cost over time, which a non-rentable guest house cannot. Get a written scope and a current price before you commit, since the metro Phoenix market can shift within a single building season.

Building with Jematell Homes

Setbacks, overlays, and ADU rules shape our design work from day one. If you are planning a custom home in Scottsdale, Rio Verde, or the greater Phoenix metro, we are happy to walk through your project.

Sources

• NAHB: Cost of Construction Survey 2024 (cost shares of a new home)
• A.R.S. 9-461.18 (Accessory dwelling units; size limit)
• City of Scottsdale: Accessory Dwelling Units (size standards)
• Phoenix Zoning Ordinance 706 (Accessory Uses and Structures)