How big can a one or two bedroom ADU be in Arizona?

Arizona law does not limit how many bedrooms an ADU (accessory dwelling unit, often called a casita) may have. It limits size. In a city of more than 75,000 people, the city must allow an ADU of at least 75% of the main home's gross floor area or 1,000 square feet, whichever is less. That is enough room for a one bedroom unit on almost any lot, and for a two bedroom unit once the main home is about 1,334 square feet or larger, since 75% of that size reaches the 1,000 square foot mark. Whether you choose one bedroom or two, each bedroom has to meet the building code on its own: a minimum room size, a ceiling height, an emergency escape window and a smoke alarm. On a rural lot with septic, the bedroom count also shapes the septic system, so it is worth deciding early.

Below is what the law says, how much space each layout has to work with in Phoenix and Scottsdale, how to choose between one bedroom and two, and what every ADU bedroom must meet.

What Arizona law says about ADU size

The city ADU statute, A.R.S. 9-461.18, sets the floor every covered city has to allow. Its words: a city must permit "an accessory dwelling unit 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." The statute applies to a municipality "with a population of more than seventy-five thousand persons," which takes in Phoenix, Scottsdale and the other large Valley cities. Counties came under a matching law that took effect on January 1, 2026.

The statute defines an ADU as a self-contained living unit with its own sleeping and sanitation facilities, on the same lot as a single-family home. It says nothing about the number of bedrooms. So a studio, a one bedroom and a two bedroom unit are all ADUs, as long as the unit fits the size cap, the setbacks and the building code.

How much space a one or two bedroom ADU has to work with

The size cap is set by the main home, so the same lot can allow a different ADU from one house to the next. A few worked examples:

• A 1,200 square foot main home. 75% of 1,200 is 900 square feet, so the city may hold the ADU to 900 square feet. That comfortably fits one bedroom and is tight for two.
• A 1,800 square foot main home. 75% would be 1,350, but the 1,000 square foot figure is smaller, so 1,000 square feet is the floor. Room for two bedrooms, a bathroom, a kitchen and a living area.
• A larger Phoenix lot. Under Phoenix Zoning Ordinance Section 706, each ADU on a lot up to 10,000 square feet caps at 1,000 square feet. On a larger lot the cap rises to the lesser of 3,000 square feet or 10% of the net lot area, and the 75% of the main home test still applies. A large home on acreage can therefore carry a roomy two bedroom unit.
• A second ADU in Scottsdale. Scottsdale limits the first ADU to "1,000 square feet or 75% of the size of the existing main single-family residential dwelling on the same lot, whichever is less," and a second or third ADU to 500 square feet. A 500 square foot unit is usually a studio or a compact one bedroom.

Lot size matters as well as house size. The City of Phoenix notes that typical Phoenix lots run 6,000 to 8,000 square feet, and on a lot that size the setbacks, the 10-foot spacing Scottsdale requires between buildings, and lot coverage limits can cap the footprint before the square foot figure does. A two-story ADU can fit more on a small lot, though Section 706 holds a detached ADU inside the required rear or side yard to 15 feet tall. Our page on ADU setback and size rules by city lists the limits city by city.

One bedroom or two: how to choose

Start with who will live there. The layout that fits the use is almost always the right one.

• An aging parent or a single relative. One bedroom is usually enough, and the space you save can go to a wider bathroom, a walk-in shower and a bigger living area. Single-level living with wide halls and doors matters more here than a second room.
• An adult child, a couple or a caregiver. A second bedroom gives a live-in caregiver or a roommate a room of their own, or doubles as an office.
• A long-term rental. State law bars a covered city from banning a long-term rental of the ADU. A two bedroom unit widens the pool of renters, while a one bedroom unit costs less to build and furnish. Check your city's owner occupancy rule before you plan around rental income.
• Guests and a home office. One bedroom plus a flexible den often works better than two small bedrooms.

The trade-off is space. Under a 1,000 square foot cap, a second bedroom takes area from the kitchen and living room. It also adds an egress window, a closet and, on a septic lot, more load on the system. If the cap is closer to 700 or 800 square feet, one bedroom usually gives the better home.

What every ADU bedroom has to meet

A room is a bedroom in the eyes of the plan reviewer only when it meets the residential code. The International Residential Code, which Arizona cities and counties adopt in their own editions, sets these minimums:

• Room size. A habitable room needs at least 70 square feet of floor area and must be at least 7 feet in any horizontal direction (IRC R304).
• Ceiling height. At least 7 feet in habitable rooms (IRC R305).
• An emergency escape window. Every sleeping room needs an opening of at least 5.7 square feet net clear, at least 24 inches high and 20 inches wide, with a sill no more than 44 inches above the floor (IRC R310). This is the most common reason a casita bedroom fails plan review; see why plan reviewers reject casita bedroom egress.
• Smoke alarms. One in each sleeping room and one outside each sleeping area (IRC R314).

Editions vary by jurisdiction: Scottsdale uses the 2021 I-codes, Phoenix the 2024 Phoenix Building Construction Code, and unincorporated Maricopa County the 2018 I-codes. These bedroom minimums read the same way in each, but confirm the details with the jurisdiction that will review your plans.

Rural lots: septic, wells and the bedroom count

On a lot with city sewer, a second bedroom changes little outside the walls. On a rural lot it can change a lot. State law lets the county require that a septic system serving a new ADU be sized for it before construction starts, and a septic system is sized by how much wastewater the home will send it, which grows with the number of bedrooms. A lot on a well also has to show it can supply a second home. In the Rio Verde Foothills, Cave Creek and the rest of unincorporated Maricopa County, settle the bedroom count before the septic design, not after. Our page on what an ADU is in Arizona covers the county rules in more detail.

Building a one or two bedroom ADU with Jematell Homes

We build casitas, guest houses and ADUs across the Phoenix metro, as a stand-alone project on a lot you already own or alongside a new custom home. If you want the casita built into the house, our 2045 plan is a single-story home of 2,045 square feet with an attached casita; see our floor plans. You get a written progress update every week of construction. Licensed under Arizona ROC# 339367. For costs, see what an ADU costs to build in Arizona, and when you are ready, tell us about your lot through our contact page.

Sources

• A.R.S. 9-461.18 (Accessory dwelling units; municipalities)
• City of Scottsdale: Accessory Dwelling Units (size limits and spacing)
• City of Phoenix: Accessory Dwelling Units
• 2018 IRC R304 (Minimum room areas)
• 2018 IRC R310 (Emergency escape and rescue openings)