A detached ADU, often called a casita in Phoenix, is a separate building on your single-family lot with its own sleeping area and bathroom. Arizona law lets the owner decide whether it has a kitchen. It gives you the most privacy of any ADU type, and usually the highest construction cost.
How large can a detached ADU be in Phoenix?
As of October 2026, the City of Phoenix ties the cap to lot size:
- Lots up to 10,000 sq ft of net area: up to 1,000 sq ft.
- Lots over 10,000 sq ft: the lesser of 3,000 sq ft or 10% of net lot area.
- On any lot, the ADU cannot exceed 75% of the main home's gross floor area.
Lot coverage for your zoning district still applies, and every roofed structure counts toward it. A big patio cover can use up room you wanted for the casita.
How tall can it be?
Phoenix allows up to 15 feet when the ADU sits inside a required yard, unless a use permit approves more. Outside the required yards, the city's amendment lets an ADU reach the same height allowed for the main house. We decide placement and height together, because one drives the other.
What setbacks apply to a detached ADU?
Under the Phoenix rules adopted to meet state law, a detached ADU may sit in the required rear yard, at least 3 feet from an interior side or rear property line and 5 feet from a street-side line. Next to a fully dedicated alley, no setback is required. Utility easements are a separate limit: state law bars building over a current or planned public utility easement without written consent from the utility.
How many can I build?
As of October 2026, Phoenix permits two ADUs on a lot with one single-family detached home. A third may be allowed on a lot of at least one acre when one of the units is income-restricted affordable housing. Lots with a duplex or triplex cannot add an ADU.
Can I use the city's free plans?
Phoenix created a Standard Plan Library from its 2024 ADU design competition. The plans are free to City of Phoenix residents and must be submitted as is; any change sends the project through regular review. When we last checked, the city listed the plans as still in review, so we confirm availability before counting on one. A custom design fits irregular lots better anyway.
What does a detached casita cost?
As of October 2026, one 2026 Arizona cost guide puts a 600 sq ft Phoenix casita at about $120,000 basic, roughly $160,000 to $220,000 mid-range and about $330,000 high-end. Trenching for sewer, water and power to the back of the lot is often the biggest swing item. See the full cost guide.
What changes the plan for a detached ADU?
- Lot size and coverage. They set the cap before design even begins.
- Utility runs. Distance to the sewer tap, panel capacity, and whether your electric provider asks for a separate service.
- Soil. Caliche, a hard calcium layer common in the Valley, can slow footing work.
- Sun. Window placement and roof color matter more here than almost anywhere. See keeping an ADU cool.
- HOA rules. City approval does not override private covenants. See the HOA guide.