ADU, casita or guest house: same idea, different rulebooks
An accessory dwelling unit (ADU) is a second, smaller home on a lot that already has a house. In Tucson most people call it a casita. The City of Tucson describes an ADU as an independent housing unit with its own kitchen, bathroom, living and sleeping space (City of Tucson ADU page). Unincorporated Pima County uses a different word and a different rulebook: a detached unit there is a guest house, covered on our guest house builder page.
The name matters less than the jurisdiction. The same backyard unit can be allowed, limited or treated differently depending on whether your lot is in the City of Tucson, unincorporated Pima County, or a town like Oro Valley, Marana or Sahuarita. We confirm that before any drawings are made.
Attached, detached or interior ADU
The City of Tucson recognizes ADUs that are detached, attached, or interior to the main home, and it lists garage, basement and attic conversions as interior units. Each type builds differently.
Attached ADU
An attached ADU shares at least one wall with the main house but has its own entry, kitchen and bathroom. It can often tie into existing plumbing and power, and it keeps more of your yard. The tricky part is the tie-in: joining a new roof to the old one and matching the stucco so it looks original. Our own crew does the roofing and stucco, so that tie-in stays under one contractor.
Detached ADU
A detached ADU is a separate building with its own foundation, walls and roof. It gives the most privacy for a tenant, an adult child or visiting family. It also needs its own utility runs, so the distance to your sewer line and electric panel affects cost. See our casita cost guide for what moves the number.
Converted space
Turning an existing garage into living space avoids a new foundation and roof shell, though the floor, walls, windows and utilities still need real work. Our garage conversion page covers what that involves.
What the City of Tucson must allow
Arizona's HB 2720 (2024) applies to cities with more than 75,000 people, and Tucson is one of them. The city adopted its updated ADU rules on October 22, 2024, effective November 21, 2024. Under the state law, Tucson has to:
- Allow at least one attached and one detached ADU on a lot zoned for a single-family home.
- Allow an ADU up to 75% of the main home's floor area or 1,000 sq ft, whichever is smaller.
- Keep side and rear setbacks to no more than 5 feet.
- Not require extra parking for the ADU.
- Allow long-term rental. If an ADU built after the law is used as a short-term rental, the owner has to live on the property.
The city also requires a cool roof on newly built ADUs. Height, lot coverage and other details still come from the city's zoning code, which we check against your lot. Our Tucson casita rules guide goes deeper on permits.
Outside the City of Tucson
Oro Valley, Marana and Sahuarita are under the 75,000 population line, so HB 2720's requirements don't apply there. Each town sets its own rules, and many neighborhoods add HOA review on top. In unincorporated Pima County, including the Catalina Foothills and much of Vail, a detached unit is permitted as a guest house under the county's guest house rules. If you're in one of those areas, see our pages for Oro Valley, Marana, the Catalina Foothills or Vail.
Designing an ADU that works in Tucson
- Size to the cap, not past it. Our casita floor plans guide shows how the 75% rule works out for different home sizes.
- Keep plumbing together. Putting the kitchen and bathroom on a shared wall shortens pipe runs.
- Plan for heat. Insulation, window placement and a dedicated mini-split matter more here than almost anywhere.
- Check the ground. Caliche, washes and septic systems all affect where a detached ADU can go and what it costs.
- Match the house. Same stucco texture, paint and roof material, so it reads as part of the property.
How we build your ADU
We walk the lot, confirm the jurisdiction and utilities, and talk through who the ADU is for. Then plans (a City of Tucson pre-approved plan or custom drawings), the permit, and the build: foundation, framing, roofing, stucco and interiors by one licensed contractor (AZ ROC #344537). You get a written estimate before anything starts. Not sure an ADU beats a simple addition? Read casita vs home addition.