An attached ADU shares at least one wall with the main house but works as its own unit, with sleeping space, a bathroom and an outside entrance. Under Arizona law, cities of more than 75,000 people, Phoenix included, must allow one attached and one detached ADU on a single-family lot.
What size and setbacks apply in Phoenix?
The same size caps as a detached unit: up to 1,000 sq ft on lots up to 10,000 sq ft, and up to 3,000 sq ft or 10% of the lot on larger ones, never more than 75% of the main home. As of October 2026, Phoenix lets an attached ADU project into the required rear yard, keeping at least 3 feet from the rear line and 5 feet from a side line.
Can the attached unit match the house?
It can, and most owners want it to. It does not have to: Arizona law bars cities from requiring an ADU to match the main home's exterior design, roof pitch or finish materials. Your HOA's architectural rules are a different matter.
Why choose attached over detached?
- Shorter utility runs. Plumbing and power are already next door.
- Less yard lost. Pool, citrus trees and play space can stay.
- Shared shade. A unit tucked against the house picks up shade the open yard does not have.
- Closer support. Families housing a parent often prefer a few steps instead of a walk across the yard.
What makes attached ADUs harder?
Tying a new roof into an old one, opening walls, and keeping the main house livable during construction. Fire separation between the units also has to meet the building code; state law does not require fire sprinklers in an ADU, but separation walls still apply. We plan the tie-in carefully so the roof does not become a monsoon leak point.
What does it cost?
A Phoenix-area builder's July 2026 example puts a roughly 450 sq ft attached unit with a kitchenette and full bath near $225,000, versus about $325,000 for a similar detached one. Published figures vary widely. See the Phoenix ADU cost guide.