What casita can you actually build?

How the size rules work

Arizona's state ADU statute, HB 2720 (effective January 1, 2025), sets a floor that most large cities must allow. It works as two caps stacked on top of each other, and your maximum casita size is whichever cap comes in smaller.

The lot-based cap is straightforward. On a lot up to 10,000 sq ft, the city must permit a casita of up to 1,000 sq ft. On larger lots, the cap becomes the lesser of 3,000 sq ft or 10% of your net lot area. So a 15,000 sq ft lot is capped at 1,500 sq ft (10%), and a 40,000 sq ft lot is capped at 3,000 sq ft (the hard ceiling).

The house-based cap is where a lot of homeowners get surprised. The statute also says an ADU can't exceed 75% of the primary home's gross floor area. A 1,200 sq ft house therefore caps out at 900 sq ft of ADU, even if the lot could physically support a 1,000 sq ft building. That's the cap the calculator flags when your house is small relative to your lot — and it's the one people miss when they buy a small older home on a big backyard expecting to build to 1,000.

Why lot size and house size both matter

The two caps exist for different reasons. The lot cap protects neighborhood scale — you can't put a 3,000 sq ft second home in the backyard of a typical suburban lot. The 75%-of-house rule protects the concept of what an "accessory" dwelling is: the ADU is supposed to be secondary to the primary residence, not larger than it. If you own a 900 sq ft cottage in a walkable Phoenix neighborhood, you can build a 675 sq ft ADU behind it, not a 1,000 sq ft one.

The Arizona casita cost guide walks through what those square footages actually cost in metro Phoenix — roughly $150 to $300+ per square foot all-in — and the financing guide covers how HELOCs, cash-out refinances, renovation loans, and construction-to-permanent loans compare for a build in this range.

What this calculator can't tell you

The calculator applies the state statute's baseline. It doesn't know about your HOA. HB 2720 preempts city zoning; it does not override private CC&Rs. If your neighborhood has an architectural review committee, that committee can still require matching materials, screening, and specific setbacks, and can slow down or effectively block a project even when the city can't.

It also doesn't know your city's specific permit process, impact fees, or rental rules — those are set locally and change often. Read the full HB 2720 explainer for the statute in plain English, and check the city page linked in your result for what we've verified about your local jurisdiction.

Finally, the calculator assumes a single-family lot in an incorporated city over 75,000 people (or an unincorporated county lot governed by HB 2928). Small towns under 75,000 aren't required by state law to allow ADUs at all — the tool flags this and points you to the rural Arizona guide.

Frequently asked questions