Casita vs. ADU vs. Guest House: What's the Difference?

If you've been researching backyard living space in Arizona, you've seen the words casita, ADU, and guest house used almost interchangeably. They overlap — but the differences matter for permits, rentability, and cost. Here's the plain-English version.

The quick definitions

• Casita: a Southwest term for a small, usually detached secondary structure. A casita may or may not have a full kitchen.
• ADU (accessory dwelling unit): a legal designation for a complete, independent living unit with its own kitchen, bathroom, and entrance.
• Guest house: an informal term for extra space for visitors — often no full kitchen, and not always a separate legal unit.

In other words: an ADU is a legal category, while casita and guest house are descriptive names. A casita that has a full kitchen, bathroom, and private entrance usually qualifies as an ADU.

Which one should you build?

If you want rental income or a fully independent space for a family member, build it to ADU standards so it's legal to occupy independently. If you just need a flexible room for guests, a home office, or a hobby space, a simpler casita or guest suite may cost less and still add value. The right answer depends on your goals, your lot, and your city's zoning.