Guest House on Arizona Horse Property: ADU Rules

The short answer: In many cases, yes, you can put a guest house on Arizona horse property. A.R.S. 11-810.01 requires counties, and A.R.S. 9-461.18 requires cities with more than 75,000 people, to allow at least one attached and one detached accessory dwelling unit where a single-family home is allowed. Building codes, the septic, the well, easements and any HOA covenants still have to line up.

A guest house on Arizona horse property is a question I hear on acreage showings. A place for an aging parent. A room for the person who feeds when you travel. A little rental that helps carry the mortgage. For years the answer was "it depends on your town."

That changed. Arizona now has two state laws that tell cities and counties what they must allow when it comes to accessory dwelling units, the official name for a casita or guest house. I read both statutes start to finish, and here is what they say in plain English.

Both laws use the same definition. An accessory dwelling unit is "a self-contained living unit" on the same lot as a single-family home that is bigger than it, with "its own sleeping and sanitation facilities," and that "may include its own kitchen facilities."

So a bedroom and a bathroom make it a dwelling unit. A kitchen is optional.

Larger ones do. A.R.S. 9-461.18 applies to "a municipality with a population of more than seventy-five thousand persons." Those cities and towns had to adopt regulations that allow, on any lot where a single-family home is allowed:

• At least one attached and one detached accessory dwelling unit as a permitted use.
• One more detached unit on a lot of one acre or more, if at least one unit on the lot is a restricted-affordable dwelling unit.
• A unit that is 75 percent of the gross floor area of the main house or 1,000 square feet, whichever is less.

"Permitted use" matters. The statute defines it as approval without a public hearing, variance, conditional use permit, special permit or special exception.

If a covered city did not adopt its rules by January 1, 2025, the statute says accessory dwelling units "shall be allowed on all lots or parcels zoned for residential use in the municipality without limits."

Your town's population and its own ADU code are things I check before we write an offer. Smaller towns are not covered by this section, so their own code is the whole story.

If your acreage is in unincorporated county land, the county law applies, and it is broader. A.R.S. 11-810.01 says "a county shall adopt regulations" allowing at least one attached and one detached unit as a permitted use, and "a minimum of one additional detached accessory dwelling unit" on a lot of one acre or more. The county may require one of the units on that lot to be restricted-affordable.

I read the full section, and the county version has no population cutoff. The size rule says a county shall allow each unit to be "at least seventy-five percent of the gross floor area" of the main home or 1,000 square feet, whichever is less, and it adds that a county may allow larger.

The county deadline was January 1, 2026. Miss it, and the statute says units are allowed on all residentially zoned lots "without limits."

Both statutes list the same core protections for the owner. A covered city or a county may not:

• Prohibit renting the main house or the guest house as separate long-term rental housing. "Long-term rental" means a lease of 90 days or longer, or month-by-month.
• Require a family, marital, employment or other relationship between you and the person living in the unit.
• Require extra parking, or fees instead of parking.
• Require the unit to match the main house's exterior design, roof pitch or finishing materials.
• Set rear or side setbacks of more than five feet from the property line.
• Require a fire sprinkler or a commercial building code.

Building codes, fire codes and health and safety rules still apply. The county statute also keeps wildfire prevention, emergency vehicle access, drainage and flood control rules in force. If your acreage sits near a wash, read our guide to buying horse property near washes and flood zones before you pick a spot for the casita.

In some county locations, yes. A.R.S. 11-820.03 says a county "shall allow" a park model trailer as an accessory dwelling unit where the zoning is one dwelling unit per three acres or greater. The statute defines a park model as 320 to 400 square feet of interior area when set up. It has to sit on a semi-permanent or permanent foundation with utility connections, meet septic, drainage and floodplain rules, and the county may require a building permit and inspection.

This is the part people skip, and on horse property it is the part that costs money.

The county statute says that if a new unit will not be on sewer, a county "may require that any septic system that will be used to serve the accessory dwelling unit be adequately sized before the construction." If the property is on septic, plan on that conversation. Our breakdown of septic systems on East Valley acreage walks through what to ask.

The county law also says it does not prohibit "shared well agreements as provided by state law" between private parties. If the casita will draw from the same well as the house, the barn and the troughs, know what that well produces first. Start with well water on Arizona horse property.

And where you put the unit matters. The statutes say a unit may not be built on top of a current or planned public utility easement without written consent from the utility. Our guide to easements and right-of-way on rural Arizona land shows you how to find those lines on a title report.

Yes. Both laws say they do not prohibit restrictive covenants about guest houses entered into between private parties. What the city or county cannot do is make a covenant a condition of your permit. If the property has an HOA, read the CC&Rs before you plan the build.

Yes. Neither statute applies on tribal land, in the territory near a military airport as the statute defines it, or near certain airports where the noise level is greater than 65 decibels. If a property is close to an airfield, I check that before a buyer falls in love with the casita plan.

• Assuming every town has to follow the state city rules. A.R.S. 9-461.18 covers cities and towns with more than 75,000 people. Smaller towns follow their own code.
• Forgetting the septic. In the county, the septic can be required to be adequately sized before construction.
• Adding a home to a well you have not checked. Know what the well produces before the casita draws from it too.
• Building on a utility easement. That takes written consent from the utility.
• Skipping the CC&Rs. Private covenants about guest houses still apply.

I am Kim Williamson, REALTOR®, with REAL Broker, and I help buyers and sellers of horse property and acreage across the Metro Plex of Arizona. If you are buying, tell me early if a guest house is part of the dream. I want to know the zoning, the septic, the well and any easements before we write the offer, not after. If you are investing, our Arizona horse property investment guide is a good next read.

If you are selling and you already have a permitted casita, say so loudly in the listing. If it was never permitted, talk to me before we list. Barns are a different animal, and our guide to barn, arena and outbuilding permits in Arizona covers those rules.

Can I build a guest house on my horse property in Arizona? In many cases, yes. A.R.S. 11-810.01 requires counties, and A.R.S. 9-461.18 requires cities with more than 75,000 people, to allow at least one attached and one detached accessory dwelling unit where a single-family home is allowed.

How big can a casita be in Arizona? Covered cities and counties must allow a unit of 75 percent of the main home's gross floor area or 1,000 square feet, whichever is less. The county statute says a county may allow larger.

Can I rent out my casita? A covered city or a county may not prohibit renting the main home or the unit as separate long-term rental housing, meaning a lease of 90 days or longer or month-by-month.

Do I need a permit for a casita? Building codes, fire codes and health and safety rules still apply, so plan on permits and inspections. Confirm with your city or county.

Can an HOA stop me from building a guest house? The statutes do not prohibit private restrictive covenants about accessory dwelling units, so read the CC&Rs.

Want to read the law for yourself? These are the primary sources this article is built on, straight from the Arizona State Legislature (links checked October 4, 2026):

• A.R.S. 9-461.18: Accessory dwelling units in cities and towns
• A.R.S. 11-810.01: Accessory dwelling units in counties
• A.R.S. 11-820.03: Park model trailer as an accessory dwelling unit

Do your own due diligence. This article is general information to help you ask better questions. It is not legal advice. Always do your own due diligence and confirm the details for your specific property with your city or county planning office before you design or build anything.

Thinking about horse property in the Metro Plex of Arizona, or want room on your acreage for family? I am Kim Williamson, REALTOR®, with REAL Broker, and I help buyers and sellers of horse property and acreage across the Metro Plex of Arizona. I have lived in Arizona 36 years, sold real estate here for 24 years, closed 1,000+ transactions, and I am an 8x WPRA World Champion. Call or text 480-206-1500, or start at www.ArizonaHorsePropertyForSale.com.