Two documents govern whether you can put a second dwelling in your Mesa backyard. The first is A.R.S. § 9-461.18, a state statute that tells cities what they must allow. The second is Mesa Zoning Ordinance § 11-31-3, the city’s own implementation. Where they conflict, the statute wins — Tempe says so in its own ordinance, and the principle holds in Mesa too.
This page walks through both. Where a rule has a subsection, we give it, so you can check us against the source rather than taking our word for it.
Can I build a casita in Mesa?
On almost any lot with a house on it, yes. A.R.S. § 9-461.18(A)(1) requires every municipality the statute reaches to allow at least one attached and one detached accessory dwelling unit on any lot or parcel where a single-family dwelling is allowed, and to allow it as a permitted use.
That last phrase is doing a lot of work. A.R.S. § 9-461.18(I)(5) defines “permitted use” as approval without a public hearing, variance, conditional use permit, special permit or special exception. A city may still review a site plan for conformance with its own zoning rules, and you still need a building permit — but there is no neighbor notification, no planning commission, and no discretionary refusal.
Mesa goes a step further at § 11-31-3(A)(2): one attached and one detached unit are permitted on any lot that already contains a single residence, regardless of the zoning district. So a house on a lot zoned for apartments, or for commercial use, still carries the entitlement.
How big can it be?
A.R.S. § 9-461.18(A)(3) sets the size as 75% of the gross floor area of the house, or 1,000 square feet, whichever is less.
“Gross floor area” has a specific meaning here. A.R.S. § 9-461.18(I)(2) defines it as the interior habitable area of the dwelling. Not the lot. Not the area under the roof. Not the footprint including the garage. If your house has 1,600 square feet of conditioned living space, three quarters of that is 1,200, which exceeds the ceiling, so you get the full 1,000 square feet. If it has 1,100, you get 825.
There is also a definitional limit that is easy to miss. A.R.S. § 9-461.18(I)(1) defines an accessory dwelling unit as a self-contained living unit on the same lot as a single-family dwelling of greater square footage. The unit has to be smaller than the house. Under the state formula that is automatic, but it matters where a city allows more than the state requires.
Mesa restates the state formula without adding to it, at § 11-31-3(C): not more than 75% of the primary residence’s interior habitable area, or 1,000 square feet, whichever is less.
How close to the fence can I build?
This is the rule most summaries get wrong, and it is worth reading carefully, because getting it right can mean a unit that is two feet wider.
There are two provisions, not one. A.R.S. § 9-461.18(B)(6) says a city may not set rear or side setbacks for an accessory unit at more than 5 feet from the property line. Separately, A.R.S. § 9-461.18(B)(5) says a city may not set restrictions on an accessory unit that are more restrictive than those for single-family dwellings in the same zoning area as to height, setbacks, lot size or coverage, or building frontage.
Read together, five feet is a ceiling, not a constant. If your district requires a house to sit 15 feet from the rear line, your unit only has to sit five. But if your district lets a house sit three feet from the side line, the parity rule means your unit should get three feet too — not five. Anything that quotes a flat five feet everywhere is applying only half the statute.
The front setback is different: the statute caps rear and side only, so the front follows whatever the district requires of a house, and the city may not make it stricter for your unit.
Mesa § 11-31-3(D)(1) requires “a minimum side and rear setback of five (5) feet” and sends the front and street-facing side setbacks to the underlying district’s standards for the house.
Where Mesa and the statute may part company
One genuinely unsettled point: whether the street-facing side yard of a corner lot counts as a “side” setback for the purposes of the five-foot cap. The statute says “rear or side” without addressing street-side yards, and the two cities read it differently — Tempe applies its five-foot figure to the street side, Mesa sends it back to the district standard. If you are on a corner lot, ask.
Do I need extra parking?
No. A.R.S. § 9-461.18(B)(3) prohibits a city from requiring a lot to have additional parking to accommodate an accessory dwelling unit, and from requiring payment of fees instead of that parking. Both halves matter: a fee in lieu is the usual workaround, and the statute closes it.
Mesa § 11-31-3(G) adds something useful: if you convert a garage or carport into a unit, you are not required to replace the covered parking elsewhere on the lot. The city’s own guidance still expects the house to retain two parking spaces, so plan for that.
Can I rent it out? Do I have to live there?
You can rent it, and you do not have to live on the property. These are the two questions homeowners thinking about rental income ask first, and the statute answers both plainly.
A.R.S. § 9-461.18(B)(1) bars a city from prohibiting the use or the advertisement of either the house or the accessory unit as separately leased long-term rental housing — defined at A.R.S. § 9-461.18(I)(3) as a lease of ninety days or longer, or month to month.
A.R.S. § 9-461.18(B)(2) bars a city from requiring any familial, marital, employment or other pre-existing relationship between the owner or occupant of the house and the occupant of the unit. Between them, those two provisions rule out the owner-occupancy conditions that used to be standard in ADU ordinances. Neither Mesa nor the statute imposes one.
Short-term rental is a separate regime, governed by city licensing rather than by this statute. Mesa requires a short-term rental license for stays of 29 days or fewer. In both cities the unit may not be sold separately from the lot.
The utility easement problem
This is the provision that catches people out, and it is the reason to look at your plat before you look at floor plans.
A.R.S. § 9-461.18(E) says an accessory dwelling unit may not be built on top of a current or planned public utility easement unless the owner receives written consent from every utility that uses that easement now or may use it in future.
Public utility easements in valley subdivisions commonly run along the rear and side lot lines, frequently eight feet deep at the rear. That is precisely where the statutory five-foot setback invites you to put a detached unit. The easement does not stop you building — it means you need consent, or you shift the structure. Your plat, recorded with the Maricopa County Recorder, shows where the easements are, and your subdivision’s map of record is on your deed.
Mesa restates the requirement at § 11-31-3(H)(3), extending it to a public utility easement “of any kind”, and requires the unit to be served by the same water, sewer and electric provider as the house.
Your HOA can still say no
The statute binds municipalities. It does not bind your neighbors.
A.R.S. § 9-461.18(C) is explicit that the section “does not prohibit restrictive covenants concerning accessory dwelling units entered into between private parties”. Your subdivision’s CC&Rs, in other words, still apply, and large parts of the East Valley are covered by them. Read yours before you spend money on design.
What the city may not do is make a covenant a condition of your permit. The same subsection prohibits a municipality from conditioning a permit, license or use of an accessory unit on adopting or implementing a private covenant, and A.R.S. § 9-461.18(B)(8) separately prohibits requiring a restrictive covenant on a lot zoned for single-family residential use.
Building codes, sprinklers and design
A.R.S. § 9-461.18(D) preserves ordinary building codes, fire codes and public health and safety regulations — with two exceptions that matter a great deal to cost. A city may not require a commercial building code to be applied to an accessory dwelling unit, and may not require a fire sprinkler system. Retrofitting sprinklers into a detached backyard structure, including the water service upgrade it usually implies, is a five-figure line item the statute removes.
A.R.S. § 9-461.18(B)(4) also bars a city from requiring the unit to match the exterior design, roof pitch or finishing materials of the house. Mesa’s ADU section contains no design standards at all, though a unit in a Historic District has to comply with the city’s historic provisions and needs a Certificate of Appropriateness. An HOA may still require a match.
Mesa does restrict construction type: § 11-31-3(B)(6) prohibits manufactured homes, recreational vehicles and trailers from being used as an accessory dwelling unit. The unit also needs its own separate, independent entrance, may not connect internally to the house, and may contain no more than one kitchen.
Can I build two? Three?
A.R.S. § 9-461.18(A)(1) guarantees one attached and one detached unit. That is a floor, and both cities match it.
A.R.S. § 9-461.18(A)(2) adds one further detached unit on a lot of one acre or more (43,560 square feet) if at least one unit on the lot is a restricted-affordable dwelling unit — defined at A.R.S. § 9-461.18(I)(6) as one that, through a deed restriction or a development agreement with the city, is rented to households earning up to 80% of area median income.
Mesa implements this at § 11-31-3(A)(3), through either a private deed restriction or a development agreement with the city.
Where the statute does not apply
A.R.S. § 9-461.18(G) carves out three categories of land, and (H) limits the statute to cities of a certain size. We run these checks before calculating anything, because an entitlement worked out on land the statute does not reach is worse than no answer.
- Tribal land — A.R.S. § 9-461.18(G)(1). The Salt River Pima-Maricopa Indian Community borders both Mesa and Tempe, so this is a live question here, not a theoretical one.
- Near a military airport — A.R.S. § 9-461.18(G)(2) excludes land in the territory in the vicinity of a military airport or ancillary military facility as defined at A.R.S. § 28-8461.
- High airport noise — A.R.S. § 9-461.18(G)(3) excludes land near an FAA commercially licensed airport, a general aviation airport, or a public airport as defined at A.R.S. § 28-8486, where the noise level exceeds 65 decibels.
- Small municipalities — A.R.S. § 9-461.18(H) applies the section only to municipalities over 75,000 people. Mesa, at 517,142, is well over.
Mesa publishes the relevant boundaries, which lets us check them properly. Its Airfield Overlay District (§§ 11-19-2 to 11-19-4) maps a runway protection zone where no building is permitted at all, plus three overflight areas around Falcon Field and Phoenix-Mesa Gateway. Overflight Area 1 is defined as the area inside the 65-decibel contour, which maps onto the statutory threshold; Area 2 lies between the 60 and 65 contours and carries use limits in the city’s own tables.
What happens if a city never adopted rules
A.R.S. § 9-461.18(F) is the enforcement mechanism: a municipality that failed to adopt conforming regulations by 2025/01/01 must allow accessory dwelling units on all lots zoned for residential use, without limits.
Both Mesa and Tempe adopted ordinances inside the deadline, so the fallback does not apply here. It is, however, why their ordinances exist and why they read the way they do.
Mesa in particular
Mesa’s lot pattern varies sharply by era. The RS-6 and RS-7 subdivisions that fill much of west and central Mesa have 10-foot front setbacks and 5-foot side setbacks, so the statutory five-foot cap changes nothing on the sides but the shallow front setback leaves a usable rear yard. The RS-9 and RS-15 neighborhoods east of Greenfield have deeper required yards for a house — 7 feet to the side, 25 to 30 feet at the rear — and that is where the five-foot cap transforms what will fit.
The RSL small-lot districts are the interesting edge case. RSL-2.5 requires only 3 feet to each interior side for a house, and RSL-3.0 and RSL-4.0 require 4 feet. Mesa’s ADU section asks for 5 feet in every district, which is more than the district requires of a house — and the state statute says an accessory unit may not be held to a stricter standard. If you are on a small lot in one of those districts, it is worth asking the city about.
Lots of an acre or more are common in the Lehi area, in the far north-east, and in pockets of RS-35 and RS-43 zoning. On those lots a second detached unit becomes available if one of them is deed-restricted as affordable to households at or below 80% of area median income.