Two documents govern whether you can put a second dwelling in your Tempe backyard. The first is A.R.S. § 9-461.18, a state statute that tells cities what they must allow. The second is Tempe Zoning and Development Code § 3-402, 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 Tempe?
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.
Tempe implements this at ZDC § 3-402(B): accessory units are permitted in every residential district listed at § 2-102, wherever the property contains a single-family dwelling, and the property keeps its single-family status. Tempe also excludes accessory units from its multi-family building design, parking, landscape and wall, and lighting chapters, and from development plan review at § 6-306.
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.
Which Tempe does. ZDC § 3-402(C)(2)(B) raises the ceiling to 2,000 square feet on lots of one acre or more, still subject to the 75% rule. On a one-acre lot with a 3,000 square foot house, state law entitles you to 1,000 square feet and Tempe allows 2,000 — so reading the statute alone would understate what you can build by half. Tempe also excludes garages, carports and exterior storage from the gross floor area calculation.
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.
Tempe ZDC § 3-402(C)(5) requires five feet to the rear, side and street side in every district, “unless a lesser setback is established by the district” — which is Tempe implementing the parity rule itself. One useful local wrinkle: where your lot abuts a dedicated public alley, the rear and side setbacks are measured from the midpoint of the alley rather than from your lot line, which on a typical 20-foot alley recovers ten feet of depth. No part of the structure may extend into the alley.
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.
Tempe ZDC § 3-402(C)(3) requires no additional parking, but adds a condition: if your unit removes or modifies a parking space that was required for the main residence, that space has to be replaced in a location the Community Development Director approves.
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 Tempe nor the statute imposes one.
Short-term rental is a separate regime, governed by city licensing rather than by this statute. Tempe permits short-term and vacation rental subject to Tempe City Code Chapter 16a, Article X, which carries its own requirements. 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.
Tempe requires residential sewer, water and utility connections to the unit built to city standards, at ZDC § 3-402(F)(1).
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. Tempe goes further and exempts accessory units from its building design chapter entirely. An HOA may still require a match.
Tempe is also unusually permissive about construction type. ZDC § 3-402(F) allows tiny-home construction on a permanent foundation, prefabricated and modular units permitted through the building safety division, and even a unit built on a trailer once the suspension and axle components are removed and the chassis is permanently attached to a foundation. Mesa prohibits manufactured homes, recreational vehicles and trailers outright at § 11-31-3(B)(6) — so a product that works in Tempe may not be permittable in Mesa.
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.
Tempe implements this at ZDC § 3-402(G) and attaches real administration to it: the deed restriction has to be recorded through the Community Development Department before the permit issues, rents follow the Arizona Department of Housing limits, and the owner files an annual compliance report. Failure to report carries a civil sanction or forfeiture of the unit.
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. Tempe, at 190,113, is well over.
Tempe is where we have to be honest about a gap. Phoenix Sky Harbor’s approach path crosses north Tempe, and the city does not publish a 65-decibel noise contour we can query. So for Tempe addresses we report airport noise as unverified rather than claiming to have checked it, and the report says so rather than returning a confident pass.
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.
Tempe in particular
Tempe’s housing stock is mostly 1950s to 1970s, on lots that are small by valley standards — the R1-6 district, which covers a large share of the city, has a 6,000 square foot minimum and a 20-foot front setback. Lots near Arizona State University and in the Maple-Ash and Mitchell Park neighborhoods are narrower still. On a 50-foot-wide lot, the five-foot side setback on each side leaves 40 feet of width, which is enough for a unit but makes the alley-adjacency rule worth knowing: where the lot abuts a dedicated public alley, setbacks are measured from the midpoint of the alley rather than the lot line, which recovers several feet.
The 45% coverage cap in most of the R1 districts is often the binding constraint in Tempe rather than the statute. A 6,000 square foot lot allows 2,700 square feet of building; a 1,400 square foot house with an attached garage can already be most of the way through that budget. This is the single most common reason a Tempe report comes back with a number well under 1,000 square feet.
Lots of an acre or more are rare in Tempe but do exist, mostly in the older areas south of Baseline and in a few pockets along the canal. Those are the lots where the city’s 2,000 square foot allowance, and the absence of a bedroom cap, change the project substantially.
One honest gap: Phoenix Sky Harbor’s approach path crosses north Tempe, and the statute does not apply to land where airport noise exceeds 65 decibels. Mesa publishes its noise contours; Tempe does not publish one we can query. So for Tempe addresses we flag airport noise as unverified rather than claiming to have checked it.