Why ADUs are receiving more attention in Arizona
Accessory dwelling units—often called casitas, guest houses, in-law suites or granny flats—can create independent living space on the same property as an existing single-family home. They are increasingly considered for aging parents, adult children, long-term rental housing, guests, caregivers and flexible living arrangements.
Arizona law has expanded ADU opportunities by requiring municipalities to allow at least one attached and one detached ADU as permitted uses on qualifying lots where a single-family dwelling is allowed. Phoenix and Tempe have updated their local rules in response. The change removes some older zoning barriers, but it does not eliminate building permits, plan review, utility requirements, setbacks, lot-coverage limits or private HOA restrictions.
What does an ADU cost in the Phoenix area?
These are preliminary construction ranges based on Ocotillo Construction's experience, not advertised bids or guaranteed prices. They exclude architectural plans, engineering and permit fees unless those services are specifically incorporated into the contract.
| Project Type | Planning Range | Typical Assumption |
|---|---|---|
| Garage Conversion | $60,000–$75,000 | Approximately 400–500 SF with bathroom, kitchenette, basic finishes and a small ductless HVAC unit; no appliances |
| New Detached ADU | $270–$350 per SF | Slab-on-grade construction with a stucco exterior; cost varies with site, utilities, design and amenities |
Garage conversion versus new detached construction
Converting an existing garage
A garage conversion can be the most economical route because the foundation, exterior walls and roof are already present. A typical 400- to 500-square-foot conversion in the $60,000–$75,000 range assumes a bathroom, kitchenette, small ductless HVAC system and basic finishes. Appliances are not included in that planning range.
Existing construction does not automatically make the project simple. The garage must be evaluated for code compliance, floor elevation, insulation, fire separation, windows and emergency egress, ceiling height, moisture protection and the condition of prior work. Correcting an unsuitable or unpermitted structure can reduce or eliminate the anticipated savings.
Building a detached ADU
A new detached ADU offers more freedom in layout, privacy and placement, but it requires a complete foundation, structure, roof, exterior envelope and utility connections. Ocotillo's broad planning range of $270–$350 per square foot assumes slab-on-grade construction and a stucco exterior. The final figure depends heavily on existing conditions and the selected amenities.
The conditions that most affect ADU pricing
- Access to the work area: Narrow side yards, occupied homes, walls, pools and limited equipment access can increase labor and material-handling costs.
- Sewer and water availability: A nearby connection may be straightforward. A distant, deep or obstructed sewer line—or a route requiring extensive demolition and replacement—can materially change the budget.
- Electrical capacity and distance: The existing service must be evaluated for capacity. A long feeder run, difficult routing or service upgrade can add cost.
- Finish selections: Cabinetry, tile, countertops, plumbing fixtures, lighting, doors and windows can move a small unit beyond a basic allowance quickly.
- Existing garage condition: Floor height, slab condition, insulation, openings, structural modifications and previous unpermitted work may require correction.
- Site and exterior work: Drainage, grading, walkways, privacy walls, gates and restoration of disturbed areas are property-specific.
What Arizona's newer ADU rules mean—and what they do not mean
Under current Arizona law, municipalities must allow at least one attached and one detached ADU on qualifying single-family lots. The law also limits certain local restrictions and generally permits an ADU up to 75% of the primary home's gross floor area or 1,000 square feet, whichever is less. Larger-lot and affordable-unit provisions can create additional options.
Phoenix states that two ADUs are generally permitted when a lot contains no more than one detached primary residence, with a possible third unit in certain circumstances. Tempe likewise allows one attached and one detached ADU on qualifying single-family properties. Tempe states that, on lots under one acre, each ADU is generally limited to 75% of the primary dwelling's habitable area or 1,000 square feet, whichever is less.
These changes make ADUs more feasible, but a general rule cannot determine whether a particular property works. Owners still need to confirm zoning, lot coverage, setbacks, building height, utility-provider requirements, drainage, fire access and permit submittals. An HOA or recorded CC&Rs may also impose private restrictions even when the city allows an ADU.
Plans and permits are still required
A garage conversion changes an area from vehicle or storage use to habitable space. A detached ADU is a new dwelling structure. Both require properly prepared plans and applicable permits. Plumbing, electrical, mechanical, structural and energy-code requirements must be coordinated before construction.
Phoenix is developing a standard-plan library for detached ADUs, and Tempe has established a standard-plan program. Standard plans may reduce design expense and review time, but they do not eliminate the need for a property-specific site plan, utility coordination and confirmation that the selected plan fits the lot. Significant modifications can require a new plan review.
A practical first-step checklist
- Confirm that the property contains a qualifying single-family home and review its zoning and lot coverage.
- Check HOA documents and recorded CC&Rs before paying for design work.
- Locate the existing sewer, water and electrical service and evaluate capacity and connection routes.
- Measure access to the proposed work area and identify walls, pools, mature landscaping and other obstacles.
- Decide whether the unit needs a full kitchen, laundry, accessibility features, separate outdoor space or upgraded finishes.
- Create a total project budget that separates construction from design, engineering, permits, appliances, furnishings and landscaping.
- Have the contractor and designer review the site before assuming that a standard plan or typical price range applies.
Frequently asked questions
Does the garage-conversion range include a bathroom and kitchen?
It assumes one bathroom and a kitchenette, along with basic finishes and a small ductless HVAC unit. It does not include appliances.
Are plans and permits included in the planning ranges?
No. Plans, engineering and permit fees are separate unless specifically included in the contract.
Can a garage always be converted?
No. Zoning may allow an ADU while the existing garage still requires structural, code, access or utility corrections. The building and property must be evaluated.
What most often raises the cost?
Difficult access, long or complicated sewer and electrical runs, inadequate electrical-service capacity, water availability, existing-condition corrections and upgraded finish selections.
Is a detached ADU cheaper per square foot than a custom home?
Not necessarily. Kitchens, bathrooms, utility connections and mechanical systems are concentrated into a small footprint, so a small dwelling can have a relatively high cost per square foot.
Can an ADU be rented?
Arizona law protects separate long-term rental use, but short-term rental licensing, owner-occupancy rules and local requirements vary. Confirm the intended use with the city before proceeding.
Plan the property before pricing the structure
Ocotillo Construction Company, Inc. has served the Phoenix metropolitan area since 1994. We construct custom homes, substantial remodels, additions and ADUs throughout Phoenix, Scottsdale, Paradise Valley, Chandler, Gilbert and Tempe. Our first concern with an ADU is not simply its square footage—it is whether the existing property, utilities and proposed use support a practical project.
If you are considering a garage conversion or detached casita, Ocotillo can help evaluate the construction approach, identify major cost drivers and develop a realistic scope before work begins.
Official resources
Ocotillo Construction Company, Inc. | ROC 163739 B-1 | ROC 163740 B | 480-857-3551 | Planning ranges and regulations are informational. Confirm property-specific requirements with the applicable jurisdiction.