How Much Does It Cost to Build an ADU in Arizona?

How much does it cost to build an ADU?

Homeowners in Arizona should plan on $250 to $450 per square foot once design, permitting, utilities, and finishes are counted.

For a 600 to 1,000 square foot casita built to luxury standards, that puts the realistic range near $200,000 to $450,000.

Attached units and garage conversions land lower, since the foundation and roof already exist.

House Bill 2720, now codified as Arizona Revised Statutes § 9-461.18, required cities with more than 75,000 residents to permit accessory dwelling units on single-family lots, with local ordinances in place by January 2025.

Scottsdale, Phoenix, Mesa, Chandler, Gilbert, and Tempe all fall under it.

The spread between those numbers is wide for a reason.

What follows breaks down where the money actually goes and which decisions move the cost of an ADU in Arizona the most.

Can You Build an ADU in Arizona?

On most single-family lots in the Valley, yes.

Arizona Revised Statutes require covered cities to allow at least one attached and one detached unit as a permitted use, without special use permits or public hearings.

Size is capped.

Your ADU can be 75% of the primary home’s gross floor area or 1,000 square feet, whichever is smaller, which means a modest main house limits your casita before you draw anything.

Lots of one acre or more can qualify for a third detached unit if one is deed-restricted as affordable.

Several constraints catch homeowners off guard during early ADU project planning:

• Recorded HOA covenants can still prohibit ADUs
• Utility easements block construction without written consent
• Airport overlay zones restrict some parcels
• Natural Area Open Space cannot be built on in northern Scottsdale

Confirm all four before you commission drawings.

Reviewing casita floor plans is the enjoyable part, but a covenant restriction discovered after design fees are spent is an avoidable loss.

How Much Does It Cost to Build an ADU?

How much does it cost to build an ADU?

The honest answer starts with square footage, though the finished product drives most of the variance.

A guest casita with quartz counters and standard fixtures sits near the bottom of the range, while a unit matching a custom home’s millwork and stonework sits near the top.

Building costs in Arizona break down roughly like this on a detached casita:

• Site work and foundation at 10% to 15%
• Framing and building envelope at 20% to 25%
• Mechanical and plumbing systems at 15% to 20%
• Interior finishes and cabinetry at 25% to 35%
• Design and permitting at 8% to 12%

Soft costs surprise people most.

Architectural drawings, structural engineering, survey work, and plan review accumulate well before a single trade arrives on site.

None of them are optional, and none of them produce anything you can walk through.

What Drives ADU Costs Up the Fastest?

Utility connections top the list.

Scottsdale requires detached ADUs to be separately metered from the main residence, which means new service runs, trenching, and utility coordination that a guest house never triggers.

Distance compounds it.

A casita placed 80 feet from the house costs meaningfully more to service than one placed 20 feet away, since every foot of trench carries labor, conduit, and restoration cost.

Site conditions round it out.

Caliche, rock, and slope all raise excavation cost, and Arizona construction pricing reflects how much of that work happens by hand rather than by machine.

A soils report before design costs far less than a change order during excavation.

Phoenix has moved to reduce one barrier.

The city now offers approved standard ADU plans at no cost to residents, which removes design fees for homeowners willing to build from a pre-approved design rather than a custom one.

A site plan submittal is still required, so the savings apply to design rather than permitting.

Frequently Asked Questions

How Long Does an ADU Take To Build?

Design and permitting typically consume three to five months before construction starts, and vertical work runs another five to eight months.

Plan on 10 to 14 months from first meeting to certificate of occupancy.

Rock excavation, utility scheduling, and plan review comments are the three variables that most often extend that window.

Understanding the full ADU building process early keeps the schedule honest and the budget stable.

Do ADUs Increase Property Value?

Appraisers generally treat a permitted ADU as additional finished square footage, which supports value directly.

Rental income potential adds a second layer, since state law protects long-term rental of ADUs in covered cities.

Unpermitted structures work against you.

An appraiser cannot credit square footage that never received a certificate of occupancy, and buyers discount it further during due diligence.

Is a Garage Conversion Cheaper Than New Construction?

Usually, by a wide margin.

You inherit a slab, walls, and roof, so the work concentrates on:

• Insulation
• Mechanical systems
• Windows
• Interior finish

The tradeoff is parking and resale.

Removing covered parking in a market where garage space is expected can offset part of the savings when you sell, particularly on higher-value properties.

Can You Rent Out an ADU in Arizona?

Long-term rental is protected by state law in the cities the statute covers, and municipalities cannot require a family relationship between you and your tenant.

Short-term rental rules differ by city and often require owner occupancy of the primary home.

Check your covenants, conditions, and restrictions (CC&Rs) regardless.

These private HOA rules can restrict rental even where city code permits it, and lenders will ask about restrictions during financing.

Planning Your Arizona Casita the Right Way

How much does it cost to build an ADU is the right question to open with, but your lot supplies the real answer.

Two identical casitas on neighboring streets can differ by six figures based on utility distance, soil conditions, and covenant restrictions alone.

McDowell Mountain Builders has built custom homes and additions across Scottsdale, Phoenix, Cave Creek, and Fountain Hills, so we evaluate your property before quoting rather than after.

Design, permitting, and construction stay under one roof, which means no costs get handed off between contractors.

Every quote starts with a site visit.

Request an estimate, and we’ll review setbacks, utility access, and soil conditions before giving you a number.

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