Does a casita pay for itself? The price-per-foot math

The two numbers that matter

Resale price per square foot is what buyers in your city paid, per finished square foot of house, in recent sales. Build cost per square foot is what a unit costs to design, permit and build, divided by its size. If the first number, times the share an appraiser credits to ADU space, is higher than the second, the unit adds more value than it costs. If not, you are buying something else: a place for family, or rental income (see rental math).

Resale price per square foot, by city

Redfin median sale price per square foot, 2026 readings (as of October 6, 2026). Neighborhoods vary widely inside each city; Phoenix alone runs from about $215 in Estrella to over $500 in Downtown, Biltmore and Arcadia. The target areas page ranks every neighborhood against the cost to build.

Source: Redfin housing market pages, Phoenix-area cities (median sale price per sq ft, 2026).

Build cost per square foot, by project type

Figures are what complete casitas cost to build in the Valley in 2026, per finished square foot; third-party cost guides are cited for comparison. These are build costs per finished square foot, not a quote: fees, financing and site conditions are added on top, and every project is priced after a site visit.

Third-party guides for comparison: Phoenix ADU cost guide; ADUs in Arizona, 2026; 2026 Phoenix renovation cost report.

Small units cost more per foot than houses because a kitchen and a bathroom cost about the same whether the unit is 400 or 1,000 square feet, and because a detached unit needs its own foundation, roof and utility trenching. A garage conversion, which already has a slab, walls and a roof, costs less to create. Where a Valley home sells for more than about $280 per finished foot, a new casita is being built below the resale rate.

The third number: how much of the resale rate an appraiser credits

An appraiser does not simply multiply your new square footage by the neighborhood price per foot. ADU square footage is reported separately from the main house and valued from comparable sales of homes with ADUs, with adjustments. In practice casita space is often credited at a discount to main-house space; a permitted unit with a full kitchen, bath and private entry, in a neighborhood with casita sales, gets closer to the full rate. We use 50% to 100% as the range and 75% as a middle case. How appraisers work is covered in appraisal and resale value.

Four worked examples

Typical Valley build cost. The first three use the city median price per foot and a 75% appraiser credit; the fourth uses the rate Phoenix homes with casitas have actually resold for in our recent sales, at full credit. Fees, financing and site conditions are not included.

Read across the rows. A garage conversion builds out for about $120 per finished foot against a Phoenix resale rate near $285, so it clears by $42,188 even when the appraiser credits only 75% of the main-house rate; it breaks even at a 42% credit. A new casita builds for about $265 to $280 per foot, which is right at the Phoenix median and well under Scottsdale's: the Scottsdale unit clears by $27,463 at the same discount, while at the Phoenix median the detached unit comes up $37,862 short until the appraiser credits 95% of the main-house rate.

The fourth row is the one that matters most, and it rests on our own sales rather than a published statistic. In our recent Phoenix sales, homes with a detached or attached casita have resold at about $325 per square foot, against a city median near $285. That is our observation from a small number of sales, not a published statistic, and an appraiser will still look for comparable casita sales near you. Priced at that rate, the 650-square-foot unit adds $34,450 more than it costs. And the rate applies to the whole house, not only the new square footage: a 1,600-square-foot home re-rated from $285 to $325 per foot gains another $64,000 before the casita's own square footage is counted. That is why a legal casita is the single improvement most likely to return more than it costs: it is the only one that changes what the rest of the house is worth per foot. The answer is still specific to your lot, your city and your neighborhood's comparable sales, which is why we review the property before quoting.

Our own two sales, checked against the neighborhood

Both of our completed casita projects are in Maryvale, where Redfin puts the median sale at $221 per square foot (November 2025). Here is what each home actually sold for, per square foot the appraiser counted, against that median.

Sale prices from the public MLS records linked on each case study. *At 3802 N 48th Dr the appraisal counted 1,289 sq ft of living area and left the roughly 300 sq ft converted garage out of the total, so the price per counted foot is what a buyer paid for the house with the studio treated as a bonus; the listing total was 1,588 sq ft. Neighborhood median: Redfin: Maryvale (Phoenix) housing market, median $/sq ft. Two sales are a pattern, not a statistic.

Run your own numbers

Costs the per-foot number leaves out

• Permit and plan review fees. Phoenix example: About $2,983 in permit, plan review and plot-plan fees on a $150,000 valuation (fee schedule effective Jan 20, 2026).
• Impact fees. No fire, police, parks, library, arterial or storm-drain impact fees in the established "Balance of City" area. About $12,000 to $35,000 in growth areas (Northwest, Deer Valley, Northeast, Paradise Ridge, Estrella, Laveen, Ahwatukee).
• Utility fees. Citywide water treatment ($4,387), wastewater treatment ($1,071) and water resource acquisition ($2,009) fees; confirm whether an ADU on the existing meter is assessed.
• Utility capacity. A panel upgrade or a long sewer run can add a meaningful amount on its own.
• Carrying cost. Interest on a HELOC or construction loan while the unit is permitted and built.
• Property tax. The assessor adds the new improvement to your valuation.

Fee figures per Phoenix ADU rules summary with ordinance citations (aduzoning.org, verified Sept 19, 2026); verify with the city.

The value that is not in the per-foot number

Two things the arithmetic above misses. First, rent: a unit that rents for $1,200 to $2,000 a month changes the picture from "value added at sale" to "income every month", which is the subject of the rental math page. Second, demand: in the only large dataset on the subject, homes with ADUs appreciated about 1.69 percentage points a year faster than homes without over a decade (California, 2013 to 2023, FHFA). That is correlation, not proof, but it points the same way buyers in the Valley do: a legal second unit widens the pool of people who want the house.