I have not built an ADU in Arizona, so I will not invent a cost number for you. But I add rentable space for a living — 2/1 to 3/1 and 3/1 to 4/2 conversions in Memphis — and the test I run there is the one I would run on your ADU.
What makes it worth it to me: the added rent has to justify the all-in build cost the same way a purchase does. I screen with the 1% rule. If the ADU rents for $1,000 a month, I want to be all-in near $100K, not $180K.
And value is not what it costs. It is what an appraiser credits at refinance, because I refinance DSCR at 70-80% LTV. If the appraisal ignores the unit, the value add is a story, not equity. Get real bids in Phoenix, then run both tests.
Braeden, I'm with Renters Choice Homes. From the management side, I'd compare rents for similar detached units rather than use the main house's rent per square foot. Access, parking, laundry, privacy and utility arrangements all matter. I'd also consider whether the ADU changes the main home's rent, plus vacancy, turnover and maintenance. Which city and unit size are you considering, and would the main home also be rented?