- Why does a small building cost so much per square foot?
- Because a casita is a complete house in miniature. It carries its own foundation, roof, envelope, kitchen, bathroom, heating and cooling, and its own connections to power, water and sewer. Those costs shrink far less than the floor area does, which is why the smallest units are the most expensive per foot and why a casita is not priced like an addition of the same size.
- Is a garage conversion cheaper than building new?
- Usually, because the foundation, the walls and the roof already exist. What it adds instead is everything a garage was never built to have: insulation, conditioned air, egress-compliant windows, a proper floor assembly, and often a rebuilt wall where the garage door was. It also costs you the garage, which matters more in some neighbourhoods than others. It is a different product from a detached casita rather than a cheaper version of one, and the published figure on this page does not price it.
- Can I build one on my lot?
- Arizona law now requires larger municipalities to permit at least one accessory dwelling by right on a single-family lot, but the local ordinance still governs setbacks, size and placement, and an HOA can still restrict what the statute permits. Which system decides what is genuinely confusing, so we wrote a separate guide on it.
- What tends to move a casita budget the most?
- Three things come up repeatedly, and which one dominates depends on the lot. Site access, because a detached unit is built at the back of a property and equipment may not be able to get there. Utility capacity, because a second dwelling adds load the existing service may not carry. And what the jurisdiction requires, because setbacks and size limits can change the building before anyone prices it. None of the three is visible on a floor plan.
- Will the existing house support a second dwelling?
- Sometimes, sometimes not. A second kitchen, bathroom and HVAC system add electrical load and drainage demand. Whether your existing service, panel and sewer connection can absorb that, or need upgrading, is determined during design and it moves the number materially.
- What do permits and design add on top of construction?
- In the City of Phoenix the building permit fee follows project valuation. For a project valued between $50,001 and $200,000, the schedule effective 20 January 2026 charges $703 on the first $50,000 plus $9 for each additional $1,000 or fraction thereof, and plan review for residential work over $50,000 is charged separately at 80% of the permit fee. Valuation is the higher of your stated figure and the City's own calculated minimum, which it works out from building area and occupancy — so a larger casita generally means a larger fee, by way of that calculation. Every other jurisdiction in the metro sets its own schedule, and impact or development fees may apply on top. Design and engineering are separate again, and whether they are inside a builder's number is worth asking directly.
- What is usually an allowance on a casita?
- Kitchen cabinetry and appliances, bathroom fixtures, flooring and lighting. Because a casita is a complete small house, the allowance schedule covers more categories than a single-room remodel would, which makes it the part of the contract most worth reading carefully. Ask what each allowance covers, whether installation is included, and when it is reconciled.
