ADU (Accessory Dwelling Unit) Cost Calculator

Converting an existing garage is almost always cheaper. A garage already has a foundation, framing, walls, and a roof, so you're mainly insulating, finishing the interior, and adding the kitchen, bath, and systems — often $130 to $220 per square foot. A detached new-build starts from bare ground with its own foundation, framing, roofing, and siding, pushing $250 to $400+ per square foot. The trade-off: a conversion costs you the garage (and some jurisdictions still want the lost parking replaced), while a detached unit gives you maximum privacy and rentability. Run both scenarios in the calculator to see the gap for your square footage.

A garage or interior conversion usually taps the home's existing water, sewer, and electrical lines a few feet away. A detached ADU in the backyard needs those lines trenched across the yard and connected — and the farther the unit sits from the house or the street, the more excavation, pipe, conduit, and backfill it takes. That's why the 'Trench Utilities to Detached ADU' add-on exists separately from the base build: it's a real, location-dependent cost that conversions rarely incur.

Generally, yes — a full kitchen is the dividing line. A true ADU is an independent dwelling: its own kitchen, bathroom, sleeping/living area, and entrance, which is what lets it be legally rented as a separate unit where permitted. Drop the full kitchen and you have an in-law suite or accessory living space attached to the main home — cheaper to build, but it usually can't be permitted or rented as a separate dwelling. The calculator's Kitchen/Bath selector reflects this: 'Full Kitchen + Bath' is the true-ADU baseline, while a kitchenette or no-kitchen suite costs less but changes the legal status.

Often, over time — but it depends on your market and how you use it. The two return levers are rental income (a long-term tenant can offset the build cost and mortgage over several years) and the value bump at resale (buyers pay a premium for income-capable, multigenerational-ready properties, frequently recovering a large share of the construction cost). In high-rent, ADU-friendly metros the math is strongest; in lower-rent areas the payback stretches out. Even without renting, housing a parent or adult child can cost far less than senior care or a separate household. Use the build estimate here as the numerator, then weigh it against local rents and comps.

Expect a building permit at minimum, plus zoning approval confirming an ADU is allowed on your lot and meets size, setback, height, and (sometimes) parking and owner-occupancy rules. Detached units and additions usually require stamped architectural and structural plans; conversions are lighter but still need permits for the new kitchen, bath, electrical, and egress. Permit and impact fees vary enormously by jurisdiction — from modest in streamlined, pre-approved-plan programs to many thousands of dollars elsewhere. That's what the 'Design / Engineering / Permits' add-on captures. Confirm your city or county's specific ADU rules before you design.

Most jurisdictions cap ADU size — commonly 800 to 1,200 square feet, and often tied to the primary home's footprint — while many also set a floor around 150 to 400 square feet. Within those limits, size is a budget and use decision: a 400–500 sq ft studio is the cheapest entry point and rents well in dense areas; 600–800 sq ft fits a comfortable one-bedroom; and 1,000–1,200 sq ft supports a two-bedroom for families. Because ADUs are priced per square foot, every extra 100 sq ft adds real money — so size to the use case, not the maximum allowed.

Prefab and modular ADUs can shorten the construction timeline (the box is built in a factory while your site is prepped) and sometimes trim labor cost, but the total isn't always lower once you add delivery, crane set, foundation, and the same utility, permit, and site-work costs a site-built unit incurs. They shine on speed and predictability more than on raw price. This calculator estimates a conventionally-built unit; if you're comparing a prefab quote, make sure it includes foundation, utility connections, and permits — vendors often quote the box alone.

A full ADU is a complete dwelling — foundation or structural work, plumbing, electrical, HVAC, insulation, and code-compliant egress — all of which require permits and inspections, and most of which require licensed trades. Even handy owners typically act as their own general contractor at most, subbing out the regulated work. DIY finishing (paint, flooring, trim) can save some money, but the structural, plumbing, and electrical scope is where the cost and the risk live, and botched work fails inspection or voids insurance. The estimate here reflects professional construction, which is the realistic path for a permitted, rentable unit.

Usually, yes — adding permitted living space increases your assessed value, so your property tax typically rises by the value the ADU adds (not the value of the whole property re-assessed, in most jurisdictions). The increase is generally modest relative to the rental income or use value the unit provides, but it's a real ongoing cost to budget alongside insurance. Rules differ by state and county, so confirm how your assessor treats ADUs before you build.

Plan for roughly 6 to 12+ months from concept to move-in. Construction itself is often the shorter part — a couple to a few months for a conversion, three to six-plus months for a detached new-build — while design and especially permitting are the unpredictable stretches. Some cities with pre-approved ADU plans turn permits around quickly; others take months. Start design and permitting early, since that phase, not the framing, is usually the bottleneck.