ADU Construction Cost Calculator

Because an ADU is a complete tiny home with all the expensive parts packed into a small footprint. A kitchen, a bathroom, plumbing, electrical, HVAC, a foundation, walls, a roof, windows, and finishes are fixed costs that don't shrink much with size — so spreading them over 600 square feet instead of 2,500 drives the per-square-foot rate up to $150–$400. A full kitchen and bath represent a far larger share of a small ADU than of a big house. The total is much less than a house, but the per-foot number is high, which is why the calculator prices by build type and adds the kitchen and utility hookups separately.

A garage conversion is the cheapest build type (around $150–$220/sq ft) because the foundation, walls, and roof already exist; you're mostly finishing the shell and adding systems. Prefab/modular comes next on speed: the unit is built in a factory while your site is prepped, so on-site work can be days instead of months — but once you add delivery, crane set, foundation, and utility hookups, the total isn't always lower than site-built, just faster and more predictable. Attached additions and detached new-builds cost the most because they're built from the ground up. Use the build-type selector in the calculator to compare them side by side for your square footage.

Site conditions capture how hard your lot is to build on, separate from the ADU itself. An easy site is flat with utilities close by. A moderate site needs some grading or longer utility runs. A difficult site is sloped, has tight access for equipment and deliveries, or requires long trenching to reach water, sewer, and power — which can add 25% or more. This matters most for detached units in a backyard, where machinery access and utility distance are real constraints. The calculator's Site Conditions selector applies this factor across the whole build.

It depends on the build type. A garage conversion reuses the existing slab (though it may need leveling, moisture proofing, or reinforcement to meet residential code). An interior conversion uses existing structure. But a detached new-build needs a complete new foundation or slab poured from scratch — which is why the calculator offers a per-square-foot Foundation/Slab add-on. Prefab units also need a prepared foundation to set on. If you're pricing a detached or prefab unit, include the foundation; for a garage conversion you usually don't.

Because their cost depends on your site, not the unit. An ADU needs water, sewer, and electrical — and sometimes gas. A conversion near the house often taps existing lines cheaply, but a detached unit may need trenching across the yard, a larger electrical panel or a new sub-panel, and confirmation that the sewer or septic has capacity for another dwelling. Those are real, variable costs that two identical units on different lots won't share, so the calculator breaks out 'New Utility Connections / Panel' as its own add-on rather than burying it in the per-square-foot rate.

More than people expect, because finishes hit every surface in a small, finish-dense space. Standard builder-grade finishes are the baseline. Mid-range — nicer flooring, better cabinets and fixtures — adds roughly 20%. High-end or custom finishes (quartz counters, custom cabinetry, premium flooring and tile) can add around 45%. Since an ADU is mostly kitchen, bath, and living space per square foot, upgrading finishes moves the total more sharply than the same upgrade would in a large home. The calculator's Finish Level selector lets you test the spread.

Almost always a building permit, plus zoning approval confirming an ADU is allowed and meets size, setback, height, and sometimes parking and owner-occupancy rules. Detached and attached builds typically need stamped architectural and structural plans; conversions are lighter but still permit the new kitchen, bath, electrical, and egress. Permit and impact fees vary widely — modest in cities with streamlined or pre-approved-plan programs, and several thousand dollars elsewhere. The 'Design & Permitting' add-on captures plans, engineering, and fees. Skipping permits means the unit can't be legally rented and creates problems at resale.

Often, over time, in markets with strong rental demand and ADU-friendly rules. The two return levers are rental income — a long-term tenant offsetting the build cost and mortgage over several years — and the resale premium buyers pay for income-capable, multigenerational-ready properties. In high-rent metros a detached unit can pay back in a handful of years; in lower-rent areas the timeline stretches. Even unrented, housing a parent or adult child can beat the cost of senior care or a second household. Use the construction estimate here as the cost side, then weigh it against local rents and comparable sales.

Plan for roughly 6 to 12+ months concept-to-completion. Construction itself varies by type: a garage conversion is fastest (about 2–4 months), an attached or detached site-built unit runs 4–8 months, and a prefab unit installs in days to weeks once the site and foundation are ready (though factory build and site prep add lead time). The unpredictable stretch is design and permitting — anywhere from one to six-plus months depending on your jurisdiction. Starting design and permits early is the single best way to keep the project moving, since that phase, not framing, is usually the bottleneck.

Some owners do, and it can trim the general contractor's markup — but ADU construction coordinates many licensed trades (foundation, framing, plumbing, electrical, HVAC) plus inspections and permit management, and mistakes in sequencing or code compliance get expensive fast. Most homeowners are better served hiring a licensed general contractor or a dedicated ADU builder, especially for detached and attached builds. Owner-builders who do succeed usually have construction experience and time to manage the schedule daily. The estimate here reflects professional construction, which is the realistic baseline for a permitted, financeable, rentable unit.