How Much Does an ADU Cost? (2026 Guide)

What drives the cost of an ADU

An ADU's price isn't one number — it's the sum of several drivers that you can estimate but not look up. The biggest is the type and size of the unit: square footage, the number of bathrooms and the size of the kitchen, and whether you're adding new floor area or reusing what exists. Next is site conditions, which are easy to underestimate. Sloped lots, poor soil, long utility runs, an electrical panel that needs upgrading, or sewer and water lines that have to be extended can all add cost that has nothing to do with the building itself.

Finish level is another major lever. The same floor plan can cost very differently depending on whether you choose builder-grade or high-end cabinets, countertops, flooring, and fixtures. Then there's location: labor and material costs, and local permit and impact fees, vary enormously from one metro to the next, so an identical unit can cost more in one city than another. Finally, soft costs — design, structural engineering, surveys, and the building permit itself — are real line items that first-time builders often forget to budget for. Taken together, these drivers are why published per-square-foot figures span so widely: cost guides such as HomeGuide and Angi generally put ADUs in the range of about $100 to $300 per square foot, with complex projects in high-cost metros pushing past $600 per square foot. Understanding these drivers is what lets you read a bid critically instead of anchoring on a single headline figure.

Typical cost ranges by ADU type

Because the drivers above stack differently for each ADU form, cost tends to follow the type of unit. Treat the following as broad, relative ranges — not quotes — and always price against your own lot and design.

Conversions are usually the least expensive path. Turning an existing garage, basement, or attic into a unit reuses the shell, foundation, and often some utilities, so a junior ADU carved from existing interior space frequently sits at the low end. HomeGuide puts garage conversions at roughly $50 to $200 per square foot, and Angi reports an average garage-to-ADU conversion around $110,000 — though a simple single-car conversion can start near $25,000 while a larger detached-garage conversion can approach $175,000 to $225,000. Attached ADUs — additions that share a wall with the house — tend to land in the middle; HomeGuide pegs them at roughly $125 to $225 per square foot, since they extend existing structure and can tie into nearby utilities. Detached new-construction ADUs typically cost the most, at roughly $150 to $250 per square foot per HomeGuide and commonly $200,000 to $400,000 all-in across industry guides, because they're a standalone building with their own foundation, utility connections, and roof. Prefabricated or modular units can shift the picture: HomeGuide places them around $80 to $160 per square foot, and factory construction may shorten the build — but you still pay for the foundation, site work, utility hookups, and delivery, so 'kit price' is rarely the all-in cost. For the structural trade-offs behind these forms, see detached vs attached ADUs. The takeaway: the cheapest unit is usually the one that reuses the most of what you already have, and the most expensive is the one built from the ground up.

These ranges line up with what owners actually report. A statewide survey of ADU owners published by the Terner Center for Housing Innovation at UC Berkeley found that roughly 37% of ADUs cost under $100,000 to build and about 71% came in under $200,000 — a reminder that, despite eye-catching high-end figures, a large share of real ADUs land well below the top of the range, especially conversions and smaller units.

The fees people forget

Hard construction costs get the attention, but government and connection fees can quietly add a meaningful share to an ADU budget. Permit fees are the first: many jurisdictions calculate the base building-permit fee — and sometimes plan-review fees — as a function of the project's permit valuation, so a larger or higher-finish unit pays more. As a sense of scale, Angi reports building-permit fees of about $1,200 to $2,000 for a garage conversion, with combined permit and review fees for a full ADU commonly running into the low thousands of dollars and up depending on the city. On top of that, some cities charge separate impact and development fees meant to offset the demand a new unit places on schools, parks, water, and sewer systems — though many places have reduced or waived these specifically for ADUs to encourage them. In California, the state's SB 13 (2019) bars local agencies from charging impact fees on ADUs under 750 square feet and scales them down for larger units, and ADUs under 500 square feet are generally exempt from school impact fees — a big reason smaller units can pencil out very differently from large ones.

Utility connection charges are another commonly missed cost: a new or upgraded water meter, sewer connection, or electrical service can each carry its own fee. And don't overlook the soft costs already mentioned — design, engineering, and survey work. None of these are universal, and several jurisdictions have deliberately lowered ADU fees, which is exactly why you should pull the actual fee schedule for your city rather than assume. Build a line for fees into your budget from the start; confirm the specific amounts with your local building department, and remember this is general information, not financial advice.

How location changes the number

Two identical ADUs in two different cities can carry very different price tags, and location is often the single biggest reason. Labor and material costs track the local construction market, so a high-cost metro will run higher than a smaller market for the same scope. Local fee schedules — permits, impact fees, and utility connections — vary just as much, and they can swing the all-in number more than most homeowners expect. The published per-square-foot numbers show the spread plainly: the same cost references put high-cost California metros well above the national norm — on the order of $300 to $400 per square foot in Los Angeles and roughly $150 to $450 per square foot across the Bay Area — against the $100 to $300 national range cited earlier.

Regulation matters too. Some states, California most prominently, have passed laws to make ADUs easier and faster to approve, and streamlined or by-right review can reduce both the time and the carrying cost of a project; related rules like SB-9 open up small-scale infill on many single-family lots. You can see where ADU activity is concentrated on our California permit data hub, and drill into specific markets like Los Angeles or Austin to gauge how busy your area is. Looking at real, recent permit activity nearby is a useful sanity check: a market with lots of comparable ADU permits usually has more builders competing for the work, which can affect both pricing and timelines.

How to get an accurate ADU cost estimate

The only number that matters is the one for your project, so the goal is to replace internet averages with real bids. Start by defining your scope precisely: ADU type, target square footage, number of bathrooms, and finish level. A clear scope is what lets contractors quote apples-to-apples instead of guessing, and vague scopes are the main reason bids come back wildly different.

From there, get multiple itemized bids from licensed contractors, and ask each to break out hard costs, soft costs, and fees separately so you can compare them line by line. Pull your city's actual permit and impact fee schedule rather than estimating, and confirm utility-connection costs with the relevant utilities. It also helps to look at what's actually being built around you: our free permit lookup tool lets you see recent permit activity in a market, which is a quick way to confirm there's an active ADU builder pool nearby. Once you have a realistic budget, plan how you'll pay for it with our ADU financing guide. If you'd like to understand ADU permit activity in your market to inform a build, request access to Igni. As always, this is general information — get professional quotes and confirm every figure locally before you commit.

Where these cost figures come from

Every dollar figure in this guide comes from third-party references, not from Igni's own data — Igni tracks permit activity, not construction pricing. The per-square-foot and by-type ranges above are drawn from widely used home-improvement cost guides, principally Angi and HomeGuide, which aggregate contractor and project data nationally. The owner-reported cost distribution comes from the statewide ADU owner survey published by the Terner Center for Housing Innovation at UC Berkeley, an academic housing-research center, and the California fee-waiver details reflect state law (SB 13, enacted 2019). These sources broadly corroborate one another, but each uses its own methodology and sampling, so treat their figures as directional benchmarks rather than precise predictions.

Because no two sources — and no two projects — measure cost the same way, the only number you can truly rely on is a current, itemized bid for your specific scope and lot. Use the ranges here to sanity-check a quote, not to replace one. To gauge how active ADU building is in your market — a useful reality check on contractor availability and pricing pressure — try the free permit lookup. Like the rest of this guide, this is general information, not financial or construction advice; confirm every figure with licensed contractors and your local building department.

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