What Does It Cost to Build an ADU? A Detailed Breakdown

By Asher Ben-Haim

Building an accessory dwelling unit (ADU) in California in 2026 typically costs between roughly $150,000 for a garage conversion and $400,000 or more for a detached new build, with most projects landing in the low-to-mid six figures. The exact number comes down to four things: the type of ADU, its size, your lot, and your finishes. At Built to Perfection, we have spent 25 years building ADUs across Los Angeles and Orange County, from garage conversions to detached units over 1,200 square feet. In our experience, the homeowners who get surprised by the final bill are almost always the ones who got a number before anyone looked at the details that actually move it.

This guide reflects California ADU costs as of July 2026 and is reviewed each year, since both pricing and state law change often.

How much does it cost to build an ADU in California in 2026?

The cleanest neutral benchmark comes from a statewide survey of ADU owners run by UC Berkeley's Terner Center, which found a median construction cost of about $150,000, or roughly $250 per square foot. That survey also found that 37 percent of ADUs cost under $100,000 and 71 percent cost under $200,000.

One caveat matters. That survey covered units built in 2018 and 2019. California construction costs have climbed sharply since then. The state's own construction cost index (tracked by the Department of General Services) has risen about 46 percent since December 2020. Adjust the older median for that increase and a realistic 2026 range for a Southern California project runs from around $150,000 for a straightforward garage conversion to $400,000 or more for a detached, ground-up build.

Those numbers cover the whole project: design, permits, labor, and materials from start to finish, not just the framing and drywall.

ADU cost by type: garage conversion, JADU, attached, and detached

The single biggest driver of your number is what kind of ADU you build. Converting space that already has a foundation, walls, and a roof costs far less than building a complete home from the ground up.

The ranges below are market-observed figures reported across California builders and cost guides. They are general planning numbers, not a Built to Perfection quote. As ADU builders and garage conversion specialists working across LA and Orange County, our own published starting point for an ADU is $120,000, which lines up with the lower, conversion end of this market.

Two patterns hold across every type. Smaller units cost more per square foot, because fixed costs like design, permits, and utility hookups don't shrink just because the floor plan does. And an attached ADU usually costs less than a comparable detached unit, since it shares a wall, a roofline, and shorter utility runs. If you are weighing a conversion specifically, we cover the tradeoffs in more depth in our look at whether garage conversions are worth it in Los Angeles.

Why Los Angeles and Orange County ADU costs aren't what most people assume

Here is the part that surprises people. You would expect Los Angeles to be the most expensive place in the state to build an ADU. It isn't.

The Terner Center data shows that hard construction costs in Los Angeles run about $35 per square foot above the state average. But the median total cost of an ADU in LA County was actually the lowest in the state, around $100,000. The reason is that a large share of LA ADUs (roughly 39 percent) are garage conversions, which pulls the median way down even though the per-unit build costs are higher. Orange County, grouped with San Diego in the same survey, came in around $130,000.

These are 2021 survey figures for 2018 to 2019 builds, so treat them as a relative map of the state rather than a current price sheet, and adjust upward for today's costs. The takeaway holds up either way: LA and Orange County are not the priciest ADU markets in California. What you build matters far more than where in Southern California you build it.

Where your ADU budget actually goes

When homeowners ask us why an ADU costs what it does, it helps to see where the money lands. A detached unit is a complete home, with a kitchen and bathroom packed into a small footprint, so the cost per square foot is high.

One line item is smaller than most people expect. Under California law, an ADU of 750 square feet or less owes no impact fees to the city, water district, or special districts, and a junior ADU of 500 square feet or less is exempt as well. Larger ADUs are charged proportionally to the main house. On a smaller unit, that exemption can save you thousands of dollars that older cost guides still build into the budget. You can read the specifics in the state's ADU guidance from the Department of Housing and Community Development.

What drives your ADU cost up or down

Two identical-looking backyards can produce very different bids. These are the factors that move your number the most:

• ADU type and size. Covered above, and it's the biggest lever by far.

• Lot conditions. A flat, open lot is straightforward and keeps costs down. A sloped or hillside lot may need grading, retaining walls, or deeper foundations, all of which add cost.

• Site access. If equipment and materials can't easily reach the build area, labor costs climb.

• Utility runs. The farther your ADU sits from existing sewer, water, and electrical lines, the more trenching and connection work it takes.

• Finish level. Standard versus high-end finishes can swing your cost per square foot significantly, since the same square footage can be finished with basic materials or with high-end ones.

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Feasibility is worth checking before you fall in love with a design, because your lot and local rules set the outer limits of what's possible. We break down the local side of this in our guide to what determines ADU feasibility in Los Angeles.

California rules that can add to your cost (and the ones the internet gets wrong)

A lot of ADU cost advice online is out of date. Two state requirements get misreported constantly, so it's worth being precise.

Solar does not apply to every ADU. Under California's 2025 energy code, which took effect January 1, 2026, a newly built detached ADU generally has to include solar panels, though some exemptions apply (for example, when there is not enough usable roof area). An attached ADU counts as an addition and is not subject to the solar requirement, and garage conversions and junior ADUs are exempt as well. You can confirm the current rules with the California Energy Commission.

Fire sprinklers usually aren't triggered. State law says an ADU does not need fire sprinklers if they aren't required for the primary residence, and building an ADU does not force you to retrofit sprinklers into your existing house. A genuinely new multi-unit building can still hit sprinkler requirements, but the common single detached ADU on a single-family lot generally does not.

Three ADU cost claims that are out of date in 2026

Before you budget around anything you read elsewhere, check these three, because a lot of pages still get them wrong.

• The $40,000 CalHFA ADU grant is closed. The state grant that reimbursed up to $40,000 of pre-development costs has been fully allocated since December 28, 2023. There is no open round or waitlist in 2026. CalHFA even warns that anyone promising to get you an ADU grant is running a scam. Do not build your budget around it.

• There is no statewide ADU property-tax exclusion. A bill (SB 1164) that would have excluded ADU value from reassessment died in the Legislature in June 2024. Building an ADU still adds to your assessed value. More on that below.

• Not every ADU needs solar and sprinklers. As covered above, both depend on the type of ADU. Blanket claims that they apply to all ADUs are wrong and can inflate an estimate.

  
  

How to pay for an ADU in California

Most homeowners fund an ADU through equity, a construction loan, or a specialized renovation loan. Here are the main options and who each one tends to suit:

• Home equity loan or HELOC. Borrows against the equity you already have, usually up to 80 to 90 percent of your home's value minus your mortgage. A HELOC works well for phased draws and keeps your current low first-mortgage rate untouched. Best for owners with substantial built-up equity.

• Cash-out refinance. Replaces your first mortgage with a larger one. This makes sense when current rates are at or below your existing rate, and it's a poor fit if you're holding a very low pandemic-era rate.

• Construction loan. Releases funds in stages as the build progresses and doesn't require large existing equity. A good fit for ground-up detached builds by owners with strong income and credit but thinner equity.

• Renovation or after-completed-value loan. Underwritten on what your home will be worth after the ADU is finished, which can boost borrowing power for newer owners. These typically require stamped plans and a signed construction contract up front.

• Government and local incentives. With the statewide CalHFA grant closed, the remaining help is mostly local. Check your city and county for pre-approved plan programs, fee reductions, or forgivable-loan programs before assuming nothing exists.

  
  

One note worth knowing: many conventional lenders won't count your projected ADU rental income when qualifying you, though construction and renovation-style products often will. Ask the question early.

How an ADU affects your property taxes

Building an ADU does not trigger a reassessment of your whole property. Under Proposition 13, your existing home keeps its protected base value, and the county assessor adds only the value of the new ADU on top.

In practice, the assessor adds the ADU's new construction value to your assessment, and your annual tax on that added value runs roughly 1.1 to 1.25 percent in most of LA County. As an example, an ADU assessed at $200,000 adds somewhere in the range of $2,000 to $2,500 per year to your tax bill. You'll get a one-time supplemental bill after completion, then the ongoing amount folds into your regular tax bill. The LA County Assessor publishes the details on how new construction is assessed.

Is an ADU worth it? Rental income and resale value

For most Southern California homeowners, the rent an ADU generates comfortably outpaces the added property tax. To keep this conservative, look at the federal Fair Market Rents that the U.S. Department of Housing and Urban Development publishes each year. Actual market rents often run higher.

Source: HUD Fair Market Rents for fiscal year 2026. A two-bedroom ADU in LA renting near $2,600 a month brings in roughly $31,000 a year, and an Orange County two-bedroom near $3,236 a month brings in close to $39,000. Against a property-tax increase of about $2,000 to $2,500 a year, a single month of rent typically covers the entire annual tax bump.

Resale value is harder to pin to one percentage, and you should be skeptical of any builder claiming a fixed uplift like "adds $300,000." A cleaner signal comes from the Federal Housing Finance Agency, which found that by 2023 the median appraised value of California homes with an ADU was about $1,064,000, versus about $715,000 for homes without one, and that ADU properties appreciated faster over the prior decade. That's an association, not a guarantee for your specific home, but it points in a consistent direction.

How long does it take to build an ADU?

From design to final inspection, most California ADUs take about 6 to 12 months. State law gives cities a 60-day clock to approve or deny a complete ADU application, and every California city is now required to offer pre-approved plans. Using a pre-approved plan can cut permit issuance to roughly three to four weeks, because the design has already been reviewed and approved by the city. Custom plans take longer. We walk through the permitting side in our guide on how to legally build an ADU in Los Angeles.

Why the right contractor keeps your ADU on budget

Most budget surprises on an ADU are not really about material prices. They come from problems found late: a utility run nobody scoped, a lot condition that needs a bigger foundation, a plan-check correction that resets the schedule. The fix is catching those things before the contract is signed, not after demo has started.

That's why we handle design and permitting together, in-house. When the same team drawing your plans is also pricing the build and pulling the permits, the budget-breaking details get caught on paper instead of in your backyard. Over 25 years and hundreds of LA and Orange County projects, that has been the difference between an ADU that lands where we said it would and one that drifts. If you're still comparing contractors, our guide to hiring a contractor in Los Angeles covers what to check before you sign anything.

We've built across the region, from The Coastal Crest ADU in Malibu to The Blue Harbor ADU in Lomita, and we bring the same 3D design and planning process to every one. If you want the full walkthrough before you start, our complete guide to planning and building an ADU covers the process end to end.

Frequently asked questions about ADU costs in California

How much does an ADU raise your property taxes?

Only the ADU's added value is assessed, at roughly 1.1 to 1.25 percent per year in most of LA County. A $200,000 ADU adds about $2,000 to $2,500 to your annual tax bill, and your main home keeps its Prop 13 base.

How long does it take to build an ADU in California?

Most ADUs take about 6 to 12 months from design to final inspection. Using a city's pre-approved plan can shorten permit issuance to roughly three to four weeks.

Which type of ADU costs the least to build?

A junior ADU carved from your existing home is usually the lowest cost, followed by a garage conversion. Both reuse existing structure, which is why they cost far less than a detached new build.

Is building an ADU worth it in California?

For most Southern California homeowners, yes. A single month of rent often covers the entire annual property-tax increase, and homes with ADUs tend to appraise higher, though returns depend on your lot, your build, and your local rental market.

Ready to price out your ADU?

Every ADU number in this guide is a starting point. The only way to know what yours will cost is to look at your lot, your goals, and your budget together. As a licensed California general contractor (CSLB License 837987) with 25 years of LA and Orange County experience, an A+ BBB rating since 2016, and recognition as an 8-time Best of Houzz Service winner, we can walk your property and give you an honest read before you commit to anything. Schedule a free consultation or call our ADU line at (818) 340-7721 to get started.