How Much Does It Cost to Build an ADU in California?

Building an ADU in California typically costs somewhere between $120,000 and $450,000 or more, depending on size, type, and site conditions. Per square foot, most projects fall in the low hundreds up to about $600. Garage conversions and prefab units sit at the low end. Detached new builds sit at the high end.

The total is only half the story. Permits, design, utility work, and contingency can add tens of thousands of dollars to a quote that only covers construction. This guide breaks down where the money goes and how to compare bids so you’re comparing the same scope.

Key Takeaways

• A building-only price per square foot is not your total ADU cost

• Garage conversions usually cost less than new detached units because the foundation and framing already exist

• Smaller ADUs cost more per square foot because fixed costs get spread over less space

• California law bars impact fees on ADUs of 750 square feet or less, but permit, plan check, and utility fees still apply

• Site work (sewer, electrical panel, sloped lots) is the most common source of budget surprises

• Get itemized bids with the same scope, not a single lump number

What Does an ADU Cost in California?

Published estimates vary a lot. Most come from builders or cost-guide sites rather than an official state average. Three ranges are the most useful for budgeting:

• Total project cost: Roughly $120,000 to $450,000+ across California, with common detached or attached units of 600 to 1,000 square feet landing in the upper-middle of that range.

• Per square foot: Commonly cited ranges run from about $150 to $400 for simpler projects and $400 to $600+ for detached builds in expensive markets. Garage conversions are typically quoted at $150 to $400 per square foot.

• A local benchmark: A 2021 San Diego Housing Commission sample of five projects ranged from $116,803 for a 224-square-foot studio to $342,078 for a 1,199-square-foot unit. That works out to about $521 per square foot for the studio and $285 for the larger unit. These are historical figures, so today’s costs will be higher.

Treat all of these as ranges to test against real bids, not as quotes.

Cost by ADU Type

Ranges come from published 2026 cost guides and builder estimates and vary by city, so use them as a starting point only.

What Is Included in an ADU Budget?

A contractor’s construction price rarely equals your total cost. Plan for these categories separately:

1. Design and plans. Typically $5,000 to $30,000 depending on how custom the design is. Some cities offer pre-approved plans that can cut this cost.

2. Permits and fees. Plan check, building permit, and inspection fees vary by city. For larger units, permit costs can approach $20,000.

3. Site work and utilities. Sewer or water connections, trenching, and electrical panel upgrades. One Bay Area builder described a 750-square-foot project whose budget grew after the lot evaluation turned up a 70-foot sewer trench, a 200-amp panel upgrade, and extra shear wall engineering.

4. Construction. Foundation, framing, roof, plumbing, electrical, and finishes.

5. Contingency. Set aside a cushion for what the plans can’t predict.

ADU Permit and Impact Fees in California

State law prohibits impact fees on ADUs of 750 square feet or less. For ADUs over 750 square feet, impact fees must be charged in proportion to the size of the primary home. That rule applies to impact fees only. You will still pay plan check, building permit, and inspection fees, and a local school district fee may apply to larger units. Some cities waive certain permit fees for a limited period, so check your city’s current rules before you budget.

Why Roof and Plumbing Costs Matter for an ADU

Two of the biggest line items in any ADU are the ones homeowners know best from repair jobs. Knowing the general price range helps you spot a bid that’s too high or too low.

Roof. National figures for a full home roof replacement vary by source. One consumer cost guide puts the average around $9,530, or roughly $4 to $11 per square foot. A 2026 industry report cited an average of $17,631 for U.S. roof replacements in 2025, a figure that includes insurance claim jobs.

A rough illustration: a 600-square-foot ADU roof at $4 to $11 per square foot would be about $2,400 to $6,600 before California labor premiums, pitch, and materials. That arithmetic is only a sanity check, not a quote. If you’re converting a garage, ask whether the existing roof can stay or needs work.

Plumbing. National averages for plumbing repair run about $180 to $600 per job, up to $4,000 for major repairs, with plumber hourly rates commonly between $45 and $150. New ADU plumbing is different from repair work. It involves rough-in, a sewer or water connection, and sometimes a water heater. Ask each bidder to price plumbing as its own line item so you can compare.

How to Compare ADU Bids

With a project this size, two quotes can look far apart simply because they include different things. Before you compare totals:

• Confirm each bid covers the same scope (design, permits, site work, appliances, finishes)

• Ask for itemized breakdowns, not one lump sum

• Ask whether the price is fixed or cost-plus

• Confirm the timeline and payment schedule in writing

Before signing, verify the contractor. In California, check license status and classification on the Contractors State License Board lookup, and confirm insurance and workers’ comp. Also keep in mind that home improvement contracts over $500 must be in writing, and a deposit is capped at $1,000 or 10% of the contract price, whichever is less.

Bottom Line

The cost to build an ADU in California depends on the type of unit, the size, the site, and what each quote includes. Start with a realistic range, get itemized bids for the same scope, and verify every contractor before you sign.

Oliebids is a competitive bid marketplace for home services. You post your project once, and verified local contractors submit their price, timeline, and cost breakdown within 60 hours. 

You review the top three bids and pick within seven days. Verified contractors upload a license, insurance, and photo ID that our team reviews, but we don’t confirm current standing with licensing agencies or insurers. Oliebids is a marketplace, not a vetting