How Much Does an ADU Cost in California?

The most common answer homeowners get when they ask about ADU costs is "it depends." That's true, but not useful. Here's a more honest breakdown of what ADUs actually cost in California, based on type, size, and location.

The Short Answer

• ›Garage conversions: $60,000 to $130,000
• ›Attached ADU: $100,000 to $250,000
• ›Detached ADU (new build): $150,000 to $400,000+
• ›JADU (Junior ADU): $30,000 to $80,000

These ranges reflect all-in costs including design, permits, construction, and basic finishes. They don't include financing costs.

What Drives the Price

Size

This is the biggest lever. Contractors typically quote ADUs on a per-square-foot basis. In most California markets, that runs between $250 and $450 per square foot for new detached construction.

A 400 sq ft studio at $350/sqft = $140,000. A 1,000 sq ft two-bedroom at $350/sqft = $350,000.

The per-square-foot cost typically goes down as you get larger, because fixed costs (foundation, roofing, utility connections) get spread over more space.

Location

Labor costs vary significantly across California. San Francisco and the Bay Area run 20–40% higher than the state average. Central Valley cities like Sacramento tend to come in closer to the low end. LA and San Diego fall in the middle.

Site Conditions

This is where budgets get blown. If your lot has:

• ›A slope: Expect to add $10,000 to $50,000+ for grading and a more complex foundation
• ›Limited access: If a crew can't get equipment through a side yard, materials have to be carried by hand
• ›Old or undersized utilities: Upgrading an electrical panel runs $3,000 to $8,000. A new sewer lateral can cost $5,000 to $20,000

Always ask your contractor to walk the site and flag potential surprises before they give you a bid.

Construction Method

Traditional stick-frame construction is what most ADUs use. It's flexible and widely understood by local contractors.

Prefab or modular ADUs offer an alternative. Companies like Abodu, Cover, and Mighty Buildings factory-build units in sections and assemble them on site. The promise is faster timelines and more predictable pricing.

Prefab units typically range from $200,000 to $350,000 all-in (including site work and permits). They tend to cost about the same as traditional construction but take less time.

Permit and Fee Costs

Building permit fees are separate from construction costs and vary enormously by city.

Rough ranges by city:

• ›Los Angeles: $15,000–$25,000 (including school fees and impact fees for larger units)
• ›San Diego: $10,000–$18,000
• ›San Jose: $12,000–$20,000
• ›Sacramento: $5,000–$12,000
• ›Oakland: $8,000–$15,000

State law prohibits cities from charging impact fees on ADUs under 750 sq ft. If your ADU is 750 sq ft or smaller, you avoid a significant cost in many cities.

Design and Architecture Costs

You'll need stamped construction drawings to pull a permit. Depending on complexity:

• ›Simple garage conversion: $2,000–$5,000 with a draftsperson
• ›New detached ADU: $8,000–$20,000 with an architect
• ›Prefab manufacturer: Often included in the purchase price

Some cities have pre-approved ADU plan libraries. If your design fits one of the pre-approved templates, you skip a large portion of the plan check process, saving both time and money.

Ongoing Costs After Construction

The build cost is one-time. The ongoing costs are:

• ›Property taxes: An ADU adds to your property's assessed value. Expect your property tax bill to increase by roughly 1% of the ADU's construction cost per year
• ›Utilities: A separate electrical meter costs a few thousand dollars to install. Some homeowners keep shared utilities and split costs informally
• ›Maintenance: Budget 1–2% of construction cost per year for repairs, appliance replacement, and upkeep
• ›Property management: If you hire a manager, expect 8–12% of monthly rent

Is It Worth It?

For most California homeowners in high-demand rental markets, the answer is yes. An ADU that costs $200,000 to build and rents for $2,000/month generates a 12% gross yield before expenses. After accounting for vacancy, taxes, and maintenance, net returns in the 6–8% range are realistic.

For homeowners building primarily for family use rather than rental income, the financial math shifts. The value is in housing a parent or adult child without paying for a separate property, which still makes sense in most cases even without rental income.

The main risk is overspending on the build relative to local rental rates. A $350,000 ADU that rents for $1,800/month in a slower rental market takes much longer to break even. Know your local rental market before you set your budget.

Getting an Accurate Number

A contractor bid is the only way to get a real number for your specific lot and project. Get at least three bids. Be specific about finishes and scope so you're comparing apples to apples.

Ask each contractor to break out the bid into: site work, foundation, framing, roofing, MEP (mechanical, electrical, plumbing), interior finishes, and landscaping/exterior. That breakdown lets you spot where estimates diverge and ask informed questions.

Budget 10–15% above your contractor's estimate for contingency. That covers unforeseen site conditions, material price changes, and the decisions you'll want to make mid-project.