Free tool · California
ADU Cost Calculator
A detached ADU in California typically costs $300–$450 per square foot to build, so a 600 sq ft unit runs about $180,000–$270,000 before design and permit fees. Garage conversions cost less ($150–$300 per sq ft), and junior ADUs least of all. Bay Area projects cost the most; inland areas the least.
Estimated total project cost
$202,000 – $302,000
Roughly $300–$450 per sq ft, all in
- Construction (hard cost)
- $180,000 – $270,000
- Design & permits (soft cost)
- $21,500 – $32,500
- Impact fees
- $0 — exempt
Under California law (SB 543 / Gov. Code 66324), an ADU under 750 sq ft is exempt from impact fees. Estimates exclude utility upgrades, site work, and financing.
This is a statewide range. Your real cost depends on your lot, setbacks, utilities, and city fees.
How much does an ADU cost in California?
Cost depends most on whether you are building new or converting existing space. New detached units carry the full price of a foundation, framing, roof, and separate utility connections, so they sit at the top of the range. Attached additions share a wall and some systems with the main house, trimming cost. Garage conversions reuse an existing shell and are usually the cheapest new-bedroom-and-bath path, followed by junior ADUs (JADUs) that are carved out inside the existing home. On top of construction, budget for soft costs — design, engineering, and city permit fees — plus any utility upgrades your lot requires.
Cost per square foot by ADU type and region
Hard construction cost per square foot, standard finishes, 2025–2026. These are the figures the calculator above uses. Design, permit, and impact fees are additional.
| ADU type | Bay Area | Los Angeles | San Diego | Sacramento / Inland |
|---|---|---|---|---|
| Detached new build | $345–$515 | $300–$450 | $300–$450 | $245–$370 |
| Attached addition | $290–$460 | $250–$400 | $250–$400 | $205–$330 |
| Garage conversion | $175–$345 | $150–$300 | $150–$300 | $125–$245 |
| Junior ADU (JADU) | $175–$315 | $150–$275 | $150–$275 | $125–$225 |
Ranges are per square foot of hard construction cost. Sources: UC Berkeley Terner Center statewide ADU survey (regional relativities), Snap ADU 2026 cost data, Golden State ADUs 2025 guide, and Angi garage conversion data.
How we calculate your estimate
The calculator multiplies your unit size by a hard-cost-per-square-foot range for the ADU type, then adjusts for region and finish level:
- Base cost per square foot is anchored to Los Angeles / San Diego metro pricing from 2025–2026 builder cost data.
- Region adjusts that base using the Terner Center regional medians — the Bay Area runs about 15% higher, while Sacramento and inland areas run lower.
- Finish level adds roughly 20% for premium materials and fixtures.
- Soft costs (design, engineering, permits) are estimated at 12% to 15% of construction cost.
- Impact fees follow California law: waived for ADUs under 750 sq ft and JADUs of 500 sq ft or less, and charged proportionally for larger units (HCD ADU Handbook, SB 543 / Gov. Code 66324).
Estimates exclude land, utility upgrades, difficult site work, and financing. They are a planning range, not a bid.
Frequently asked ADU cost questions
How much does it cost to build an ADU in California?
A detached ADU typically runs $300–$450 per square foot of hard construction cost in most California metros, so a 600 sq ft unit lands around $180,000–$270,000 before design and permit fees. Garage conversions are cheaper at $150–$300 per square foot.
Are garage conversions cheaper than detached ADUs?
Yes. Converting an existing garage reuses the foundation, walls, and roof, so it usually costs $150–$300 per square foot versus $300–$450 for a new detached build — often 30% to 50% less overall.
Do I have to pay impact fees on my ADU?
Under California law (SB 543 / Government Code 66324), ADUs under 750 sq ft and JADUs of 500 sq ft or less are exempt from impact fees. ADUs of 750 sq ft or more are charged impact fees proportional to your primary home's square footage.
What are soft costs for an ADU?
Soft costs cover architectural design, structural engineering, and city plan-check and permit fees — typically 10% to 18% of construction cost, or roughly $12,000–$30,000 for a standard ADU. Utility upgrades and site work can add more.
How much does a junior ADU (JADU) cost?
A JADU is carved out of your existing home (maximum 500 sq ft) and shares a wall, bathroom, or kitchen, so it is the cheapest path — usually $150–$275 per square foot, or roughly $30,000–$90,000.
Why do Bay Area ADUs cost more than inland California?
Labor and permitting cost more in high-demand coastal metros. UC Berkeley's Terner Center statewide survey found Bay Area ADUs at a $329 per square foot median versus roughly $200 per square foot in Los Angeles and San Diego counties.
