How Much Does an ADU Cost in the Bay Area? (2026 Guide)
ADUs in the Bay Area cost $120,000-$500,000+ in 2026, depending on type and city. Garage conversions start at $120K. Attached ADUs run $150K-$350K. Detached ADUs cost $250K-$500K+. Costs vary by city, with premium markets adding 15-25% over South Bay averages.
How much does an ADU cost in the Bay Area in 2026?
Bay Area ADU costs in 2026 range from $120,000 for a garage conversion to $500,000+ for a large detached unit. The average detached ADU costs $300K-$400K. Attached ADUs average $200K-$300K. Costs are highest in Palo Alto, Saratoga, and Los Altos, and lowest in San Jose and Fremont.
ADU Costs in the Bay Area: 2026 Overview
ADU construction is booming in the Bay Area. California’s streamlined permitting laws and strong rental demand make ADUs one of the smartest investments Bay Area homeowners can make. But costs vary widely depending on ADU type, location, and finishes. This guide breaks down what you should expect to pay in 2026.
Cost by ADU Type
| ADU Type | Cost Range | Timeline | Typical Size |
|---|---|---|---|
| Garage Conversion | $120,000-$150,000 | 4-6 months | 400-500 sqft |
| Attached ADU | $150,000-$350,000 | 6-10 months | 400-800 sqft |
| Detached ADU | $250,000-$500,000+ | 8-14 months | 500-1,200 sqft |
These ranges include construction costs. Permits, design fees, and utility connections are additional (see “Beyond Construction Costs” below).
City-by-City ADU Costs
Labor rates, permit fees, and local requirements vary across the Bay Area. Here is what a typical 600 sqft detached ADU costs in different cities:
| City | Estimated Cost | Permit Timeline | Notes |
|---|---|---|---|
| San Jose | $250,000-$350,000 | 6-10 weeks | Reduced ADU permit fees |
| Fremont | $260,000-$360,000 | 8-12 weeks | Standard processing |
| Sunnyvale | $270,000-$380,000 | 8-12 weeks | Active ADU market |
| Campbell | $260,000-$370,000 | 6-10 weeks | Streamlined process |
| Cupertino | $290,000-$400,000 | 8-14 weeks | Higher land values increase costs |
| Mountain View | $290,000-$400,000 | 8-12 weeks | Strong rental demand |
| Palo Alto | $320,000-$450,000 | 10-16 weeks | Design review possible |
| Saratoga | $320,000-$460,000 | 10-16 weeks | Hillside lots add cost |
| Los Gatos | $310,000-$440,000 | 10-14 weeks | Design review in some areas |
Cost differences between cities reflect local labor markets, permit fee structures, and the finish levels typical in each community. Homeowners in premium cities tend to choose higher-end finishes, which pushes average project costs up. For location-specific details, see our pages on ADU construction in San Jose, ADU builder in Fremont, and ADU construction in Palo Alto.
What Drives ADU Costs Up or Down
ADU Type and Size
The biggest cost factor is the type of ADU you build. Garage conversions are the most affordable because the structure already exists. Every additional square foot of a detached ADU adds $350-$500 to the project. The sweet spot for cost efficiency is 600-800 sqft: large enough to be a comfortable one-bedroom and small enough to keep costs manageable.
Site Conditions
Flat lots with easy access for equipment keep costs low. Sloped lots, tight access (common in older Bay Area neighborhoods), and poor soil conditions add to the foundation and site work budget. Hillside properties in cities like Saratoga and Los Gatos can add 15-25% to base costs.
Finish Level
Builder-grade finishes (vinyl plank flooring, laminate countertops, standard cabinets) keep costs at the low end of each range. Mid-range finishes (quartz countertops, hardwood-look tile, soft-close cabinetry) add $20,000-$40,000. High-end finishes (custom cabinetry, natural stone, designer fixtures) can add $50,000-$100,000 to a detached ADU.
Utility Connections
Detached ADUs require new connections for water, sewer, and electrical. This typically costs $15,000-$30,000, with sewer connection being the most expensive component. Attached ADUs and garage conversions can often tie into existing utility lines, reducing this cost significantly.
Beyond Construction Costs
Budget for these additional expenses beyond the construction estimate:
- Permits and fees: $5,000-$25,000 (varies significantly by city)
- Architecture and engineering: $10,000-$25,000
- Utility connections: $15,000-$30,000 (detached only)
- Landscaping restoration: $5,000-$15,000
- Contingency: 5-10% of construction cost
How Custom Home Controls ADU Costs
Custom Home’s two-phase process is designed to eliminate budget surprises. In Phase 1 (Design), we deliver complete 3D visualizations of your ADU, an itemized budget breaking down every cost category, and engineered plans ready for permit submission.
When you approve Phase 1 and move to Phase 2 (Build), the price is locked in. This is critical for ADU projects because utility connections, site conditions, and permit requirements can create unexpected costs if they are not identified upfront.
Our clients typically save 10-15% compared to homeowners who hire a designer and contractor separately, because our integrated process eliminates the miscommunication and rework that happens when design and construction are handled by different firms.
Next Steps
If you are considering an ADU in the Bay Area, the best first step is a site assessment. We will evaluate your lot, discuss your goals (rental income, family housing, property value), and provide a preliminary cost range specific to your property and city.
Contact Custom Home for a free consultation to discuss your ADU project.
Frequently Asked Questions
What is the cheapest type of ADU to build in the Bay Area?
Garage conversions are the most affordable at $120,000-$150,000. They use the existing structure, so you skip foundation and framing costs. The trade-off is that size is limited by your garage footprint (typically 400-500 sqft for a two-car garage).
Are permits included in ADU cost estimates?
Permit costs are often listed separately. Bay Area ADU permits range from $5,000-$25,000 depending on the city. Some cities like San Jose have reduced ADU permit fees to encourage construction. Always confirm permit costs with your local building department.
Can I finance an ADU in the Bay Area?
Yes. Common financing options include home equity loans (HELOCs), cash-out refinance, construction loans, and renovation loans (FHA 203k, Fannie Mae HomeStyle). Some Bay Area homeowners use the projected rental income to qualify for the loan.
How much does an ADU increase property value?
Bay Area ADUs typically increase property value by $150,000-$300,000 or more, depending on the ADU type, quality, and location. Detached ADUs with separate entrances add the most value. The income potential from rent further increases the effective return.