ADU construction costs in California range from $50,000 to $300,000+ depending on type, size, and location. Here's a realistic breakdown to help you plan your budget.
Accessory Dwelling Units — commonly called ADUs or "granny flats" — have become one of the most popular home improvement projects in California. They add living space, generate rental income, and increase property value. But one of the first questions every homeowner asks is: how much does it actually cost?
The honest answer is that costs vary significantly depending on the type of ADU, your lot conditions, local permit fees, and the level of finishes you choose. In this guide, we break down realistic cost ranges for 2025 so you can plan your budget with confidence.
ADU Cost by Type
| ADU Type | Typical Cost Range | Best For |
|---|---|---|
| Detached ADU | $180,000 – $320,000 | Maximum privacy, highest rental value |
| Attached ADU | $120,000 – $220,000 | Shared wall with main house, lower cost |
| Garage Conversion | $60,000 – $130,000 | Fastest & most affordable option |
| Junior ADU (JADU) | $30,000 – $80,000 | Interior conversion, no new foundation |
These ranges reflect construction costs in the Central Valley (Ceres, Modesto, Turlock area). Costs in the Bay Area or Los Angeles typically run 30–50% higher.
What Drives the Cost Up?
- Size — every additional square foot adds cost. A 400 sq ft studio costs significantly less than an 800 sq ft one-bedroom.
- Foundation type — a new slab foundation adds $15,000–$30,000 to the budget vs. using an existing concrete floor in a garage conversion.
- Utility connections — hooking up water, sewer, and electrical from the main house to a detached ADU can cost $15,000–$40,000 depending on distance and local fees.
- Permit and impact fees — California has reduced ADU fees, but you can still expect $5,000–$20,000 in permit costs depending on your city.
- Finishes — standard finishes (basic cabinets, vinyl flooring, standard fixtures) keep costs low. Upgraded finishes can add $20,000–$50,000.
Hidden Costs to Budget For
Beyond construction, make sure you budget for these often-overlooked expenses:
- Architectural plans and engineering: $5,000–$15,000
- Soil report (if required by city): $1,500–$4,000
- Title 24 energy compliance report: $800–$2,000
- Landscaping and site cleanup after construction: $2,000–$8,000
- Furnishing the unit if renting it out: varies widely
How to Maximize Your Investment
A well-built ADU in the Central Valley typically rents for $1,200–$2,000 per month depending on size and location. That means a $180,000 garage conversion ADU can pay for itself in 8–12 years through rental income alone — while also adding $150,000–$250,000 to your property value.
Pro Tip: The most cost-effective ADU is often a garage conversion. If you have a detached garage that's at least 400 sq ft, converting it eliminates the need for a new foundation and can cut your budget by 40–50% compared to a detached build.
Why Hire a Licensed Contractor for Your ADU?
ADU construction in California requires permits, inspections, and work that meets building code. An unlicensed contractor might quote you a lower price, but unpermitted work can result in fines, forced demolition, and major headaches when you try to sell or refinance.
At C Hernandez Construction (Lic. #1106454), we've built ADUs across Ceres, Modesto, Milpitas, and the Central Valley. We handle permits, framing, drywall, roofing, and coordinate with specialty trades so you have one point of contact from start to finish. Contact us for a free, no-obligation estimate on your ADU project.
Ready for a Free Estimate?
C Hernandez Construction · Lic. #1106454 · Ceres, CA · (209) 241-3765