ADU pricing varies widely based on scope and site conditions. Pricing starts at $300,000 and goes up based on size, scope, and the finishes you choose. We provide early-stage cost modeling to help you make informed design decisions before committing to plans — including whether a garage conversion or new build is the better value for your property.
ADU Builders in Marin, Napa, and Sonoma Counties
Homeowners across Sonoma, Napa, and Marin counties are turning to ADUs for flexible living, income potential, and smart use of space. Whether you’re building for family, converting a garage, or planning for future rental income, we build high-quality ADUs tailored to your needs.
With over 20 years of experience building in Wine Country, we’re known for excellent client service, clear communication, and homes built to last. If you’re looking for a seasoned ADU contractor, or a granny flat builder that understands the region, we’re here to help.
Types of ADUs Allowed in California
California law allows several ADU types on most residential properties:
- Detached ADU: A stand-alone unit separate from the main home.
- Attached ADU: Shares at least one wall with the main residence.
- Garage or Basement Conversion: Turns existing space into a livable unit.
- Junior ADU: Up to 500 sq ft within the primary home, includes a small kitchen.
We’ve built each of these and can guide you toward the best approach based on your lot, budget, and goals.
ADU Construction Costs
ADU costs vary depending on size, location, and scope. Here are typical California ranges.
Garage conversion ADUs typically cost between $100,000 and $250,000, depending on the existing structure and site conditions. Detached ADUs generally start around $350,000 and can exceed $600,000 for larger or more custom builds. In Sonoma County, where site-specific factors like grading, access, and finish level come into play, most ADUs fall within the $350,000 to $750,000 range. Our team at Brandywine Builders provides clear, upfront cost guidance early in the process so there are no surprises.
We don’t push one-size-fits-all pricing. Instead, we help you understand the options, then provide clear estimates that match your priorities and timeline.
Working with the Experts
Brandywine Builders makes adding an ADU straightforward and rewarding by guiding you through every step, from initial ideas to move-in-ready space. Your project begins with a complimentary consultation to discuss how you plan to use your ADU, whether it’s for extended family, rental income, or a home office. We prepare a detailed budget, collect bids from trusted partners, and review costs to align with your goals.
Design-Build Teamwork
Our team stays current on local regulations, setbacks, and permitting requirements throughout Sonoma, Napa, and Marin counties, so your ADU meets all code and zoning rules without delays. After refining plans and signing a clear contract, we host a kick-off meeting to introduce our project management platform for smooth communication. During the process, we stay in communication with your architect team and confirm all your selections to keep everything on track. During construction, you’ll receive weekly updates and walkthroughs.
At completion, we present your ADU, review operations, and share warranty information so you can enjoy your new space with confidence.
What to Expect in Sonoma, Napa & Marin Counties and the State of California
California has removed many barriers to building ADUs. Here’s how that benefits you:
- No minimum lot size.
- 4-foot side and rear setbacks for detached ADUs.
- No owner-occupancy requirement through 2025.
- No impact fees for ADUs under 750 sq ft.
- Cities must approve compliant plans within 60 days.
We stay current on all regulations and use them to help you move your project forward without delays or surprises.
Each county applies state ADU rules a little differently:
- Marin County: Stricter review processes in some towns (like Mill Valley) and more fire safety/environmental overlays.
- Napa County: ADU-friendly overall, though rural properties may face septic and access limitations.
- Sonoma County: Supportive, with some variability based on zoning, slope, or location.
We’ve worked in each county and understand how to navigate local requirements and keep your project on track.
Accessory Dwelling Unit (ADU) FAQs
What Types of ADUs Are Allowed in California?
California allows several types of ADUs: detached units, attached units, garage or basement conversions, and small interior units within the main home, often called junior units. Garage conversions are especially popular because they use existing space and may have faster approvals. Brandywine Builders has experience with all of these options and can help you choose the right type based on your lot, goals, and the requirements in Sonoma, Napa, or Marin Counties.
How Much Does It Cost to Build an ADU?
How Long Does It Take to Build an ADU?
Most ADU projects take 6 to 12 months, from planning to move-in. Garage conversions may finish sooner, since much of the structure already exists, while detached units often require full site prep. Brandywine Builders creates a clear project schedule at the start and keeps you updated from day one, whether you’re on-site or managing your project remotely from a second home or out-of-town location.
What Are the ADU Permitting Rules in Sonoma, Napa, and Marin Counties?
Each county has its own codes, zoning rules, and environmental considerations. The best way to stay aligned is to work with a builder who knows the area. We have remodeled homes across Sonoma, Napa, and Marin Counties for more than two decades. We handle everything from permitting to inspections, and we build to meet (and often exceed) local standards. Our team ensures your remodel is compliant, high quality, and right for the region.
What’s One Drawback of Building an ADU?
One drawback is the upfront cost and time are still notable factors. Site conditions, utility upgrades, and permitting complexity can increase that investment. Still, many homeowners find the long-term value worth it. At Brandywine Builders, we walk you through all the variables early, so you understand the full picture before moving forward with design or construction.
Why Choose Us for Your ADU Construction
We focus on quality over volume, and relationships over transactions. We bring decades of local knowledge, design expertise, and dependable project management to every ADU we build. You can take a look at our projects page to learn more. Whether you’re constructing an ADU in Marin, Napa, or Sonoma, we bring a thoughtful, hands-on approach to every project.
Contact Us
If you’re interested in building a custom ADU, contact us today for a consultation. We’d love to hear from you.