ADU Contractor in Santa Rosa, CA

Serving Sonoma County, Marin County, East Bay, and Fairfield/Vallejo  

Licensed General Contractor | Lic #1133241

⭐ ⭐ ⭐ ⭐ ⭐ Overwhelmingly Rated 5/5 on Thumbtack ⬅️

Building an ADU Is Not Like Any Other Home Project

Most home projects, a kitchen remodel, a room addition, even a full renovation, involve one structure. An ADU means building a complete, independent dwelling unit on a property that already has a home on it. That's a separate foundation, separate framing, a second electrical service, a second set of plumbing, and in most cases a separate utility meter. It also means a permit process that runs through the planning department, the building department, and the utility company, sometimes simultaneously.


The coordination problem is real. An ADU touches more trades than any standard renovation. If you're managing those trades separately, you're also managing the sequencing, the gaps between scopes, and the permit timeline, on a project where a mistake at the foundation stage doesn't show up until framing is done.


Most homeowners starting an ADU project don't know what they don't know yet. The ones who've been through it before hire a contractor who manages the full scope, not a collection of specialists they have to coordinate themselves.

Modern backyard with a small gray cabin, patio seating, and a lush green lawn at sunset

How JCW Approaches Your ADU Build

Licensed General Contractor | Lic #1133241

I'm John Collins. I've been doing residential construction in Sonoma County and across the North Bay for over 25 years. JCW handles ADU construction as a complete project, every trade, one contract, managed by me personally from the first estimate through the final inspection.

1. ADUs require a different level of coordination than a standard renovation

You're building a complete dwelling unit on a property that already has a home on it. That means two electrical services, two sets of plumbing, a separate utility connection, and a permit process that runs through planning, building, and the utility company simultaneously. A contractor who manages this as a complete scope is not the same as one who handles trades separately.

2. The permit process is part of the build, not a separate problem

ADU permits in Sonoma County involve multiple departments and can take two to four months to approve. JCW manages the application and all follow-up as part of the project scope. You don't track down the county. You don't coordinate with the utility company. That's in the contract.

3. Rental-ready and family-ready mean different things, both matter

A unit built for a tenant needs to hold up under occupancy without generating maintenance calls. A unit built for a parent or adult child needs to feel like a real home, not a garage conversion with a bathroom added. JCW approaches both with the same standard: built the way John would want it done for his own family.

4. The estimate covers what a ADU project actually involves

ADU projects grow when scope isn't defined clearly upfront. Before work starts, you get a complete, itemized estimate covering every trade, what's included, what it costs, and what's genuinely hard to predict based on site conditions. What's in the estimate is what's in the contract. Changes are discussed before they touch the invoice.

What JCW Handles

  • Foundation, slab, raised, or crawlspace

  • Framing, sheathing, roofing

  • Separate electrical service and panel

  • Full plumbing rough-in and finish

  • Insulation, drywall, flooring

  • Interior finish, cabinetry, paint

  • Permit application and all inspections

Three Types of ADU Builds.

One Standard.

Detached ADU

A fully independent structure on the same lot. Separate entrance, separate utility service, complete living space. The most common ADU type in Sonoma County.

Attached ADU

Built as an addition to the existing home, sharing a wall but with its own entrance and living space. Lower site impact, same full-scope build.

Garage Conversion

An existing attached or detached garage converted into full living space. Often the fastest path to a permitted ADU, but only if the conversion is done right.



Why Homeowners Build ADUs

  • Monthly rental income on an existing lot

  • Housing a parent or adult child nearby

  • Long-term property value increase

  • No landlord responsibilities off-property

  • Multi-generational living without shared space

  • California ADU laws now make it easier than ever

  • One contractor, one contract, one point of contact

How an ADU Build With JCW Gets Done

No two builds are identical, but the process is consistent.

1. Site assessment, what your property actually allows

John comes out, walks the lot, and looks at setbacks, utility access, lot coverage, and existing site conditions. You find out what type of ADU is feasible, what the permit path looks like, and what a realistic build costs, before you spend a dollar on design.

2. Design coordination and permit application

JCW works alongside your designer or architect to confirm the plans are buildable, then manages the permit submission and county follow-up. ADU permits in Sonoma County can take two to four months. John handles the process and keeps you informed, you don't chase the county.

3. Full build, managed to certificate of occupancy

Once permits are approved, John manages every trade personally, sequencing the work, keeping the schedule moving, and communicating directly with you when decisions need to be made. The job doesn't close until the final inspection is done and the CO is in hand.

⭐⭐⭐⭐⭐ Kate

They did a fantastic job taking down the old fan/light and installing the new one. They also installed a new light hanging from a 14ft slanted ceiling. Super punctual and polite, we will definitely hire for more small and large renovation needs! Already thinking of a bathroom that needs updating.

What Homeowners Say About Working With JCW

Man smiling while entering a code on a wall-mounted security keypad.

I've been doing this work for 25 years, and I've built my reputation one project at a time.


Google Reviews: ~ 14 five-star reviews

Thumbtack Reviews: ~75 five-star ratings


Homeowners hire me because I listen, stay involved, and treat their homes with respect. Here's what a few have said:

Frequently Asked Questions About ADU Construction in Santa Rosa

  • How much does ADU construction cost in Sonoma County?

    ADU construction costs in Sonoma County vary significantly based on size, design, site conditions, and finish level. A modest detached ADU typically starts around $200,000 and moves up from there depending on scope. Attached ADUs and garage conversions can come in lower. JCW starts every project with a complete, itemized estimate, call to discuss your specific site and get a realistic number.

  • How long does it take to build an ADU in California?

    From the start of the permit application to final inspection, most ADU builds run six to twelve months. Permit approval in Sonoma County typically takes two to four months on its own. Construction time after permit approval depends on size and scope but generally runs three to six months. John gives you a realistic timeline at the estimate stage, not an optimistic one that slips.

  • Do I need an architect or designer, or does JCW handle that?

    ADU construction requires permitted plans, which means you'll need a designer or architect. JCW manages the construction scope and the permit process but does not provide design services. John can refer you to local designers who know Sonoma County ADU requirements, and works closely with your design team to make sure what gets drawn is actually buildable.

  • What types of ADUs does JCW build?

    JCW handles detached ADUs, attached ADUs, and garage conversions. The right type depends on your lot, your goals, and what local zoning allows. John will walk through the options at the site assessment and give you an honest read on what makes sense for your property.

  • Can I rent out the ADU right after construction?

    Once the certificate of occupancy is issued, the unit is legally habitable and can be rented. JCW manages the project through to CO before closing the job, you're not left to figure out final inspections on your own.

  • What's the difference between an ADU, a JADU, and a garage conversion?

    An ADU (accessory dwelling unit) is a full independent dwelling, its own kitchen, bathroom, and living space, built on a property with an existing home. A JADU (junior ADU) is a smaller unit, typically converted from part of the existing home, with some shared facilities. A garage conversion repurposes an existing attached or detached garage into living space. JCW handles all three types. John can help you figure out which makes the most sense for your property at the site assessment.

  • Do you work in Marin County and the East Bay?

    Yes. JCW handles ADU construction across Sonoma and Marin County, the East Bay, and the Fairfield/Vallejo corridor. John manages the work personally in all service areas.

Thinking About Building an ADU? Start With a Site Assessment.

The first step is finding out what your property actually allows and what a realistic build looks like, before you commit to anything. Call or fill out the form. John will come out, walk the site, and give you an honest assessment of what's feasible, what it involves, and what it costs.