Home Additions & ADU in Fairfax - One Fixed Price, Permits Handled

Fast, Reliable Home Additions & ADU Across Fairfax

Adding square footage to a Fairfax home usually costs between $45,000 and $130,000 for a room addition, and $90,000 to $220,000 for a full detached ADU. Those numbers are higher than what you’d pay in San Rafael or Novato, and there’s a reason: Fairfax sits inside a Very High Fire Hazard Severity Zone, so nearly every project triggers CAL FIRE fire-hardening requirements and defensible-space reviews before Marin County will even look at your permit. That’s the reality we deal with every week. If you’re on a hillside lot off Bolinas Road or up in the Cascade Canyon area and you’ve been told your addition will take a year, call us at (877) 610-1318 and let’s talk about what’s actually true for your property.

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Barrett & Sons Construction has been building in Marin County since 1998. We’re not a franchise sending strangers from another state. Tom Barrett and his sons run the crews, and we’re on Fairfax job sites most days. We know what the county building department asks for on hillside additions, we know how to stage materials when the street is twelve feet wide and the lot drops forty feet, and we know which legacy structures will trigger a full fire-hardening retrofit before the county issues a new permit. That knowledge is the difference between a job that starts in June and one that starts in March.

Why Barrett & Sons Construction Is Fairfax’s Preferred Home Additions & ADU Company

We’ve been at this for 29 years, and Fairfax homeowners keep calling us back for the same reason they recommended us to their neighbors: the price we quote is the price you pay, and every change along the way goes on paper before a nail moves. That matters more in Fairfax than almost anywhere else in Marin. When the inspector flags an unpermitted 1980s deck and tells you it needs a full fire-hardening retrofit before your ADU permit moves forward, the last thing you need is a contractor who shrugs and hands you a bigger invoice later. With us, that scope change is a written change order, explained before work proceeds.

Ask anyone on your block off Sir Francis Drake Boulevard, or look up the license number yourself. Tom publishes his name, his address, and his license number on every estimate. He wants you to verify him. And when you call, a live person answers, 24/7, not a voicemail box and not an out-of-state call center.

We also carry the safest brands in the trade. When your Fairfax addition needs a new patio or walkway, we install Techo-Bloc and Cambridge Pavers, not mystery materials off a van. When the county requires Class A roofing on your addition, we spec GAF and CertainTeed without hesitation. You can look up every product we put in.

Our Home Additions & ADU Services in Fairfax

ADU Design & Permitting

Most Fairfax ADUs stall at the permit counter, not the construction site. Marin County requires fire-hardening documentation, hillside geotechnical review, and often flood-zone analysis if you’re near Fairfax Creek. We handle the full design and permitting package in-house, including CAL FIRE defensible-space plans and the written documentation the county wants before it stamps anything. Your permit gets pulled, inspections get signed, and you see every submittal before it goes in.

Garage Conversion ADU

Converting a Fairfax garage usually runs $30,000 to $80,000. The range depends heavily on what we find when we open the walls. Garages from the 1950s and 1960s here often have undersized footings, zero insulation, and old knob-and-tube wiring hiding behind wood siding. If your garage sits below the main house on a Cascade Drive hillside, drainage and retaining work can add real cost. We’ll tell you what we see before we start, in writing.

New Detached ADU

A new detached ADU in Fairfax typically runs $90,000 to $220,000. The upper end shows up on steep hillside lots where materials have to be hand-carried or craned in, and where the foundation requires drilled piers instead of a standard slab. We build detached units that meet current Wildland-Urban Interface codes: ember-resistant vents, ignition-resistant siding, dual-pane tempered glass, the whole package. It’s built to survive a Fairfax fire season, not just to pass an inspection.

Junior ADU & Interior Conversion

For smaller budgets, an interior conversion can add a rental unit or an in-law suite without changing your home’s footprint. A junior ADU carved out of an existing bedroom or a finished basement space usually lands on the lower end of our Home Additions & ADU pricing. The catch in Fairfax is that any interior work touching exterior walls or your roof often pulls the whole structure into fire-hardening compliance. We’ll map that out before you spend a dollar on drawings.

Room Additions, Second Stories & Sunrooms

Beyond ADUs, we build Home Additions & ADU of every shape: ground-floor master suites, family room bump-outs, second-story additions over existing single-story ranchers, and enclosed patio or sunroom conversions. Each one in Fairfax faces the same three forces: steep lots, fire-code retrofits, and legacy unpermitted work that has to be fixed before the county signs off. We’ll put that reality in your estimate, not surprise you with it later.

Trusted Brands We Service in Fairfax

We stock and install Oldcastle and Quikrete products for foundations, retaining walls, and hardscape work tied to your addition. We like them because the specs are consistent and the local supply chain through Marin keeps turnaround short. When a Fairfax job needs a retaining wall before the foundation goes in, we aren’t waiting a week for materials to show up from another region. The team at Home Additions & ADU in Fairfax has been doing this long enough to know which suppliers carry what, and which products hold up to wet winters and hot, dry fire seasons.

Common Home Additions & ADU Problems We See in Fairfax Homes

• Unpermitted work from the 1970s through 1990s. Fairfax hillside neighborhoods are full of decks, sunrooms, and bonus rooms that were built without permits decades ago. When you apply for a new addition or ADU, Marin County often requires a fire-hardening retrofit on those legacy structures before issuing anything new. Budget and schedule both expand, and most homeowners find out at the worst possible moment.
• Hillside foundation movement. Fairfax’s inland valley position, wet fog-driven winters, and steep lots near Fairfax Creek mean foundations shift. Before we add a second story, we check for cracking, settling, and drainage problems. If the base moves, your addition moves, and nobody wants that three years after the drywall goes up.
• Fire-code surprises. Almost every Fairfax project triggers some mix of Class A roofing, ember-resistant venting, ignition-resistant siding, and defensible-space work. Some of it you can see. Most of it is buried in the county’s submittal requirements. We handle it on the front end so it isn’t a change order nightmare on the back end.
• Narrow access and material staging. Many Fairfax streets, especially off Bolinas Road and up in the canyon areas, are tight. A standard delivery truck cannot always make the turn, and staging a foundation pour or a framing package can require a crane or hand-carrying. It costs more and takes longer, and any contractor who doesn’t tell you that up front is guessing.

Pricing for Home Additions & ADU in Fairfax, CA

We Also Serve Cities Near Fairfax

Our Fairfax crews regularly work in Kitchen Remodeling in Fairfax adjacent communities as well. If your project sits in San Anselmo, San Rafael, Kentfield, or Lucas Valley-Marinwood, we handle those permits and hillside reviews too, and we know the same Marin County building officials. We also handle Bathroom Remodeling in Fairfax jobs that often ride along with an addition or ADU project. The same crew, the same written guarantee, no matter which side of the Fairfax line you’re on.

FAQs - Home Additions & ADU in Fairfax