Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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AfterFast, Reliable Home Additions & ADU Across Kentfield
Building an addition or an ADU on a sloped, rain-soaked Kentfield lot is not the same as building one on a flat parcel in Novato or central San Rafael. The permitting runs through Marin County, the drainage has to handle 50 to 60 inches of annual rainfall, and the hillside near Kent Woodlands carries fire-resistant construction rules most flatland towns never see. We know those rules because we work them every month. If you are pricing a room addition, second story, or detached ADU in Kentfield, a realistic budget for the work we do here generally falls between $45,000 and $130,000 for a home addition, and $90,000 to $220,000 for a full new ADU. Call (877) 610-1318 to schedule a free estimate, and Tom Barrett or one of his sons will walk the property with you and give you a written line-item quote before anything is signed.
Why Barrett & Sons Construction Is Kentfield’s Preferred Home Additions & ADU Company
Barrett & Sons Construction has been owner-operated since 1998. Tom Barrett answers his own phone, and his sons run the crews that work the uphill streets above Sir Francis Drake Boulevard and around the College of Marin edges. That matters in Kentfield because the county building department, the fire marshal, and the drainage reviewers all have their own expectations here, and an owner who has pulled permits on these hillsides before will not be surprised by a WUI review or a slope-stability letter.
Our Home Additions & ADU work in Kentfield has included everything from a junior ADU carved out of a mid-century split-level near the flats to a full detached unit on a stepped hillside lot where the drainage plan mattered as much as the framing. We write every scope, every allowance, and every change order on paper before it happens. If something is not done right, the 90-Day Done Right Promise means one call brings Tom back. That is a written guarantee on the contract, not a sales line.
If you live in Kentfield and you want to check us out first, look up the license number. It is published, along with Tom’s name and address. That is the whole point. A contractor working in Kentfield should be willing to be found.
Our Home Additions & ADU Services in Kentfield
ADU Design & Permitting
Most Kentfield ADU projects start with the county, not with a hammer. Marin County Community Development reviews the plans, the fire district reviews the location, and the drainage reviewer wants to see where all that rain goes. We handle that whole path: site assessment, plans, structural details for sloped lots, WUI-compliant exterior choices, and permit submittal. Then we wait on the county with you and keep the schedule honest. A detached ADU in Kentfield runs $90,000 to $220,000 depending on site access and foundation work; a garage conversion is typically $30,000 to $80,000.
Garage Conversion ADU
Half the Kentfield homes we walk were built in the 1950s and 1960s with attached garages on posts and piers. Those garages have shifted, and the slab edges have often absorbed decades of hillside runoff. Converting one into a legal one-bedroom ADU means leveling the floor system, adding seismic ties where the county requires them, and reworking the drainage so water stops following the old slab toward the house. We have done enough of these off College Avenue and the Woodside area to know what the inspector will flag before he arrives.
New Detached ADU
Building new in Kentfield usually means working on a slope with a stepped foundation or a retaining wall plan the county geotech reviewer will scrutinize. Because the orographic lift along the Marin hills dumps so much rain here, temporary waterproofing during framing and excavation is non-negotiable from October through April. A new detached ADU with one bedroom, a full bath, and a small kitchen typically runs $90,000 to $220,000 in this market. That includes permit fees, foundation work, and the fire-resistant vents and cladding the fire district requires near the hill-edge parcels.
Junior ADU & Interior Conversion
A junior ADU often makes the most sense for Kentfield homeowners working with an existing bedroom wing or a lower level that already has its own exterior door. The work stays inside the existing footprint, which keeps the county review simpler and the budget lower than a detached build. We still pull permits and still write every finish allowance on paper. If your lower level has felt damp in heavy winters, we address the moisture source during the conversion rather than covering it with drywall and hoping for the best.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Trusted Brands We Service in Kentfield
We install materials you can look up before you sign, not mystery brands off a van. For Kentfield ADU kitchens and baths, that often means KraftMaid cabinetry and Cambria countertops. For exterior work that must meet Marin County Fire’s WUI standards, we use GAF roofing systems with the fire-rated ridge and venting details the hillside parcels require. We know these products in the specific configurations Kentfield inspectors expect, which saves review time. If you want to compare materials, we will show you the spec sheets and let you make the call.
Common Home Additions & ADU Problems We See in Kentfield Homes
- Undersized drainage on 1950s-1970s hillside homes. The original downspout and footing-drain systems were often sized for flatland rainfall, not the 50 to 60 inches Kentfield gets every year. By the time a homeowner calls us, water has usually been following the foundation line for years. Any addition or ADU plan has to fix that first, not build on top of it.
- Settled post-and-pier foundations under older additions. Decades of heavy seasonal rain have shifted a lot of the post-and-pier systems on the sloped streets above Sir Francis Drake Boulevard. Framing a new room addition without addressing that settlement just transfers the movement into the new work.
- WUI fire-resistant requirements catching people off guard. Homes near Kent Woodlands and the hill-edge parcels fall under Wildland-Urban Interface rules that dictate roof class, vent mesh, and exterior cladding. We plan for those from day one and include the added material cost in the written estimate, so the county review does not send you back to square one.
- Damp lower levels that complicate ADU conversions. The same rainfall pattern that makes Kentfield so green also keeps crawl spaces and lower floors wet. A junior ADU conversion without a moisture plan will smell musty and fail an inspection. We address the intrusion source first, then build.
Pricing for Home Additions & ADU in Kentfield, CA
In Kentfield, a room addition or second story typically runs $45,000 to $130,000. A garage conversion ADU runs $30,000 to $80,000, and a new detached ADU runs $90,000 to $220,000. What shifts the number most in this town is site access, foundation condition, and the WUI exterior requirements near the hills. A flat lower-level junior ADU with good access will land at the lower end; a hillside detached unit with a retaining wall and fire-resistant roofing and cladding will land higher. You will never get a phone number with no range from us. You will get a written line-item quote with every allowance on paper, and every change order issued in writing before work proceeds. Call (877) 610-1318 to schedule a free estimate.
We Also Serve Cities Near Kentfield
Our crews work throughout central and southern Marin, so if you are in Larkspur, San Anselmo, Corte Madera, or Fairfax, we can run the same county-permit and drainage-aware process on your project. And if your Kentfield project starts with the existing house rather than new square footage, we also handle Kitchen Remodeling in Kentfield and Bathroom Remodeling in Kentfield under the same written-change-order rules.
Serving Kentfield, CA - Our Local Coverage Area
We’re based in the Kentfield area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
FAQs - Home Additions & ADU in Kentfield
Timing depends on two things: the Marin County building department review queue and the site-specific engineering your hillside lot needs. A junior ADU or interior conversion can often start within a few weeks of permit approval; a new detached ADU with slope-stability and WUI review may take longer. What we can tell you up front is that we do not start framing until permits are in hand and every allowance is on paper. Call (877) 610-1318 and we’ll give you an honest timeline based on your specific lot and project.
Yes, we pull every permit and work with Marin County Community Development and the Marin County Fire reviewers on every eligible job, and we do not skip that step because a previous contractor told you it was optional. The permit inspector works for you, not for me. That is the safest way to run a project in an unincorporated area like Kentfield, and it is how we have operated since 1998.
A new detached ADU in Kentfield typically runs $90,000 to $220,000, which is often a bit higher than a comparable build in flatland parts of Novato or San Rafael because of hillside drainage, foundation work, and the WUI fire-resistant materials required near the hill-edge parcels. A garage conversion runs $30,000 to $80,000. For an exact number on your property, call (877) 610-1318 and we will walk it with you for free.
Not without a written change order that you sign first. Every scope, allowance, and price change arrives on paper before a single nail moves. If a material costs more than the allowance we set, you will see that number in writing and approve it or choose something else. There are no surprise invoices at the end. That is a Barrett & Sons rule, and it applies to every job in Kentfield and everywhere else we work.
Yes. Every completed job carries the written 90-Day Done Right Promise: if it is not done right, one call brings Tom back to make it right. That promise is written on the contract, not buried in fine print. We also stand behind the materials we install, and because we use named brands like GAF and KraftMaid, the manufacturer warranties are real and verifiable. Call (877) 610-1318 to talk through your project, and bring your written estimates; we will give you a free second opinion on any of them.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Kentfield and the surrounding Marin County communities since 1998.