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 Kensington
Adding square footage or an ADU to a Kensington home starts with a simple fact: this isn’t Berkeley, and it isn’t El Cerrito. Kensington sits unincorporated in Contra Costa County, which means every addition, garage conversion, and detached ADU permit runs through the County Building Department in Martinez, not a city hall ten minutes away. Most addition projects here fall between $45,000 and $130,000, with detached ADUs running $90,000 to $220,000. Our crews know the county path, the geotechnical triggers on steep lots, and what those 1920s-to-1950s hillside homes hide behind the walls. Call (877) 610-1318 and we’ll walk the lot with you before you sign anything.
Why Barrett & Sons Construction Is Kensington’s Preferred Home Additions & ADU Company
Tom Barrett founded this company in 1998, and his sons run crews on Kensington jobs every week. We’re not a franchise dispatching strangers from three counties away. When you call about a second-story addition off Arlington Avenue or an ADU design review on a slope above Colusa Circle, the person who answers knows what those words mean.
Kensington homeowners tell us the same thing, usually after a bad experience with an out-of-area outfit: they got a phone quote, then a different number at the door. We do it differently. Every scope, every allowance, every price change arrives as a written change order before a single nail moves. Tom publishes his name, his address, and his license number. Look it up before you sign. That’s the point.
We also pull permits on every eligible job and stand with you through the county inspection. In Kensington, that means knowing when Contra Costa County requires a geotechnical report before a hillside foundation permit moves. Most contractors learn that the hard way, after your timeline slips a month. We know it before we write the proposal.
Our Home Additions & ADU Services in Kensington
Our Home Additions & ADU team covers the full range of work Kensington homeowners call us for, from a junior ADU inside an existing floor plan to a full second-story addition with seismic upgrades. Here’s what we handle most in the Kensington hills.
ADU Design & Permitting
Kensington is unincorporated Contra Costa County, so ADU plans route through the County Building Department in Martinez. That single fact reshapes your timeline. Hillside grading or foundation work on a sloped lot frequently triggers a county-reviewed geotechnical report before a permit is issued. We handle that review, the plan set, and the counter paperwork so you don’t lose weeks figuring out which office to call.
Garage Conversion ADU
Many Kensington homes have detached or tuck-under garages from the 1930s and 1940s built on the same post-and-pier foundations as the house itself. Converting one into a livable ADU means addressing seismic anchoring, cripple-wall reinforcement, and often replacing rotted mudsill redwood that has absorbed decades of hillside fog moisture. We scope that work upfront, in writing, with allowances built into the contract.
New Detached ADU
A new detached ADU on a steep Kensington lot starts with drainage and grading, not framing. Nearly all of Kensington sits within a Very High Fire Hazard Severity Zone, so new construction must meet WUI standards: ember-resistant venting, ignition-resistant siding, and Class A roofing. With that, you’re looking at $90,000 to $220,000 depending on size, site access, and slope engineering. We price every line item before we dig.
Junior ADU & Interior Conversion
A Junior ADU inside an existing home, typically a converted bedroom wing or lower floor with its own entrance, runs considerably less than a detached build because you’re working within already-conditioned space. On Kensington’s hillside homes, interior conversions often reveal knob-and-tube wiring and single-pane steel-sash windows that must come out for code. We write those allowances into the contract day one, so the discovery doesn’t become a surprise invoice.
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 Kensington
When your addition needs windows, we install Simonton and Ply Gem because they hold up to the damp, fog-heavy hillside conditions that rot cheaper frames in under a decade. For windows on the windward side or any south-facing exposure, we often specify PGT for impact-rated glass that handles both storms and wildfire ember exposure. We keep a local parts pipeline for these brands, which means fewer two-week waits on backordered frames when your framing inspection is already scheduled.
Common Home Additions & ADU Problems We See in Kensington Homes
- Rotted redwood framing from hillside fog. The Kensington hills trap coastal moisture that flatland El Cerrito and Richmond don’t get. Original redwood studs and sills in pre-war homes often look fine on the surface and crumble when you open the wall. That’s why our proposals always include a structural repair allowance, in writing, before demo starts.
- Knob-and-tube wiring behind original walls. A 1930s bungalow off Arlington Avenue nearly always has some knob-and-tube still live. County code requires it removed or disconnected for any new permitted addition. We budget for rewiring in the initial scope, not as an afterthought.
- Failing hillside retaining walls and drainage. Additions on steep Kensington lots put new loads on old retaining walls, many of them built fifty years ago without proper drainage. A new ADU foundation that settles or cracks after the first rainy season is usually a drainage plan problem, not a concrete problem.
- Wrong-counter permit assumptions. Contractors used to Berkeley or El Cerrito assume city permits. In Kensington, every permit goes through Contra Costa County in Martinez, and hillside work often needs a geotechnical report first. If your contractor is surprised by this, your schedule will be too.
Pricing for Home Additions & ADU in Kensington, CA
Kensington pricing runs higher than flatland cities for one reason you can’t negotiate away: slope. Steep lots mean more foundation engineering, more retaining wall work, more geotechnical review, and often longer material carries. A room addition in Kensington typically runs $45,000 to $130,000. A detached ADU runs $90,000 to $220,000. A garage conversion lands between $30,000 and $80,000. These ranges assume the hidden conditions we already know to look for: knob-and-tube removal, rotted redwood replacement, and drainage correction on sloped ground. Line-item pricing looks like this:
| Project Type | Typical Kensington Range |
|---|---|
| Home Addition | $45,000 - $130,000 |
| ADU Construction | $90,000 - $220,000 |
| Garage Conversion | $30,000 - $80,000 |
Every proposal includes written allowances for the structural and electrical surprises Kensington homes are known for. If the allowance isn’t on paper, it’s a surprise waiting for a date. The estimate is free. Call (877) 610-1318 and we’ll put together a written, line-item proposal for your specific lot and floor plan.
We Also Serve Cities Near Kensington
While this page focuses on Home Additions & ADU in Kensington, our crews regularly work in El Cerrito, Albany, Richmond, and Berkeley. We know the permit counters, the soil types, and the inspectors in each. If you’re a few blocks over the line, call (877) 610-1318 and we’ll point you to the right office for your job.
Serving Kensington, CA - Our Local Coverage Area
We’re based in the Kensington 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 Kensington
Yes, in most cases. Because Kensington is unincorporated Contra Costa County, foundation and grading permits for steep lots run through the County Building Department in Martinez, and a county-reviewed geotechnical report is routinely required before the permit is issued. We handle that report as part of our ADU Design & Permitting scope. Call (877) 610-1318 and we’ll tell you whether your specific lot triggers the requirement before you spend a dime.
We stop, document it, price the removal and rewiring as a written change order, and don’t resume until you approve the updated scope. That’s exactly what happened on a 1930s bungalow off Arlington Avenue where our crew opened a load-bearing wall for a second-story addition and found original redwood studs rotted from trapped hillside fog moisture, plus live knob-and-tube running through the ceiling joists. We priced the reframe and rewiring in writing, the homeowner approved it, and the job stayed on schedule. No surprise invoice at the end. Call (877) 610-1318 to talk through your specific addition scope.
Because Kensington homes were built on steep hillside lots between the 1920s and 1950s, most with post-and-pier or cripple-wall foundations that have endured decades of fog-driven moisture and seismic movement from the nearby Hayward Fault trace. Opening up a wall almost always reveals some combination of rotted redwood, settlement, or outdated framing that flatland homes in Berkeley or El Cerrito simply don’t have. We write that structural allowance into the contract up front, so the final number matches the real conditions on your lot. Call (877) 610-1318 for a written quote that reflects actual Kensington ground, not a generic county average.
Sometimes, but it depends on the scope of what the county requires once the walls are open. Attached garage conversions in Kensington often trigger seismic anchoring and cripple-wall reinforcement on the garage structure itself, because that’s what the new livable space sits on. Whether the county extends that requirement to the main house depends on the connection between the two structures and what the plans show. We evaluate that during design and tell you before you spend money on permits. Call (877) 610-1318 and we’ll assess your garage conversion with a free on-site visit.
We design the drainage and grading plan into the addition scope from the start, not after the foundation is poured. A new addition or ADU adds load to soil that’s already moving on Kensington slopes, which is why so many older retaining walls fail during construction. We inspect the walls, identify whether they need repair or replacement, and price that work as a separate line item you approve before we touch it. That’s the difference between an addition that settles straight and one that cracks in the first rainy season. Call (877) 610-1318 to schedule a free drainage and lot assessment.
Written by Tom Barrett, Owner at Barrett & Sons Construction. Serving Kensington hillside homes with permits pulled, change orders in writing, and a 90-Day Done Right Promise on every job.
Ready to talk about your Kensington addition or ADU? Call (877) 610-1318 for a free estimate. A live person answers, not a voicemail box. We’ll schedule a walkthrough of your lot, talk through the county permit path, and give you a written line-item proposal with no surprises at the end.