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
A home addition or ADU in Kensington typically runs $45,000-$130,000 for a room addition and $90,000-$220,000 for a detached ADU, with the final number shaped by hillside grading, seismic requirements, and what the walls reveal once they’re open. We’re Barrett & Sons Construction, and we’ve been walking Kensington’s steep streets long enough to know that a 1930s Tudor on Arlington Avenue behaves nothing like a flatland house in El Cerrito. If you’re planning a second story, a garage conversion, or a new ADU in the Kensington hills, call (877) 610-1318 and we’ll give you a straight answer on what your project actually requires.
The biggest mistake we see in Kensington? Contractors who quote the job before they know what’s behind the lath and plaster. That’s the expensive way to do it. Our estimate process starts with the property, the slope, and the county’s rules, not with a generic square-footage multiplier.
Why Barrett & Sons Construction Is Kensington’s Preferred Home Additions & ADU Company
Tom Barrett started this company in 1998, and his sons run crews across the East Bay hills today. When a Kensington homeowner calls, they talk to a person who answers the phone, and when the work starts, the name on the truck matches the name signing the permit. That matters in a town like Kensington, where the permitting process goes through Contra Costa County in Martinez, and a contractor who treats it like Berkeley is going to burn your timeline before the first inspection.
Every scope, every allowance, every price change on a Kensington addition arrives as a written change order before work happens. No surprise invoices at the end. The permit inspector works for you, not for me, and we pull permits and sign inspections on every eligible job. Ask anyone on your street, or look up the license number yourself. We publish Tom’s name, address, and license number because we want you to verify exactly who you’re hiring before you sign.
Our Kensington customers get the same person who walked the job on day one showing up to check the work. Tom lives close enough that if something isn’t right, he wants to know about it. That’s the 90-Day Done Right Promise, written on every contract: one call brings Tom back.
Looking for a contractor who treats Home Additions & ADU work as more than a bolt-on service? That’s all we do, and we do it with permits pulled and inspections signed. Our crews are background-checked, uniformed, and they carry the Barrett & Sons name because they’ve earned it.
Our Home Additions & ADU Services in Kensington
Kensington’s housing stock, built mostly between the 1920s and 1950s, demands a specific approach to additions. Here’s how we handle the work that fits these homes:
ADU Design & Permitting
Because Kensington sits on unincorporated Contra Costa County land, every ADU permit goes through the County Building Department in Martinez, not Berkeley or El Cerrito city hall. We handle that from day one. On hillside lots, the county frequently requires a geotechnical report before a permit is even issued. We get the soil engineer out early so your design doesn’t stall at the counter.
Garage Conversion ADU
Converting a 1940s garage in Kensington usually means addressing the slab, the roof, and the electrical service all at once. Those old detached garages along the Arlington corridor were never built for living space, and the county will hold the conversion to current code the moment you file. We price the conversion honestly, including the upgrades you’ll actually need to pass inspection.
New Detached ADU
A new detached ADU on a Kensington hillside lot means grading, drainage, and retaining walls before the foundation pour. The coastal fog that rolls into these hills keeps soil moisture high, and getting drainage wrong means foundation problems in two fog seasons. Our ADU Site Prep & Utilities work includes proper swales, French drains where needed, and a foundation spec that accounts for the slope and the Hayward Fault trace nearby.
Junior ADU & Interior Conversion
For Kensington homeowners working with an existing footprint, a junior ADU inside the main house is often the fastest path to rental income or an in-law suite. Interior conversions typically trigger fewer grading requirements, but they still open walls, and opening walls in pre-war Kensington homes means confronting knob-and-tube wiring and aging redwood framing. We handle the upgrades in writing before rough-in, so the inspection sign-off comes on schedule.
Second Story Additions
Second-story additions in Kensington are the most demanding project type we take on here. The original post-and-pier foundations on these steep lots were never engineered for a second floor. Seismic shear requirements under current Contra Costa County code often mean foundation reinforcement, not just framing. We bring the engineer in before we quote the structure, because the last thing you want is a second story that settles differently from the first.
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 Kensington addition calls for windows, doors, or weather-sealing against the fog that moves through these hills, we install brands with names you can look up. Simonton windows hold up under East Bay humidity, and Ply Gem doors handle the temperature swings between the ridge and the flatlands. We don’t install mystery brands off a van. Your materials are written into the contract, so you know exactly what you’re paying for before we order anything.
Common Home Additions & ADU Problems We See in Kensington Homes
Three decades of working the East Bay hills have taught us what goes wrong here more often than anywhere else. Here’s what we routinely find in Kensington:
- Contractors pulling permits at the wrong counter. They assume Kensington is Berkeley or El Cerrito territory. It’s unincorporated county land, and every permit goes through Martinez. We’ve seen projects stall for six weeks waiting on a correction that should have been filed correctly the first day.
- Skipped geotechnical reports on hillside additions. The Hayward Fault runs through this neighborhood, and the county knows it. Build on a steep Kensington lot without a geotech report, and you’re gambling the foundation on soil nobody tested. We don’t take that bet, and we don’t let our clients take it either.
- Fog-driven dry rot hiding behind lath and plaster. Kensington’s hills trap coastal moisture, and the redwood framing in these 1920s-1950s homes absorbs it for decades. On a 1930s Tudor above Arlington Avenue, our crew opened a wall for a second-story addition and found knob-and-tube wiring and redwood framing with 40% rot from trapped coastal fog. We wrote the code-triggered electrical upgrade and engineered shear wall into a change order, priced it in writing, and got the owner’s sign-off before rough-in. The inspection sign-off came on schedule because there were no surprises left.
- WUI fire hazard requirements nobody mentioned. Nearly all of Kensington sits in a Very High Fire Hazard Severity Zone. New construction and substantial remodels must meet wildland-urban interface standards: ember-resistant venting, Class A roofing, ignition-resistant siding. A contractor who doesn’t know this will underbid the job and then try to pass the upgrade as a change order later. We price it from the start.
Pricing for Home Additions & ADU in Kensington, CA
Kensington additions and ADUs cost more than flatland projects in El Cerrito or Richmond, and they should. The slope, the seismic engineering, the county’s geotechnical requirements, and the WUI fire standards all add legitimate cost that cheaper bids simply ignore. Here’s an honest look at market pricing for this hillside town:
| Project Type | Typical Kensington Range |
|---|---|
| Home Addition | $45,000-$130,000 |
| ADU Construction | $90,000-$220,000 |
| Garage Conversion | $30,000-$80,000 |
What moves that number up? Steeper lots, foundation reinforcement for a second story, the condition of the existing redwood framing, and how much code-triggered electrical or plumbing work the county requires once walls are open. We price allowances in writing, and every change order is signed before the work happens. No surprise invoices, ever. Call (877) 610-1318 for a free estimate, and we’ll give you a line-item scope you can actually compare.
We Also Serve Cities Near Kensington
Barrett & Sons Construction serves Kensington and the surrounding East Bay hills, including Kensington neighbors like El Cerrito, Albany, Richmond, and Berkeley. If you’re in one of those communities and planning an addition or ADU, the same written-scope, permits-pulled approach applies. Call (877) 610-1318 and we’ll come take a look.
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
In Contra Costa County, a geotechnical report is typically required for any new structure on a slope in Kensington, and the county’s Department of Conservation and Development will tell you that before the permit is issued, not after. A flat garage conversion on level ground may not trigger one, but any detached ADU on the hillside almost certainly will. Call (877) 610-1318 and we’ll tell you whether your specific lot needs one, before you pay for plans.
Kensington is unincorporated Contra Costa County, so every permit goes through the County Building Department in Martinez, not a city hall. That means different review timelines, different plan check requirements, and a different set of inspectors than neighboring cities use. A contractor who has only pulled permits in Berkeley or El Cerrito will learn this the hard way, and your timeline will pay for it. We file in Martinez because that’s where Kensington’s permits have always gone.
Opening walls in older Kensington homes routinely triggers electrical upgrades to replace knob-and-tube wiring, structural upgrades to address redwood framing that’s undersized or partially rotted, and insulation and moisture barriers required by current code. The county doesn’t grandfather existing conditions once you expose them. We write those into the contract as allowances, so the cost is on paper before the first wall comes down.
Most of Kensington sits in a Very High Fire Hazard Severity Zone, which means your ADU must meet wildland-urban interface standards: Class A roofing material, ignition-resistant siding, and ember-resistant venting on every opening. These are not optional upgrades, and a designer who skips them will get the plans bounced at county review. We include them from the first drawing, so your permit doesn’t stall.
Kensington’s coastal fog settles into these hills and keeps redwood framing damp for much of the year, especially on north-facing slopes and homes with original single-pane windows. We open walls during the inspection phase and tell you exactly what’s rotten before we frame anything new. The rotten sections get replaced, the healthy framing gets a moisture barrier, and the whole scope appears in a written change order with a price, before we touch it.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Kensington and the East Bay hills.