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 Hillsborough
Home additions and ADU construction in Hillsborough typically run between $45,000 and $130,000 for a room addition, and $90,000 to $220,000 for a full detached ADU. The Hillsborough Architectural Review Board adds a layer most Peninsula contractors never deal with, so getting the design right before the first permit filing saves you weeks and thousands. We’re Barrett & Sons Construction, and we’ve spent 29 years navigating exactly this kind of town-level review. Call Tom at (877) 610-1318 for a free estimate, and we’ll walk your project through the ARB without surprises.
Why Barrett & Sons Construction Is Hillsborough’s Preferred Home Additions & ADU Company
We’re not a franchise dispatching strangers to your door. Tom Barrett’s name is on the truck, his sons run the crews, and he lives close enough to Hillsborough that a callback on the 90-Day Done Right Promise doesn’t mean waiting on an out-of-state office. When a Hillsborough homeowner calls about a garage conversion on a narrow upper hillside road, we already know the crane staging conversation is coming. When they mention heritage oaks near the addition site, we know a certified arborist report lands before the grading permit. That’s not something a national outfit picks up from a call script.
Hillsborough is a small town in the way that matters to a homeowner spending six figures. People talk. They ask their neighbors who pulled permits, who showed up when they said they would, who put every change order in writing before a single nail moved. We’ve built our reputation in this ZIP code on that exact standard, and our Hillsborough customers tend to see us again at the hardware store, not just on the job site.
Our Home Additions & ADU Services in Hillsborough
ADU Design & Permitting
Most Hillsborough ADU projects stall before construction ever starts. The Town of Hillsborough requires architect-stamped plans for substantial remodels and ADUs, and the local ARB enforces its own standards on massing, materials, and landscaping preservation. We handle the full design-to-permit package. That means meeting with the ARB, submitting stamped drawings, coordinating with the certified arborist when heritage trees are near the site, and bringing every inspection to sign-off. Our Home Additions & ADU team handles this process from first survey to final inspection.
Garage Conversion ADU
Converting a detached garage in Hillsborough makes particular sense on the large estate lots that dominate 94010. Many of these garages were built alongside homes from the 1920s through the 1960s, which means slab foundations, outdated electrical, and sometimes no insulation at all. We convert them into conditioned living space, pull the permits, upgrade the foundation as engineering requires, and match the exterior to the main house so the ARB signs off without a fight. A Hillsborough garage conversion typically runs $30,000 to $80,000 depending on the existing structure.
New Detached ADU
A new detached ADU on a Hillsborough hillside lot means grading plans, drainage plans, and often a crane staging plan if the road is one of the narrow private lanes cut for 1920s-era vehicles. We’ve done it. On one upper hillside project, we closed the lane for a single day to pour the foundation, kept two heritage oaks protected through the entire build, and finished with ARB-approved Spanish Colonial stucco and clay tile. Every step was documented in the permit file before inspection. That’s the difference between a compliant build and a stop-work order.
Junior ADU & Interior Conversion
For homeowners who don’t want a separate structure, a junior ADU inside the existing footprint can add a legal rental unit or extended-family suite without touching the property’s massing. In Hillsborough, that often means converting a portion of a large custom home’s lower level or a wing with its own entrance. These projects require careful plumbing and electrical work through thick plaster walls, plus the same ARB review on any exterior changes. We scope everything in writing first.
Second Story Additions
Hillsborough’s mid-century modern homes and Spanish Colonial estates often sit on post-and-pier foundations that were never designed for a second story. Before we add upward, we assess the load path, evaluate the cripple walls, and engineer the lateral bracing the hillside demands. Skipping that assessment is how people end up with unpermitted structural changes and a foundation that moves under the new load. We don’t skip it.
Room Additions
Ground-floor room additions on Hillsborough’s multi-acre lots give us room to work, but they also bring the clay soil and slope drainage issues this town is known for. Expansive Bay Area clay heaves with winter rains, and a room addition that ignores drainage planning is a future foundation problem. We plan grading and drainage as part of the scope, not as an afterthought.
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 Hillsborough
We install Windows & Doors - Hillsborough products from Simonton, Ply Gem, and PGT on our Hillsborough additions and ADUs. These names matter when you’re matching new construction to a 1930s Tudor or Spanish Colonial, because the ARB wants exterior materials to look like they belong. We keep local parts access for warranty work across Hillsborough’s 94010 ZIP, so a window seal or door hardware issue doesn’t sit for weeks waiting on a national parts chain. Mention your preferred brand on the call and we’ll line-item it in the written scope.
Common Home Additions & ADU Problems We See in Hillsborough Homes
- ARB design kickbacks. Homeowners start ADU designs without confirming the Architectural Review Board’s massing and material rules, then watch the design get returned weeks into the schedule. Happens constantly with out-of-town designers who’ve never worked in 94010.
- Slope drainage and expansive clay. Hillsborough’s hillside terrain channels winter rains into foundation and retaining wall problems. A room addition that doesn’t plan grading and drainage up front is a heaving foundation waiting for the next wet season.
- Post-and-pier foundations under second stories. The older estate homes on the upper hillside roads weren’t framed for vertical loads. Adding a second story without a full load-bearing assessment leads to unpermitted structural changes and a foundation that tells you about it later.
- Tree ordinance surprises. The Town’s tree-protection ordinance means a certified arborist report is often required before any grading permit near mature heritage oaks. Contractors who don’t know this stall the job at permit filing.
Pricing for Home Additions & ADU in Hillsborough, CA
Here’s what current Hillsborough market pricing looks like for the work we’re discussing on this page. These ranges reflect the town’s ARB documentation, engineering, and hillside site conditions.
| Project | Typical Hillsborough Range |
|---|---|
| ADU Construction | $90,000-$220,000 |
| Home Addition | $45,000-$130,000 |
| Garage Conversion | $30,000-$80,000 |
What moves the number isn’t a mystery. A second story on a post-and-pier foundation costs more than a ground-floor room addition. A detached ADU on a steep hillside lot with heritage oaks and crane staging runs higher than a flat-lot garage conversion. Every line item goes in writing before work starts, and every change order is on paper before it’s approved. Call (877) 610-1318 and we’ll give you a written, line-item quote for your specific property. Estimates are free.
We Also Serve Cities Near Hillsborough
Hillsborough’s ARB is unique in this stretch of the Peninsula, but we work the same way in the surrounding towns. If your project sits across the border, we serve Home Additions & ADU in Hillsborough and the neighboring communities of Burlingame, Millbrae, San Mateo, and San Bruno, with the same written scopes and the same 90-Day Done Right Promise.
Serving Hillsborough, CA - Our Local Coverage Area
We’re based in the Hillsborough 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 Hillsborough
Plan on an additional 4 to 8 weeks for ARB review on top of standard San Mateo County building permit timelines in Hillsborough. The ARB meets on its own schedule and reviews massing, materials, and landscaping preservation, which means your project won’t get a building permit until the ARB signs off. Call (877) 610-1318 to get your design in front of the board the right way the first time.
Hillsborough’s upper ridgeline lots fall into higher wind-load zones than the flatlands, so structural framing, windows, and our Roofing in Hillsborough details must meet engineered wind-uplift requirements that a flat-lot quote often omits. The specific rating depends on your exact ridge location and exposure, but the engineering is part of every compliant scope we write. Call (877) 610-1318 for a free estimate on your specific lot.
Sometimes yes, sometimes no. Many Hillsborough garages from the 1920s through the 1960s sit on thin slabs with no rebar and no footing depth for living space. The structural engineer’s report decides, and it’s not a corner we cut. If your garage slab won’t carry the load, the scope accounts for it in writing before work begins. Call (877) 610-1318 and we’ll tell you what’s on your property.
A compliant Hillsborough ADU quote includes the ARB application and stamped architect plans, certified arborist reports when heritage oaks are near the work, hillside drainage and grading plans, engineered wind-load details for ridgeline exposure, and the full permit and inspection file to sign-off. A cheap quote skips the ARB, the arborist, and the drainage plan, and you find out at the permit counter. Call (877) 610-1318 for a quote that already accounts for 94010.
Yes. A permitted second-story addition with signed inspections creates a documented file your insurer can review, which is far stronger than an undocumented structural change when a claim arises. If you sell the home, the unpermitted work becomes a disclosure problem. Permits aren’t optional paperwork; they’re the record that your addition was built to code. Call (877) 610-1318 and we’ll pull them on every eligible job.
Tom Barrett built Barrett & Sons Construction in 1998, and he still answers the phone. If you’re planning a Hillsborough home addition or ADU, call (877) 610-1318 for a free estimate and a written, line-item scope. No surprise invoices, no unwritten allowances, and a 90-Day Done Right Promise on every completed job. Ask anyone on your street, or look up the license number yourself.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Hillsborough since 1998.