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
A home addition or ADU in Hillsborough typically costs $45,000 to $130,000 for a room addition and $90,000 to $220,000 for a new detached ADU, and nearly every project in the 94010 ZIP requires Town of Hillsborough Architectural Review Board approval before a building permit is issued. Barrett & Sons Construction has framed additions and ADUs on Hillsborough’s upper hillside roads where the narrow lanes make crane access a real puzzle and the heritage oaks mean an arborist report lands on the desk before the first grading permit. Call Tom Barrett at (877) 610-1318 and we’ll walk the property with you, talk through what the ARB will ask for, and put every number in writing before you sign anything.
Why Barrett & Sons Construction Is Hillsborough’s Preferred Home Additions & ADU Company
We’re not the biggest name in the Bay Area, and that’s the point. Barrett & Sons has been owner-operated since 1998, and Tom Barrett’s sons run the crews. When we quote a second-story addition on a Tudor Revival estate off Ralston Avenue, the man who signs the contract is the same man you’ll see at the lumber desk in San Mateo on Saturday morning. Hillsborough homeowners check us out before they call: Tom’s name, his license number, and the company address are published and verifiable. Look us up. We want you to.
Hillsborough’s building requirements are different from Burlingame’s or Millbrae’s, and we’ve spent enough time in front of the Town’s Architectural Review Board to know what gets flagged on the first pass: roof pitch, window proportion, the way a new wing sits against the original massing. A contractor who does most of his work two towns over files the wrong permit first and loses three weeks. We file in the right order the first time, because we’ve done it here. Our Hillsborough customers call the office and a live person answers, 24/7. Not a call center. One of Barrett’s people.
Our Home Additions & ADU Services in Hillsborough
New Detached ADU
A detached ADU in Hillsborough runs $90,000 to $220,000, and the biggest cost drivers are foundation work and design review. Most of these projects go onto hillside lots where expansive Bay Area clay demands a geotech report before we pour anything. A compliant quote includes that report, the shear-wall bracing the structural engineer specifies, and architect-stamped plans that clear the ARB’s massing and material standards. Skip any of that and you’ll be pouring a foundation that fails inspection on the first visit. We handle the full scope: design, ARB submittal, permits, utilities, foundation, framing, and the final inspection sign-off.
Second Story Additions
Second-story additions in Hillsborough typically land between $45,000 and $130,000, and on the older estate homes that dominate this town, that number almost always includes seismic work. The original post-and-pier foundations under those 1930s Spanish Colonials were never designed to carry a second floor in a high seismic-hazard zone. We’ve pulled permits on upper hillside roads where the load-bearing changes required full shear-wall retrofitting before the first floor joist went in. The permit inspector works for you, not for me, and on these jobs that’s exactly what you want: someone independent checking our lateral bracing against current code.
ADU Design & Permitting
Hillsborough adds a layer no neighboring city has. The Town of Hillsborough Architectural Review Board reviews every ADU for massing, materials, and landscape preservation before a building permit gets issued, and that’s on top of standard San Mateo County building code. Our ADU design process starts with that ARB submittal in mind, not as an afterthought. We work with architects who have drawn ADUs in Hillsborough before, so the plans that go to the ARB come back approved instead of bounced for a second round with redlines on the roof pitch and window trim. Tom will walk you through the exact submittal path and what it costs in both dollars and weeks.
Junior ADU & Interior Conversion
A junior ADU is the quiet workhorse of Hillsborough’s housing stock: converting an existing bedroom or a basement-level space inside those big mid-century homes on the upper hillside roads, without touching the exterior envelope. That matters here, because the moment you change the exterior, the ARB is involved. A conversion that stays inside the existing walls can sometimes move through with far less design-review friction. We’ll tell you honestly which route your floor plan supports, and we’ll put the answer in writing. Prices for interior conversions fall under the broader home addition range of $45,000 to $130,000 depending on bathroom scope, kitchenette work, and structural headers.
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 premium brands we can stand behind when a Hillsborough homeowner asks a hard question at the hardware store. On ADU and addition jobs that means Simonton windows for their wind-load ratings and PGT for impact-rated glazing, because Hillsborough’s ridgeline exposures and the Town’s design standards both ask more of a window than a flat-lot subdivision home ever would. We don’t stock mystery materials off a van. Your contract names the brands, the models, and the rating each product carries, so when insurance asks what the new windows were rated to, you have an answer on paper.
Common Home Additions & ADU Problems We See in Hillsborough Homes
- Wind-load and impact-rated glazing requirements: Hillsborough sits on exposed ridgeline slopes where the Town enforces wind-load ratings more strictly than many neighboring cities. A second-story addition or ADU that specs builder-grade windows fails the plan check, and switching products mid-job means a change order you didn’t budget for. We quote Simonton or PGT from the start, with the rating written into the contract.
- Expansive clay and hillside drainage: The clay under much of Hillsborough swells in winter rains and shrinks in summer drought, which is why a second-story addition without a geotech report is a cracked subfloor waiting for its second rainy season. Any site disturbance here needs grading and drainage planning, or the addition settles unevenly against the original house.
- Unreinforced foundations on pre-1960 estates: The Tudors and Spanish Colonials on multi-acre lots were built before seismic codes existed in any meaningful form. Adding a second story means retrofitting the original cripple-wall and post-and-pier foundation with plywood shear panels, and skipping that step fails the structural inspection outright.
- Heritage oak tree-protection conflicts: A certified arborist report is often required before any grading permit is issued near mature oaks, and Hillsborough enforces this more aggressively than most Peninsula cities. We’ve had projects where the addition footprint moved four feet to protect a root zone, and that adjustment happened on paper before any dirt turned.
Pricing for Home Additions & ADU in Hillsborough, CA
Here’s what Hillsborough homeowners should expect to see on a written line-item quote from us, not a number thrown out over the phone. These ranges reflect the town’s ARB review process, seismic retrofit work, and the load-bearing engineering that older estates require.
| Service | Typical Hillsborough Range |
|---|---|
| New Detached ADU | $90,000 - $220,000 |
| Second Story Addition | $45,000 - $130,000 |
| ADU Design & Permitting | Included with full-scope contract |
What moves the number on a Hillsborough addition: the amount of seismic retrofit the original foundation needs, whether the site requires a geotech report and grading plan for expansive clay, and how much architectural work the ARB’s massing review pushes back on. A compliant quote includes the engineering stamps, the arborist report when heritage oaks are within the disturbance zone, and written allowances for every material line. Call (877) 610-1318 for an exact quote. Estimates are free, and the number you hear is the number we honor in writing.
We Also Serve Cities Near Hillsborough
While this page covers Home Additions & ADU in Hillsborough, our crews also work regularly in Burlingame, Millbrae, San Mateo, and San Bruno. Each town has its own planning quirks; Hillsborough’s ARB is its own animal entirely. If you’re planning an addition or ADU anywhere on the Peninsula, the same written change orders and the same 90-Day Done Right Promise apply. Our Home Additions & ADU team is ready to walk your property and give you the real numbers.
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
Yes, the Town of Hillsborough Architectural Review Board reviews every ADU, addition, and substantial remodel in the 94010 ZIP for massing, materials, and landscape preservation before a building permit is issued, and this is an extra layer that neighboring Burlingame or San Mateo do not have. Hillsborough is one of the only fully incorporated cities in California with zero commercial zoning, meaning every structure is a private residence and every project is judged against residential design standards. Plan for an added two to six weeks in your timeline and architect-stamped plans. Call (877) 610-1318 and Tom will walk you through the exact submittal path.
Hillsborough’s upper hillside exposures sit in a wind zone that requires engineered wind-load ratings on windows, roof connections, and wall sheathing, and a second-story addition that specs unrated builder-grade windows will fail plan check. You need products with documented ratings: Simonton and PGT both carry the ratings we specify on Hillsborough ridge work. The rating goes into the contract, and the inspection sign-off documents it. Call (877) 610-1318 for a quote that includes compliant windows on the first pass, not a change order later.
A compliant Hillsborough ADU quote includes the geotech report for expansive clay, the shear-wall engineering for seismic retrofitting, the wind-load-rated windows, the arborist report when heritage oaks sit near the footprint, and the ARB design review submittal package. A cheap quote leaves all of those out, then calls you from the jobsite with a change order when the foundation fails inspection or the plans get bounced back with redlines. Every Barrett & Sons quote comes as a written line item, and every price change arrives as a written change order before work proceeds.
Yes, and this matters more than most homeowners expect. When you carry documentation that your addition’s windows, roof connections, and shear panels were installed to Hillsborough’s wind-load ratings and signed off by the Town’s inspector, an insurance adjuster can verify the claim faster and with less pushback. We provide that documentation as part of every completed job, because the permit inspector’s sign-off is your best evidence that the work was done right. It’s one of the reasons Tom publishes his license number and wants you to look it up before you sign.
Yes, a garage conversion ADU in Hillsborough still requires Town of Hillsborough permits and, because it changes the exterior envelope of a structure, it typically must clear ARB design review as well. The conversion involves new foundation work at the garage slab, electrical and plumbing rough-ins, and insulation to code, all of which the Town inspects. Prices for garage conversions fall between $30,000 and $80,000, and the full design and permitting path applies. Call (877) 610-1318 and we’ll tell you exactly what your garage floor plan can support and what the Town will ask for.
If you’re planning an addition or ADU in Hillsborough, start with a conversation, not a contract. Call Tom Barrett at (877) 610-1318 to schedule your free estimate. We’ll walk the property, talk through the ARB path, and put every number in writing before a single nail moves. And every job we complete carries the written 90-Day Done Right Promise: if it’s not done right, one call brings Tom back.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Hillsborough and the San Francisco Bay Area since 1998.