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 Half Moon Bay
A room addition or ADU in Half Moon Bay typically runs $45,000 to $130,000 for an addition and $90,000 to $220,000 for a new detached ADU. And before you ask: yes, almost every one of these projects needs a Coastal Development Permit from the California Coastal Commission, not just a city permit. That’s the difference between a contractor who has done this work on the coast and one who learned about it the hard way.
Barrett & Sons Construction has been building on the San Mateo County coast long enough to know what happens when salt air meets wood framing, and what happens when a project gets frozen mid-construction because someone skipped the coastal review. We’re owner-operated, Tom Barrett and his sons run the crews, and every permit gets pulled before a single nail moves. If you’re planning a room addition, garage conversion, or detached ADU in Half Moon Bay, call (877) 610-1318 and we’ll walk you through what the Coastal Commission will actually require.
Why Barrett & Sons Construction Is Half Moon Bay’s Preferred Home Additions & ADU Company
This is not a franchise dispatching strangers to your door. Tom Barrett lives twenty minutes away, and his sons are on-site most days. When a Half Moon Bay homeowner calls with a question about their addition, the person who answers has likely been inside their walls. That matters more here than it does inland, because the person who built your addition needs to understand Coastal Commission requirements, not just building code.
Every Barrett & Sons job includes written change orders before any scope changes, permits pulled and inspections signed on every eligible project, and a 90-Day Done Right Promise in writing. We’ve had Half Moon Bay customers pull our license number and check it before signing. We encourage that. Tom publishes his name, address, and license number, and a live person answers the phone 24 hours a day.
Our Home Additions & ADU team has done work from the Old Town cottages near Main Street to the ranch homes off Higgins Canyon Road and the newer builds near Miramar. When we quote a project in the 94019 zip code, we scope for the coastal review, the salt-air corrosion, and the fog drip that inland contractors never think about.
Our Home Additions & ADU Services in Half Moon Bay
ADU Design & Permitting
The design phase on a Half Moon Bay ADU is where we earn our keep. Almost every ADU in Half Moon Bay falls within the Coastal Commission’s coastal zone, which means the design has to clear Coastal Development Permit review on top of standard city permits. We handle the full permit package: site plans, coastal setback analysis, environmental review documentation, and the dozen little forms the Coastal Commission’s San Francisco District office wants to see. If you’ve heard stories about ADU projects near the coast stalling for months, it’s usually because the designer didn’t know what the Coastal Commission flags first.
New Detached ADU
A detached ADU on a Half Moon Bay lot runs $90,000 to $220,000, but the cost depends heavily on what your parcel actually allows. On larger acreage properties off Higgins Canyon Road or up Highway 92, we often deal with septic and well setbacks that dictate where the ADU can sit, not where you’d prefer it. A detached ADU built to withstand salt-laden fog requires stainless or hot-dipped fasteners, upgraded flashing, and a vapor barrier system that would be overkill in Hillsborough but is standard practice here.
Garage Conversion ADU
Converting an attached garage in Half Moon Bay usually runs $30,000 to $80,000. The catch: even an interior conversion that doesn’t expand the footprint can trigger Coastal Development Permit review if it changes exterior square footage or alters a nonconforming structure. We parse the California Coastal Act exempt categories before we scope any garage conversion, because a conversion that gets flagged mid-project is a conversion that sits idle while the Coastal Commission’s review clock runs.
Junior ADU & Interior Conversion
Junior ADUs inside an existing home are the fastest path to a rental unit on a Half Moon Bay property, typically $45,000 to $130,000 depending on scope. But don’t assume interior work escapes coastal review. The moment a conversion touches exterior square footage, you’re back into Coastal Development Permit territory. We handle the analysis upfront so you know what you’re dealing with before you spend a dollar on design.
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 Half Moon Bay
We install materials you can look up, not mystery brands off a van. For hardscape and exterior surfaces on Half Moon Bay additions, we regularly specify and install Techo-Bloc and Cambridge Pavers, and use Quikrete for foundation and concrete work where it suits the coastal conditions. Our crew keeps local stock runs to Half Moon Bay suppliers, but the reality of building here is that most long-haul material deliveries come over Highway 92, which means lead times and prices run higher than inland bids.
Common Home Additions & ADU Problems We See in Half Moon Bay Homes
- Coastal Development Permit surprises. Out-of-area GCs commonly quote a Half Moon Bay addition without the coastal review baked in, then discover after demolition begins that the Coastal Commission needs months to process the application. We scope the permit path before we quote, so your schedule reflects the actual review timeline.
- Salt-air rot in existing framing. Decades of marine layer fog and salt exposure eats sill plates, sheathing, and exterior trim in Half Moon Bay’s older homes. Once walls open, remediation is standard practice, not an exception. We budget for it, and our written change orders cover what we find behind the drywall.
- Fog drip and moisture damage. Half Moon Bay’s coastal cypress and eucalyptus belt keeps rooflines and foundation perimeters wet even without rain. New additions need aggressive vapor barrier and moisture management details, and skipping them means premature rot inside five years.
- Underestimated material and hauling costs. There’s no big-box lumberyard in Half Moon Bay. Material deliveries come over Highway 92, and long hauls push delivery prices and lead times beyond what inland allowance math predicts.
Pricing for Home Additions & ADU in Half Moon Bay, CA
Here’s honest money talk. A new detached ADU in Half Moon Bay runs $90,000 to $220,000. A garage conversion ADU runs $30,000 to $80,000. A home addition runs $45,000 to $130,000. These are all-in numbers for a properly permitted project with coastal review, written change orders, and materials suited to the salt air.
| Project Type | Typical Range in Half Moon Bay |
|---|---|
| New Detached ADU | $90,000-$220,000 |
| Garage Conversion ADU | $30,000-$80,000 |
| Home Addition | $45,000-$130,000 |
What drives the price up here more than inland: Coastal Development Permit fees and review time, seismic retrofitting on older foundations, salt-air material upgrades, septic and well setback compliance, and grading and drainage work on open lots. We price all line items in writing before we start, and any change gets a written change order before work proceeds.
Call (877) 610-1318 for a free estimate. We answer live, and there’s no trip charge inside the county.
We Also Serve Cities Near Half Moon Bay
Our crews regularly work in El Granada, Hillsborough, Burlingame, and Millbrae. If you’re in one of those cities and weighing a coastal-adjacent addition, our Home Additions & ADU in Half Moon Bay team can help you understand whether coastal review will reach your property. Same owner, same crews, same written-change-order guarantee.
Serving Half Moon Bay, CA - Our Local Coverage Area
We’re based in the Half Moon Bay 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 Half Moon Bay
Yes, it can. The California Coastal Act’s exempt categories are narrow, and if an interior conversion incidentally changes exterior square footage or alters a nonconforming structure, the Coastal Commission can require full Coastal Development Permit review. We parse the exempt categories before quoting any project in Half Moon Bay, and we’ve seen out-of-area GCs learn this rule only after demolition started. Call (877) 610-1318 if you’re unsure whether your conversion will trigger coastal review. We’ll tell you straight.
Septic and well setbacks dictate the ADU’s location before any design work happens. On larger Half Moon Bay parcels, the leach field, septic tank, and well each carry specific setback distances that limit where a detached ADU can sit. We measure and map those setbacks during the initial site visit, because a plan that ignores them is a plan the county will reject or the Coastal Commission will bounce back. Call (877) 610-1318 for an on-site look at your property.
Open lots in Half Moon Bay typically drain poorly because the persistent marine layer keeps the soil saturated and the coastal fog prevents evaporation. Adding square footage changes the site’s drainage pattern, and the Coastal Commission will want to see a stormwater plan that handles runoff without eroding the surrounding acreage. We scope grading and drainage before quoting, because cutting a corner here leads to foundation moisture and a red-tagged inspection. Call (877) 610-1318 and we’ll walk your site before you commit.
Only if standard hardware is used. Half Moon Bay’s salt-laden marine layer corrodes galvanized fasteners and standard flashing much faster than inland Bay Area cities. We spec stainless or hot-dipped fasteners and upgraded flashing on every coastal addition we build, and the vapor barrier details get designed for fog drip as well as rain. On a detached ADU off Higgins Canyon Road, we opened the walls of a 1960s ranch and found rotted sill plates from decades of salt-laden fog drip from the coastal cypress belt. The fix added a week to the schedule, but it prevented a future punch list disaster. Call (877) 610-1318 to discuss materials that survive this climate.
We front-load the design and permit fees and stage construction draws to match actual milestones, not calendar dates. A Half Moon Bay addition can sit in Coastal Development Permit review for months, and the last thing you want is a contractor demanding a large draw on work that hasn’t started. Our draw schedule is written into the contract and tied to inspections passed, not invoices sent. That’s the only way to handle a long coastal review without burning a client’s cash flow. Call (877) 610-1318 for a copy of how we structure payments on a coastal project.
If you’re already researching additions and ADUs, you’re ahead of the curve. The next step is a free estimate with a contractor who has pulled permits in Half Moon Bay and knows what the Coastal Commission will require. Call (877) 610-1318 and ask for Tom. He answers his own phone.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Half Moon Bay.