A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateFast, Reliable ADU Builders Across Half Moon Bay
Adding an ADU in Half Moon Bay usually costs $275 – $450 per square foot for a site-built unit, and a prefab studio from a brand like Abodu or Tuff Shed typically lands between $180,000 and $320,000 all-in. That’s the honest range before anyone quotes you. The bigger variable here isn’t square footage. It’s salt air, the Coastal Act, and whether your existing garage slab can support a conversion. Our ADU Builders team works in Half Moon Bay weekly, and we’ve learned what fails here. Call (424) 328-5746 to get a written price for your property.
Half Moon Bay isn’t like building in Burlingame or Castro Valley. The fog off the Pacific settles on every fastener, window track, and sill plate. A standard galvanized hanger that lasts forty years inland will rust through in three winters out here. Before we touch a single wall, we inspect what’s already standing. That inspection changes the quote, the materials list, and how long your unit survives.
Why Benson ADU San Francisco Is Half Moon Bay’s Preferred ADU Builders Company
Half Moon Bay homeowners keep calling us for one reason: we put the coastal spec in writing before anyone swings a hammer. Most contractors quote a generic ADU price and then discover the slab is spalled or the garage was never engineered for living space. We start by sounding the concrete and checking for salt corrosion on the existing anchors. That’s not extra work. That’s the first ten minutes of the site visit.
Our crew has built and converted ADUs from the flats near Main Street up through the hillside lots west of Highway 1. We know which neighborhoods sit inside the coastal zone and which don’t, and that single fact changes your permitting path more than any design choice. Troy Eastman, who leads our team, insists every Half Moon Bay estimate includes a line item for coastal-grade hardware. If you’ve had a contractor tell you standard galvanized is “fine for this area,” get a second opinion.
Every project gets The Relay Visibility Guarantee: a written price before work starts, real-time tracking, and a 365-Day Done Right Promise. If something fails within a year, we return and fix it. Free. That guarantee is stamped on every Half Moon Bay contract we write.
ADU Plans in Half Moon Bay
ADU Plans in Half Moon Bay are the first thing we produce on nearly every project, because the Coastal Act doesn’t care what you want to build. It cares what already exists. Most homeowners assume they’ll add a detached unit in the backyard. Then we check the parcel and find an old garage or pump house is nonconforming, and the Commission will not allow a net increase in nonconforming square footage. The plan shifts to a conversion within the existing footprint. That’s the plan that gets approved.
Coastal Zone Plan Review
If your parcel sits west of Highway 1, your ADU Plans almost certainly require coastal development review. We prepare plans that account for view-shed restrictions, height caps, and the no-net-increase rule. A well-prepared plan shows the reviewer you understand the constraints. A rushed plan gets bounced back and costs you four to eight months.
Septic and Lot Coverage Planning
Many Half Moon Bay lots east of the highway run on septic, which limits how much impervious surface you can add. We map existing structures, calculate new lot coverage, and design the ADU so the system isn’t overloaded. If a JADU inside the main house works, that can bypass septic expansion entirely. We’ll tell you if it does.
ADU Contractor in Half Moon Bay
An ADU Contractor in Half Moon Bay has to understand concrete failure before they understand framing. The salt air here doesn’t just rust the screws. It gets into porous concrete and corrodes rebar from the inside out, which is why so many old garages near the water have spalled slabs. You can’t anchor new walls to crumbling concrete and expect the unit to settle evenly. We inspect first, then quote.
Garage Conversion Assessment
Before we quote a garage conversion, we sound the slab with a hammer and probe the sill plates for rot. If the concrete spalls, we spec a repair with epoxy-coated reinforcement and hot-dip galvanized anchors. That adds $8,000 – $18,000 to a typical conversion, and it’s the difference between a unit that lasts thirty years and one that shifts in the first winter storm.
Marine-Grade Material Spec
We specify stainless-steel flashing, marine-grade window frames, and sealed paver joints on every coastal ADU. Interior-grade vinyl windows with weep holes clog within two years out here. The water backs up, finds the sill, and you get mold inside a brand-new unit. The marine-spec version costs more up front. The cheap version costs more by year ten.
Trusted Brands We Service in Half Moon Bay
We’re authorized to build with Plant Prefab, Abodu, and Tuff Shed, and we use James Hardie cladding on site-built units because it holds up to fog and salt better than wood or stucco. For Half Moon Bay customers, that means you get curated options instead of whatever the contractor feels like using. If you want a Plant Prefab unit that arrives as panels and assembles in weeks, we handle the site work and foundation. If you prefer a Tuff Shed studio converted to living space, we manage the envelope upgrade, electrical, and finish. We keep the spec sheets for these brands on hand so you can verify performance claims yourself.
Common ADU Builders Problems We See in Half Moon Bay Homes
- Rusted standard-galvanized hardware: Fasteners through siding panels corrode within three winters. Panels loosen in the first big storm, and you’re paying for rework before the unit is even rented.
- Spalled concrete under garage sill plates: Decades of salt spray degrade the slab, especially in converted pump houses and boat storage in the western neighborhoods. New walls anchored to crumbling concrete settle unevenly and crack drywall within a year.
- Mold from clogged window weep holes: Inland-spec vinyl windows trap wind-driven salt and moisture. The result is water intrusion, mold inside the ADU, and a tear-out of a brand-new window installation.
- Uninsulated garage envelope conversions: Older garages in Half Moon Bay were built for cars, not people. Without a full envelope upgrade, you’ll get cold drafts, condensation, and mold in corners the first winter.
Pricing for ADU Builders in Half Moon Bay, CA
A detached site-built ADU in Half Moon Bay runs $275 – $400 per square foot, which puts most one-bedroom units between $250,000 and $420,000. A prefab studio from Abodu or Plant Prefab typically totals $180,000 – $280,000 including site work and permits. A garage conversion within an existing footprint costs $150,000 – $260,000 depending on slab condition and how much of the original structure can be saved. The coastal-spec version of the same job costs 10 – 18 percent more than the inland-spec version. That’s the cost of stainless hardware, marine-grade windows, and sealed joints. The cheap spec costs more within a decade. We write the price before work starts and we don’t change it. Call (424) 328-5746 for a free estimate on your property.
We Also Serve Cities Near Half Moon Bay
We build ADUs across the coastside and up the Peninsula, including El Granada, Burlingame, San Francisco, and Castro Valley. If you’re in Half Moon Bay and wondering whether your parcel falls inside the coastal zone, the same team handles it. See our ADU Builders in Half Moon Bay page for local specifics or call directly for a site visit.
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 About ADU Builders in Half Moon Bay
It means you can’t add a new detached unit on a parcel where an existing structure is already nonconforming, unless the ADU fits within that structure’s existing footprint. In practice, that usually means converting a garage or pump house instead of building new in the backyard. Our team checks your parcel status before you spend money on design. Call (424) 328-5746 and we’ll pull the coastal commission records for your lot.
Fastener corrosion. Standard galvanized roofing screws rust through within three winters, and flashing joints fail where wind-driven salt gets behind the edge. We spec stainless-steel or hot-dip galvanized fasteners and stainless flashing on every coastal ADU. The material cost is a few hundred dollars more, and it prevents a re-roof within a decade.
Not always. If the spalling is shallow and the rebar hasn’t lost significant cross-section, we can repair the slab with epoxy-coated reinforcement and a new concrete overlay. If the rebar is deeply corroded and the slab is cracked through, a partial or full replacement is the honest fix. We assess this before quoting so the price doesn’t change mid-project.
Marine-grade window frames use non-corroding hardware, sealed weep systems that drain without clogging, and glass coatings that resist salt haze. Interior-grade vinyl windows clog, trap moisture, and fail within two years in Half Moon Bay. The coastal-spec version costs 15 – 25 percent more up front and lasts three to four times longer. Call (424) 328-5746 for an exact quote on your window package.
We’re authorized to build with Plant Prefab, Abodu, and Tuff Shed. All three hold up well in Half Moon Bay when the site work and foundation are done correctly, which means stainless anchors, proper drainage, and a sealed envelope. The unit itself is factory-built to tight tolerances. The failure points are the site connections, and that’s what we control.
Written by Troy Eastman, Owner at Benson ADU San Francisco, serving Half Moon Bay and the surrounding coastside.
Ready to price your ADU? Call (424) 328-5746 for a free, written estimate. We’ll inspect your slab, check your coastal zone status, and give you a real number before you commit to anything.
