A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateTrusted Home Addition Contractor for San Francisco Homeowners
Adding square footage to a San Francisco home usually costs between $250 and $450 per square foot for a site-built addition, while a well-specified prefab panel system from a brand like Boxabl or Plant Prefab can bring that down to the $200 to $350 range once foundation, utility hookups, and finishes are factored in. Those are the honest numbers most contractors will not name until you are three meetings deep. Benson ADU San Francisco, led by Troy Eastman, puts the price in writing before any work starts, and backs the build with a 365-Day Done Right Promise. If you are researching a Home Addition Contractor in San Francisco, you are likely weighing a growing family against the cost of moving, and the math usually favors building. Call (424) 328-5746 for a free estimate with a written line-item quote.
Key Takeaways
- San Francisco home additions run roughly $200 to $450 per square foot depending on whether you go prefab, site-built, or a hybrid. Written pricing before work starts is standard here.
- Benson ADU San Francisco builds with brands you can research yourself, including Plant Prefab, Boxabl, and James Hardie cladding, so you are never locked into one take-it-or-leave-it product.
- The 365-Day Done Right Promise covers your addition for a full year. If something is not right, we make it right. In writing, on every project.
- A live person answers at (424) 328-5746, 24/7. No voicemail loops, no phone tag, no waiting until Monday morning.
Sunroom Addition
A sunroom addition is the most cost-efficient way to add usable square footage without the full weight of a traditional room addition: it typically costs $180 to $300 per square foot in San Francisco, uses lighter foundation work, and can often be permitted faster than a full second-story project. For larger projects, a Home Addition Contractor - San Francisco can help you compare options. You consider one when you want daylight, a reading room, a plant space, or a dining nook that feels connected to the yard. Benson ADU San Francisco handles the permit drawings, the glazing selection, and the thermal envelope so the room is comfortable in a foggy Outer Sunset morning and a 75-degree Potrero afternoon alike.
Glass Selection and Thermal Comfort
We specify insulated low-e units and, where the site allows, operable walls that open to a deck. The goal is a room you use ten months a year, not a greenhouse that cooks in August. We will show you the U-factor and solar heat gain coefficient for each glass option in plain English before you commit.
Foundation and Flashing Details
San Francisco slopes mean many sunroom additions sit on a stepped or pier foundation, and bad flashing is the number one call-back we see from other contractors’ work. Our crew details the roof tie-in and sill flashing in the written plan, and the 365-Day Done Right Promise covers any leak that appears in the first year.
Second Story Addition Contractor
A Second Story Addition Contractor builds upward when the lot will not let you build outward, and in San Francisco, a vertical addition typically costs $350 to $500 per square foot because it involves structural engineering, stair relocation, and often a temporary roof. You need one when your lot coverage is maxed, when you want to preserve the backyard, or when the views from a second level will change how your family lives. Benson ADU San Francisco sequences the work to keep the ground floor livable as long as possible, and we explain the structural load path in terms you can follow, from new beams to new footings.
Structural Engineering and Load Paths
Every second-story project starts with an engineer’s letter and a load calculation, not a guess. We show you where the new steel or LVL beams land and why, so you know the house has not just been stretched, but actually strengthened.
Phasing to Keep You in the Home
We can usually keep the first floor occupied through most of the framing, with a temporary weather barrier and a set schedule you see in advance. Before a wall opens, you will know which rooms are affected, for how long, and what the dust-control plan is.
Garage Addition Contractors
Garage Addition Contractors add living space by converting or expanding the garage footprint, and in San Francisco that work typically runs $200 to $350 per square foot depending on whether you are converting within the existing shell or extending it. You go this route when you need an ADU for rental income, an in-law unit, or a workshop without touching the main house. Benson ADU San Francisco builds garage conversions as full living spaces: insulation, ventilation, a proper subfloor, and a heating source, never just drywall over concrete.
Conversion vs. Expansion Trade-Offs
A conversion within the existing garage shell is faster and cheaper because the foundation and roof are already there. An expansion gives you more square footage but triggers new foundation work and a longer permit path. We lay out both numbers side by side so you can choose with your budget, not a sales pitch, in front of you.
Moisture and Ventilation Upgrades
Most San Francisco garages were never built for habitation, which means vapor barriers, under-slab insulation, and mechanical ventilation have to be added, not skipped. We include those line items in writing because we know they are where cheap conversions fail.
Bathroom Addition Contractor
A Bathroom Addition Contractor adds a new full or half bath where one does not exist, and in San Francisco you should budget $25,000 to $60,000 for a modest addition, with the higher end reflecting new plumbing runs, drain line work, and waterproofing. You need this when a growing household is competing for one bathroom, or when an ADU or new bedroom makes the existing bath inadequate. Benson ADU San Francisco handles the DWV plumbing, the wet-area waterproofing, and the ventilation exhaust so the new bath is not just pretty, but dry and code-compliant.
Plumbing Stack and Drain Line Reality
Any plumber can tie in a sink; the skill is in the venting and drain slope that keep a new bath from gurgling or smelling. We map the existing stack and show you exactly where the new lines run before we cut the slab.
Waterproofing Standards
We use a full membrane system on the wet walls, not just cement board and tile, and we flood-test the pan before any finish goes on. It is the difference between a bath that lasts decades and one that rots from the inside.
Bedroom Addition Contractor
A Bedroom Addition Contractor adds a legal sleeping room, and in San Francisco the cost typically lands between $250 and $400 per square foot, driven by egress requirements, closet framing, heating, and exterior wall construction. You pursue one when a new child arrives, when aging parents move in, or when a home office needs to become a guest room. Benson ADU San Francisco builds bedrooms to code for egress and light, which matters for resale and for anyone’s peace of mind about a legal bedroom count.
Egress and Legal Bedroom Requirements
A legal bedroom in California needs a minimum window opening for escape and rescue, a closet, and a heat source. We size the egress window and position it in the plan from day one, so you never end up with a “bonus room” you can not legally call a bedroom when you sell.
Sound and Privacy Details
Between the new bedroom and the rest of the house, we specify solid-core doors, acoustic insulation, and air-sealing. It is the kind of detail that matters when the room is for a baby, a shift worker, or an overnight guest.
Brands We Service
Benson ADU San Francisco is authorized to build with Boxabl, Plant Prefab, and Tuff Shed systems, which means you are not limited to whatever stick-frame crew happens to be available. Boxabl’s folding panel modules can shorten on-site framing time substantially; we handle the foundation set, crane coordination if required, and site utility connections. Plant Prefab’s panelized wall systems arrive factory-cut, which reduces weather exposure and material waste, and we manage the crane or lift logistics on tight San Francisco streets.
For exterior cladding, we specify James Hardie siding and trim because it holds paint longer in salt air and fog than wood, and it does not rot. If you have a particular brand in mind, we can build with it. Whether you choose a prefab system from our brand partners or a fully site-built addition, we can help you get it permitted, built, and done right.
Signs You Need a Pro Right Now
- You are adding a bathroom or bedroom and touching the main sewer or water lines. A botched drain tie-in is expensive to fix and can contaminate the soil under your foundation. A licensed contractor with a written scope prevents the worst of it.
- Your foundation has visible step cracks or you are building up. A second story adds real structural load. If the foundation has already been moving, adding weight without an engineer’s review is a safety issue, not a cosmetic one.
- Your garage slab has moisture wicking through or no vapor barrier. Converting that space into a living area without addressing the moisture path will lead to flooring failures and mold. We see it in Outer Sunset and Excelsior garages regularly.
- You are weighing an ADU or addition on a sloped lot. San Francisco hills mean retaining walls, stepped footings, and drainage plans. A good contractor starts with the geotech and grading, not a glossy rendering.
- You have had a contractor hand you a verbal estimate and disappear. If you cannot get a written line-item price before you sign anything, you are being set up for a change-order negotiation mid-project. That should be the dealbreaker.
Our Process, Step by Step
- 1
Phone consultation and rough budget. You call (424) 328-5746 and a live person answers, day or night. We ask about the lot, the square footage you are picturing, and whether you want prefab or site-built. You get a rough dollar range on that first call.
- 2
Site visit and written scope. Troy Eastman or a project lead walks the property, checks access, the foundation, the panel, and the sewer location. Within days you get a written line-item price, not a verbal ballpark. This is the number we stick to.
- 3
Plans, engineering, and permits. We handle the drawings, the structural calcs, and the permit submittal. You see the plan before we file it, and we explain any redlines the city sends back.
- 4
Build with progress tracking. Every job has a set schedule with photo updates and a live arrival time for the crew. Our technicians are background-checked and in uniform, and the same project lead you met stays on your job.
- 5
Final walkthrough and 365-Day Done Right Promise. We walk the finished addition with you, demonstrate everything, and hand over the written guarantee. If anything is not right within a year, we come back and make it right. Free.
How Much Does Home Addition Contractor Cost in San Francisco?
A typical home addition in San Francisco runs $200 to $450 per square foot, with the final number driven by whether you go prefab or site-built, how much foundation and utility work the lot needs, and the finish level you choose. A compact sunroom might come in around $60,000 to $90,000, while a full second-story addition with a new bath can reach $250,000 to $400,000 or more depending on the view-grade finishing. A garage ADU conversion typically lands in the $120,000 to $180,000 range when it includes a kitchenette and a full bath.
What moves the price most in San Francisco is soil and access. A sloped lot in Noe Valley or Twin Peaks needs retaining and stepped footings that a flat lot in the Marina does not. A narrow street in the Richmond may need a smaller crane or manual material handling, which costs more in labor. The way to avoid overpaying is to get a written line-item quote and compare it against another written quote, not a verbal estimate. Our free estimate includes the full scope, the brand of materials, and the permit fees broken out, so you can see the math before you commit. Call (424) 328-5746 and we will walk you through a real number for your specific lot.
Our Service Area Near San Francisco
Benson ADU San Francisco serves the city and the surrounding Bay Area, including Home Addition Contractor in Berkeley, Home Addition Contractor in Vallejo, plus Piedmont, Burlingame, Castro Valley, Mill Valley, Danville, Albany, Foster City, Lafayette, and Tamalpais-Homestead Valley. Our team dispatches from the city and coordinates with our home office network for scheduling and material delivery. If your project is in San Francisco itself, we can typically schedule a site visit within a few days.
Serving San Francisco, CA - Our Local Coverage Area
We’re based in the San Francisco area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
Frequently Asked Questions About Home Addition Contractor in San Francisco
A Home Addition Contractor - Albany plans, permits, and builds new living space attached to or adjacent to your existing home, from sunrooms and second stories to garage conversions and new bedrooms. In San Francisco, that means managing structural engineering, foundation work, utility tie-ins, and the city’s planning and building code requirements. We handle the entire process from the first written quote through the final inspection.
Most San Francisco additions take 4 to 9 months from first site visit to final inspection, with prefab systems like Boxabl or Plant Prefab sometimes shortening the on-site framing phase by weeks. Permit review at the Department of Building Inspection can add 2 to 4 months depending on the project’s complexity and whether it triggers design review. Our written schedule shows the critical path before we start, so you know which weeks are city waiting time and which are actual construction.
You should budget $200 to $450 per square foot depending on the type of addition, with a small sunroom starting around $60,000 and a full second story potentially exceeding $400,000 with high-end finishes. Prefab and panelized systems tend to land at the lower end of that range, while fully custom site-built work with steel moment frames lands at the top. Call (424) 328-5746 for a written quote on your specific project.
Yes, we are an authorized builder for Boxabl, Plant Prefab, and Tuff Shed systems, and we specify James Hardie cladding as a standard exterior option. Each brand has different lead times and crane or lift requirements, and we will show you the real numbers on each before you choose. If you already have a brand preference, we can work with it.
A live person answers (424) 328-5746 around the clock, every day of the year, and we can often schedule a site visit in San Francisco within a few days. Urgent structural concerns, like a failing foundation under a planned second story, get prioritized for an engineer’s review. For emergencies like storm damage or a leaking roof that threatens an existing addition, call us and we will get someone out.
If any part of your new addition fails within a full year of completion, we come back and make it right at no charge, backed by a written guarantee on every project. That includes leaks, door and window operation, flooring defects, and any workmanship issue tied to our build. If a covered repair fails again, we return for a free second visit.
Start by defining your must-haves versus nice-to-haves, then check your lot’s current square footage and any existing permit history through the San Francisco Property Information Map. Gather your budget ceiling, not your budget floor, so we can show you what is actually buildable at that number. Then call (424) 328-5746 and talk through the project with a live person before anything gets drawn.
Schedule Your Home Addition Contractor Service in San Francisco Today
Get a written line-item price from a Piedmont Home Addition Contractor before you spend another hour guessing. Call Benson ADU San Francisco at (424) 328-5746 and talk to a live person about your project, your lot, and your budget. The estimate is free, the guarantee is in writing, and the number we quote is the number you pay. Call today and get the plan in front of you, not a verbal promise.
Written by Troy Eastman, Owner at Benson ADU San Francisco, serving the Bay Area since 2006.
