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 $180,000 and $320,000 for a detached ADU, while attached room additions run $120,000 to $250,000 depending on scope and site conditions. Sutton ADU San Francisco has handled these projects across the city since 2006, from Sunset District garage conversions to Bernal Heights second-story additions, and we follow our Home Addition Contractor Maintenance Checklist for San Francisco Homeowners on every build. We send you the name and photo of every crew member before they arrive, we hand you a written price before work starts, and we back the finished build with a 365-Day Done Right Promise in writing. Call (831) 272-7212 to talk through your project.
Miles Hartwell leads our San Francisco team, and the standard he set has not changed since day one: no unknown faces at your door, no vague estimates, no surprise invoices. You get a fixed line-item price, a background-checked crew that shows photo ID at the door, and a guarantee that actually says what it covers in plain English.
Key Takeaways
- Written, fixed price before any work begins, on every project, no exceptions
- Every technician is background-checked, identity-verified, uniformed, and photo-badged; name and photo sent to you before arrival
- 365-Day Done Right Promise covers workmanship for a full calendar year from project completion
- Founded 2006, part of the Threshold Group family operating under Threshold Verified standards
- Authorized experience with Abodu, Cover, Dvele, Mighty Buildings, Boxabl, Plant Prefab, Tuff Shed, and James Hardie systems
Sunroom Addition
A Sunroom Addition is a glass-heavy room attached to your home’s exterior wall, built on a foundation and framed like a standard room but with large windows or glazed panels that let in natural light while giving you usable living space year-round. Homeowners in San Francisco often choose a sunroom addition when they want a bright reading room, a plant-filled sitting area, or a flexible space that feels like part of the yard without sitting out in the fog and wind. A sunroom addition in San Francisco typically runs $85,000 to $180,000 depending on glazing type, foundation work, and whether you want heating run to the space.
Framing and Foundation Options
We build sunrooms on concrete slab foundations or elevated piers, depending on your lot’s slope and drainage. San Francisco hillside properties in areas like Noe Valley and Twin Peaks often need pier foundations, which add cost but prevent the settling cracks you see in cheaper sunroom kits.
Glazing and Climate Control
Low-E double-pane glass is standard on our sunroom additions because it keeps heat in during the foggy months and blocks UV damage to furniture and flooring. We can also run a mini-split heat pump so the room stays usable during cold Sunset mornings and warm Mission afternoons.
Second Story Addition Contractor
A Second Story Addition Contractor is a general contractor who specializes in adding a full upper floor to an existing single-story home, which means removing the roof, reinforcing or replacing the foundation, and building new walls, floor systems, and roofing above your current living space. Homeowners in San Francisco look into second-story additions when their lot is too small for a ground-level extension but the neighborhood zoning allows extra height. A second-story addition in San Francisco typically runs $250,000 to $450,000 when you include structural engineering, foundation upgrades, and full interior finish work.
Structural Engineering Requirements
Your existing foundation and first-floor framing have to be evaluated by a licensed structural engineer before we touch the roofline. In neighborhoods like the Richmond District with older 1940s construction, this often means adding steel moment frames or upgrading the foundation footings, which is required for permitting and protects your home during a seismic event.
Seismic and Code Compliance
San Francisco has some of the strictest seismic requirements in the country, and second-story additions trigger a full structural review. We pull all permits, work with the engineer on the shear wall and lateral bracing design, and schedule inspections with the city before framing is closed in.
Garage Addition Contractors
Garage Addition Contractors are builders who convert or expand an attached or detached garage into livable square footage, often keeping a smaller garage bay for parking while adding a bedroom, bathroom, or studio apartment to the remaining footprint. Homeowners in San Francisco choose garage additions when they need an extra bedroom or rental income but do not want to give up their entire parking space. A garage addition in San Francisco typically runs $100,000 to $220,000 depending on whether you are converting existing space or adding a new garage and living area from scratch.
Conversion vs. New Construction
If your existing garage has a concrete slab and framed walls, a conversion is usually the fastest and most cost-effective option. If you are adding a completely new garage with living space above it, you are looking at full foundation, framing, electrical, and plumbing work, which pushes the project toward the higher end of the range.
Moisture Control and Insulation
Garages in San Francisco’s coastal fog zones need proper vapor barriers and closed-cell insulation to prevent mold and dampness in the converted living area. We install perimeter drainage and seal the slab before framing, which is a step many low-bid contractors skip and regret later.
Bathroom Addition Contractor
A Bathroom Addition Contractor is a builder who adds a new full or half bathroom to your home, handling the plumbing rough-in, waterproofing, fixture installation, and finishing work as one coordinated project. Homeowners in San Francisco need bathroom additions when a growing family is sharing one bath, when aging parents move in, or when a primary bedroom lacks an en-suite. A bathroom addition in San Francisco typically runs $45,000 to $95,000 when the new bathroom is placed near existing plumbing lines; costs rise to $110,000 or more if significant drain line work or a new stack is needed.
Tying Into Existing Plumbing
Placing the new bathroom adjacent to an existing kitchen or bath stack keeps costs down because the drain and vent lines are already close. If your addition is on the far side of the house, we run new drain lines under the slab or through the crawlspace, which adds labor and material but gives you full placement flexibility.
Waterproofing Standards
Every bathroom addition includes full shower pan waterproofing with a membrane system, not just tile over cement board. This is where cheap work fails within a few years; our 365-Day Done Right Promise covers the waterproofing and all workmanship for a full year after completion.
Bedroom Addition Contractor
A Bedroom Addition Contractor is a builder who adds one or more new bedrooms to your home, either as a ground-level extension, a second-story room, or a converted existing space like a garage or attic. Homeowners in San Francisco add bedrooms when they are expecting a child, moving in an aging relative, or need a dedicated home office that can count as a legal bedroom for resale value. A bedroom addition in San Francisco typically runs $70,000 to $150,000 for a single attached bedroom, with costs varying by foundation type, egress requirements, and whether a closet is built in.
Egress and Ventilation Requirements
A legal bedroom in San Francisco must have a window large enough to serve as an emergency exit under city code. We size and place egress windows correctly the first time, so you do not end up with a finished room that fails inspection or cannot be listed as a bedroom when you sell.
Closet and Finish Details
Every bedroom addition includes a closet with proper depth and door hardware, plus insulation and drywall finished to match the rest of your home. We keep the trim profile, door style, and paint consistent with your existing rooms so the addition does not look like an afterthought.
Brands We Service
We have direct experience with prefab and modular systems from Boxabl and Plant Prefab, two brands that approach San Francisco lots differently. Boxabl’s folding modules work well on tight urban lots where a traditional build would struggle with crane access or setback restrictions (the required gap between your new structure and the property line), and we also work as a Brisbane Home Addition Contractor for similar compact sites. Plant Prefab’s panelized system suits larger additions and second-story work where you want factory-precision walls delivered and assembled on site.
We also work with Tuff Shed structures as starting points for ADU conversions and James Hardie fiber cement siding for exterior cladding that holds up against San Francisco’s salt air and fog. Whether you have one of these systems or any other make, we can handle the foundation, site work, and interior finish. If you are still deciding between brands, our team can walk you through the cost and timeline differences honestly.
Signs You Need a Pro Right Now
- You are planning to pull permits for a major addition. San Francisco’s Department of Building Inspection requires stamped structural drawings for most additions, and errors in the plan set delay the project by months. A contractor who has built in your specific neighborhood knows what the plan reviewers will flag before you submit.
- Your existing foundation shows cracks or settling. Adding weight on top of a failing foundation turns a $200,000 addition into a $400,000 structural problem. We inspect the existing slab or pier condition during the estimate and tell you what the foundation actually needs before you commit.
- You need a second story but your home was built before 1980. Older San Francisco homes often lack the shear wall and bolting required for a vertical addition under current code. Retrofitting first is mandatory and skipping it is not an option if you want to pass inspection.
- You want an ADU but your lot has a slope or drainage issue. Hillside lots in neighborhoods like Potrero Hill and Diamond Heights need engineered retaining walls and drainage plans before the ADU slab goes in. Guessing at this stage causes water intrusion and cracked foundations down the line.
- You have hired contractors before and felt uneasy about who was in your home. If you have ever looked out your window and not recognized the person walking up your driveway, you know the exact feeling we eliminate. Every Sutton crew member arrives in a clean uniform with a photo ID badge you have already seen in advance.
Our Process, Step by Step
- 1
Free on-site consultation and written estimate. We walk your property, measure the lot and existing structure, and discuss your goals. Within a few days, you receive a written line-item price, not a ballpark figure. The price is fixed; if anything changes, we tell you before the work happens and get your sign-off.
- 2
Permit drawings and city submittal. Our design team prepares the architectural and structural drawings, and we submit the full package to the San Francisco Department of Building Inspection. We attend all plan review meetings and resubmit corrections until your permit is issued.
- 3
Site preparation and foundation work. We mark the property lines (verifying the required setback from each boundary), excavate as needed, and pour the foundation or install the pier system. Every crew member on site has been background-checked and identity-verified under The Threshold Standard, and you know who is coming before they arrive.
- 4
Framing, rough-in, and inspections. We frame the addition, run electrical and plumbing, and call for the required city inspections at each stage. You are welcome to attend any inspection and ask questions. We do not close up walls until the city inspector signs off.
- 5
Finish work and final walkthrough. Drywall, paint, flooring, trim, and fixtures go in, then we do a final walkthrough with you to build a punch list. We do not consider the job done until you have walked every room and signed off. Then your 365-Day Done Right Promise starts.
How Much Does Home Addition Contractor Cost in San Francisco?
A typical attached room addition in San Francisco runs $120,000 to $250,000, while a detached ADU in the 400 to 600 square foot range runs $180,000 to $320,000 before site work. Bathroom additions run $45,000 to $95,000, bedroom additions run $70,000 to $150,000, sunroom additions run $85,000 to $180,000, and second-story additions run $250,000 to $450,000. Site work, including foundation upgrades, retaining walls, and drainage, can add $20,000 to $60,000 depending on your lot.
What affects the price most is whether your existing foundation can support the new load, how far the new plumbing and electrical runs must travel, and whether your addition triggers a seismic retrofit. Finishes also swing the number: a basic bathroom with fiberglass surround costs far less than a custom tile shower with a glass enclosure.
To avoid overpaying, get a written line-item price before signing anything. A fixed price prevents the slow creep of change orders that happens when an estimate was vague from the start. Our free estimate includes a full written breakdown so you can see exactly what each phase costs. Call (831) 272-7212 to schedule yours.
Our Service Area Near San Francisco
Sutton ADU San Francisco serves homeowners across the city and the surrounding Bay Area, including home addition projects in Redwood City, room additions in San Mateo, Walnut Creek, Alameda, South San Francisco, San Pablo, Novato, Pleasant Hill, Pacifica, Benicia, and San Anselmo. We schedule on-site estimates across this entire footprint, and because our home remodeling team is local, we can usually get to your property within a week of your call.
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 - San Francisco plans, permits, and builds new livable square footage attached to or detached from your existing home. That includes foundation work, framing, roofing, electrical, plumbing, insulation, drywall, and finish work. Sutton ADU San Francisco handles everything from the first measurement to the final inspection. Call (831) 272-7212 to talk through your project scope.
Most attached room additions take 4 to 7 months from the first consultation to final inspection, with permitting accounting for 2 to 4 months of that timeline. Detached ADUs typically run 6 to 10 months depending on site work and utility connections. Every project gets a written schedule before work starts, and we update you if city inspections shift the dates. Call (831) 272-7212 for a timeline specific to your project.
Room additions run $120,000 to $250,000, ADUs run $180,000 to $320,000, and second-story additions run $250,000 to $450,000 for the full project with finishes. Your exact price depends on your lot, foundation, and finish choices. We provide a written line-item price before work begins, no exceptions. Call (831) 272-7212 for a free written estimate.
Yes, we install James Hardie fiber cement siding on additions where the existing home already uses it or where the homeowner wants a fire-resistant, low-maintenance exterior. Hardie plank and panel systems hold up well in San Francisco’s coastal climate and come with the manufacturer’s material warranty. We can also match existing siding profiles so the addition blends with the original structure.
If something goes wrong mid-project, a tarp comes off, or a pipe leaks before the addition is finished, call us immediately and we will send a crew member to address it. Our 365-Day Done Right Promise covers completed work for a full year, and if any element fails, a second visit is free, no haggling, no fee. Call (831) 272-7212 and a live person answers 24 hours a day.
The 365-Day Done Right Promise covers defects in workmanship for one full calendar year from project completion. If anything we built is not right, we come back and make it right at no charge. It is a written guarantee stamped on every project, not a verbal assurance. That is The Threshold Standard, and it applies to every addition we build in San Francisco.
Clear the area around the planned addition, move vehicles away from the work zone, and secure any pets during work hours. We send you the name and photo of every crew member before they arrive, so you know exactly who will be walking onto your property. If you have specific access concerns, tell us during the estimate and we will build them into the plan.
Schedule Your Home Addition Contractor Service in San Francisco Today
Call (831) 272-7212 to schedule your free on-site estimate. You will get a written line-item price before any work begins, a background-checked crew with a name and photo sent to you in advance, and a 365-Day Done Right Promise on the finished build. No pressure, no vague numbers, just straight answers from a team that has been building additions in San Francisco since 2006.
Written by Miles Hartwell, Owner at Sutton ADU San Francisco, serving San Francisco since 2006.
