Service
Home Addition Contractor in San Jose & the Bay Area
Add the bedrooms, the suite, or the second story your family needs, without giving up the street you love.
UniqHaus designs, engineers, permits, and builds home additions in San Jose, Sunnyvale, Campbell, and across the Bay Area. Most additions run $100,000 to $500,000+, and your itemized price is fixed once you approve it.
Why one team
One studio for the whole addition.
One number you can plan around.
An addition is new construction attached to an old house. That is where budgets slip: the architect draws it, an engineer sizes it, someone else runs the permit, and a contractor prices it last.
When those people work for different firms, every gap between them becomes a change order or a delay, and you are the one making the calls between them. At UniqHaus, the people who design your addition are the same people who price and build it. You get one contract, one schedule, and one team accountable for the result, from first sketch to final build.
- Architecture and 3D design, so you see the new rooms before anything is framed
- Structural engineering for the foundation, framing, and tie-in to your existing house
- Title 24 energy compliance prepared with the drawings, not after plan check asks for it
- Permit submittal and city coordination, including San Jose filings through SJPermits.org
- Construction by our own crew, from new footings to final inspection
What you get in writing
Fixed price
Itemized; fixed once you approve it. No hidden fees.
Warranty
Cosmetic workmanship / structural (foundation and framing)
Consultation
Lot, goals, and budget review. No obligation.
3D design
Credited back when you build with UniqHaus
Licensed and proven
CSLB #1158697 · 21+ years in the Bay Area · 162+ projects · Top 1% of CA contractors (BuildZoom Verified) · 4.9 on Google
Addition types
Five ways to add space to a Bay Area home
The right addition depends on what your family is missing, how much yard you have, and what your lot allows. These are the five we build most often.
- Room addition. A new bedroom, family room, or office of 200-400 sq ft on its own foundation, usually off the back or side of the house. Room addition cost in San Jose
- Second-story addition. A whole new floor that keeps your yard intact. It needs the existing foundation and framing checked and often reinforced first. Second-story cost and feasibility
- Primary suite addition. A bedroom, ensuite bathroom, and walk-in closet planned as one private wing, so the parents finally get a door that closes. Primary suite addition guide
- Bump-out. One wall pushed out a few feet for a larger kitchen, nook, or bathroom, often without a full new foundation. Bump-out and addition cost by type
- Addition with an attached garage. New living space and a garage designed together, so the roofline, driveway, and entry read as one house. See the Bird Ave addition with attached garage
How an addition comes together
Five steps, in this order.
Lot and zoning check. We confirm setbacks, lot coverage, and height limits so you know what your lot can hold before you pay for design.
Design and 3D. You walk through the new rooms in 3D and settle layout and finishes while changes are still free to make on screen.
Engineering and permits. Structural plans, Title 24, and the permit set are prepared in-house, submitted, and taken through plan check by us.
Itemized fixed price. You get a line-by-line price for the approved design. Once you sign, that is the number you build to.
Build. Our crew handles foundation, framing, tie-in, systems, and finishes, with inspections scheduled as each stage closes.
If you ask for a change after approval, we price it as a written change order before any work happens. Nothing shows up on an invoice you did not see first.
Typical addition timeline
Real additions
Three families who stayed and built.
- Hawthorne Ave, Campbell (2025). A whole-home remodel and addition that took the house from about 1,700 to about 3,117 sq ft, with 5 bedrooms, 5 bathrooms, an open floor plan, full electrical and plumbing replacement, and custom cabinetry.
- Bird Ave, San Jose (2021). A major addition and whole-home remodel that more than doubled the home, from about 1,046 to about 2,207 sq ft, with 4 bedrooms, 3 bathrooms, a new porch, a rear patio, and an attached garage.
- Blair, Sunnyvale (2024). A full-home remodel and addition that turned a 3 bed / 1 bath house into 4 bedrooms and 3.5 bathrooms, growing from about 1,518 to about 1,918 sq ft, with every room rebuilt from the studs.
Cost overview
What a home addition costs in the Bay Area.
Expect about $400 to $700+ per finished square foot in 2026. Most of our addition projects land between $100,000 and $500,000+.
Bay Area pricing runs roughly 20-30% above the national average. An addition costs more per foot than a remodel because none of the structure exists yet: new footings, framing, roof, and systems all go into the number. Permits are their own line, often $3,000 to $10,000+ in Santa Clara County for plan check, the building permit, school fees, and any impact fees.
The biggest swings come from three choices: building up instead of out, how much plumbing you add (a bathroom or kitchen costs more than a bedroom of the same size), and finish level. We recommend carrying a 10-20% contingency until the design is final. For the line-by-line breakdown, read our home addition cost guide for the Bay Area.
Bay Area 2026 cost by addition type
Bump-out
Under 150 sq ft
Room addition
Bedroom, family room, or office, 200-400 sq ft
Primary suite
Bedroom, bath, and closet
In-law suite
Attached living, kitchen, and bath
Second story
A whole new floor
Your number
These are planning ranges. Your itemized price comes after design and is fixed once you approve it.
Where we build
Home Additions Across the Bay Area
Each city has its own setbacks, height limits, and plan review queue, and those rules decide whether your addition goes up or out. Our office is at 2570 N First St in San Jose, and we build additions across Santa Clara County and the Peninsula, including San Jose, Sunnyvale, Campbell, Cupertino, Los Gatos, Saratoga, Los Altos, Mountain View, Palo Alto, and Menlo Park.
Still deciding whether to add on or sell? Moving in the Bay Area means roughly 6% in agent commission, transfer taxes, and a pricier replacement home, plus leaving your schools and your property-tax basis behind. Our home addition vs moving guide walks through that math.
Questions
Home Addition FAQ
How much does a home addition cost per square foot in San Jose?
About $400 to $700+ per finished square foot in 2026, roughly 20-30% above the national average. A 200-400 sq ft room addition typically runs $100,000 to $250,000, and a second story runs $200,000 to $500,000+. Kitchens and bathrooms sit at the high end because of plumbing and waterproofing. See our room addition cost guide for San Jose.
How long does a home addition take?
Most additions take 4 to 8 months: 1 to 2 months of design, 2 to 4 months of permitting and plan review, and 3 to 5 months of construction. An addition paired with a whole-house remodel usually runs longer. Starting with a complete, code-ready permit set is the biggest lever on the schedule.
Do I need a permit for a home addition in San Jose?
Yes, always, because you are adding square footage. In San Jose the application is filed through SJPermits.org, and plan check, the building permit, school fees, and possible impact fees often total $3,000 to $10,000+. Plan review commonly takes 2 to 4 months. We prepare and submit the set and answer plan check comments for you. Details are in our home addition permit guide for Santa Clara County.
Is a second story addition better than a ground-floor addition?
It depends on your lot. A second story keeps your yard and works when setbacks or lot coverage leave no room to build out, but it costs more because the existing structure usually needs reinforcement and the roof comes off for a time. A ground-floor addition costs less and keeps single-level living, but it uses yard space. Our second-story addition guide compares both.
Can we live at home during a home addition?
Often yes for a ground-floor addition, since much of the work happens outside the existing walls until the tie-in. A second-story addition often means moving out for part of construction while the roof is open. We tell you which applies during design, so you can plan housing with real dates. See when to move out during a remodel.
Should I build a home addition or an ADU?
Build an addition if your family needs more room inside the main house, such as bedrooms or a primary suite that shares the kitchen and living areas. Build an ADU if you want a separate unit with its own entry and kitchen for parents, adult children, or rental income; ADUs typically run $150,000 to $400,000. Compare both in our ADU vs home addition guide, or see our ADU service.
Free consultation
Outgrowing the house, not the neighborhood?
Tell us about your lot and what your family needs. We will tell you what fits and what it is likely to cost.