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Local, fully licensed home addition experts trusted by San Mateo County since 2004.

Why San Mateo County Home Additions Are Different

Here are the actual, on-the-ground factors that dictate how a home addition is built in our specific market:

  • Strict Lot Constraints: Peninsula neighborhoods (like San Carlos, Belmont, and Burlingame) often have smaller lots that are already maximizing allowed coverage. Before you pay for any design work, we assess what your lot and city zoning will actually accommodate.
  • Varying Soil Conditions: Hillside areas in San Carlos and Bay-adjacent parcels often require a geotechnical engineering report before a foundation can be designed or permitted. We identify this at the initial site visit and build it into your quote—never as a mid-project surprise.
  • Complex Local Permitting: San Carlos, Redwood City, Burlingame, Belmont, Foster City, and San Mateo each have unique building departments, plan check timelines, and fee schedules. We manage every single application, plan check response, and inspection so you never have to navigate the city building department yourself.

How a GKB Addition Gets Built

Most contractor pages claim to "manage your project." Here is exactly what that means when you work with us:

  1. Site Assessment & Feasibility: Before any design or fees, Brian personally walks your property to assess lot constraints, existing structures, and city zoning. You get a direct, honest answer on what is actually realistic.
  2. Design & Engineering: We develop plans built for city approval. For second-story additions and complex builds, we coordinate with licensed structural engineers, building their fees and review timelines into the initial project plan.
  3. Permit Preparation & Submission: We handle all applications, including architectural drawings, structural calculations, and Title 24 energy compliance. We manage the entire city plan check dialogue until your permit is officially issued.
  4. Construction: The same dedicated crew starts and finishes your addition. We coordinate every licensed trade (framing, plumbing, electrical, HVAC) and provide weekly updates so you are never left in the dark.
  5. Final Inspection & Closeout: We schedule the final building department inspection, obtain the closed permit, and complete a final walkthrough. Your property retains a fully permitted, documented record—which is absolutely crucial for resale value.

Types of Home Additions We Build

Home additions don't come in one shape. Here is what we build most often across San Mateo County—and what makes each project type distinct:

Room Additions

Extend your home horizontally with a new bedroom, home office, or family room. These are our most cost-predictable additions since they don’t require altering the existing roof. We design them to connect seamlessly so they look like part of your original house.

Second Story Additions

Ideal for tight lots across the Peninsula, San Carlos, and Belmont. This highly structural scope involves reinforcing your foundation, adding a staircase, and rebuilding the roofline. We partner with licensed structural engineers to ensure these vertical additions are safe and fully permitted.

Primary Suite Additions

Add a spacious master bedroom, private bath, and walk-in closet as a connected unit. These additions dramatically improve your daily life and significantly boost resale value. We seamlessly coordinate the structural framing with the luxury bathroom remodeling scope to deliver a perfect retreat.

In-Law Suites

Add independent or semi-independent living quarters, whether attached to the main house or in a repurposed structure. These flexible spaces range from a simple bedroom-and-bath suite to fully self-contained units featuring their own kitchen, laundry, and private entrance.

Interested in building an in-law unit as a fully permitted Accessory Dwelling Unit (ADU) to unlock legal rental income and maximum property value? We've created a dedicated guide that covers the entire process: ADU Construction & Guest Houses

Learn More About Our Process

What a Home Addition Returns in the Bay Area

The financial case for adding onto a house in San Mateo County is entirely different than the rest of the country. Here is the honest truth about the math:

The Cost of Moving vs. Adding: Buying a larger home on the Peninsula means facing massive transaction costs, new mortgage rates, and uprooting your kids’ schools. A $250,000 addition in San Carlos is frequently a far better financial decision than a move that carries $400,000 to $600,000 in total friction.

The Return on Investment (ROI) at Resale: While the real value is in enjoying your home for the next decade, here is how different additions perform on paper.

  • Bedroom & Family Rooms: Typically return 50–70% of their cost at resale.
  • Second Story Additions: Return approximately 65%, offering the maximum raw square footage per dollar on tight lots where building out isn’t an option.
  • Primary Suite Additions: Return 60–80%. A well-executed master suite dramatically impacts how quickly a home moves in a competitive listing situation.

The Honest Summary: If you’re planning to stay in your home for five or more years, the math almost always favors the addition. We’ll walk through the specific case for your property during your consultation.

Planning an ADU specifically? Accessory Dwelling Unit economics are in a different category because rental income changes the return calculation significantly. ADU Construction & Guest Houses

What Determines the Cost of a Home Addition in San Mateo County?

Bay Area home addition costs run meaningfully higher than the national averages you’ll find on home improvement websites. Current market data puts standard horizontal room additions in the $300–$500+ per square foot range, with second-story projects running higher due to the structural work involved.

Here is exactly what drives where your project lands:

  • Direction (Building Up vs. Building Out): Horizontal additions are typically less expensive per square foot since you aren’t rebuilding a roofline or reinforcing the foundation for a new floor load. However, they require available lot coverage, which many Peninsula properties don’t have.
  • Plumbing, Electrical & Mechanical Scope: A bedroom addition is structurally similar to a primary suite, but adding bathrooms or kitchens involves licensed trades (plumbing, electrical, HVAC). Skilled labor carries a real cost in the Bay Area.
  • Structural Engineering: Required for all second-story additions and complex soil conditions. We include licensed structural engineering fees in your scope. If a contractor quotes a second story without this, they are either leaving the cost out or planning to skip it.
  • Permit Costs & Timelines: San Mateo County cities charge permit fees that scale with project valuation. We build these exact fees and plan check timelines into your budget from day one.
  • Integration with the Existing Home: How the new structure ties in, whether finishes need to perfectly match, and if the addition triggers mechanical upgrades in your original home.

 

Brian provides a realistic cost range during the free in-home consultation—based on your specific property, your city’s requirements, and current Bay Area construction costs—before you commit to anything.

How We Approach Home Addition Pricing

Every quote is itemized, fixed, and given to you in writing before work begins. This matters more on additions than on any other project we take on—the dollar amounts are larger, the timelines are longer, and the consequences of a vague contract are proportionally more severe. No open allowances. No surprise structural discoveries that weren't part of the original scope.

Our home addition projects generally fall into three scopes:

Single-Room Additions: New bedrooms, home offices, family rooms, or attached sunrooms. These are our most cost-predictable scope. Structural complexity is well-defined by the connection to the existing house, and permit requirements are typically straightforward.

Multi-Function Additions: Primary suites and full in-law suites. Because these involve plumbing, electrical upgrades, and HVAC integration, they require more coordination across licensed trades and longer permit timelines—both of which we account for explicitly in your project plan.

Second Story & Complex Additions: Full-scale construction requiring structural engineering, roof removal, potential foundation reinforcement, and complete mechanical systems. These are our most demanding projects. The heavy planning and engineering work we do upfront is exactly why the project executes the way it’s supposed to.

Call Brian at 650-966-4190 to talk through which tier fits your goals and your home.

Home Addition FAQs

It depends on project type. A single-room horizontal addition typically runs 3–5 months from permit approval to final inspection. A second story or multi-function addition runs 5–8 months or longer, depending on structural complexity and your city’s plan check timeline. We build a complete project calendar before work begins — including the permit phase, which is often the most time-variable element. You get an honest schedule, not an optimistic one.

Yes, always. Any addition that changes the footprint, height, or mechanical systems of your home requires permits from your city’s building department — no exceptions across San Mateo County. Genuine Kitchen & Bath handles every permit application, plan check, and inspection as a standard part of every project.

For most room additions and primary suite builds, a licensed architect isn’t legally required in California — a licensed contractor with the appropriate classification can submit plans for permit. Second story additions, hillside builds, and projects with significant structural complexity benefit from an architect’s involvement during the design phase. We work with trusted local architects and structural engineers on projects that warrant it, and we coordinate their scope and fees as part of the overall project plan.

For most horizontal additions, no — we phase the work to keep the existing living space functional throughout. Second story additions require roof removal over a portion of the home, which creates a period of more significant disruption — typically 1–3 weeks with temporary weather protection in place. We discuss the specific construction sequence at the consultation so you can plan around it before work begins.

Yes, incrementally. In California, home improvements trigger a supplemental assessment on the added value only — the existing assessed value of your property is not reassessed. For an addition that adds $200,000 in assessed value at San Mateo County’s approximate rate of 1.1%, the annual tax increase would be roughly $2,200. We’ll give you a direct answer on what to expect for your specific scope at the consultation.

A closed permit is the documented proof that your addition was designed, inspected, and built to code. Buyers’ lenders and agents will ask for it. An unpermitted addition — or one where the permit was pulled but never finaled — creates real problems at sale: lenders may refuse to finance the purchase, and buyers may demand a significant price reduction or walk away. We don’t consider a project finished until the final inspection is signed off and the permit is closed.

What Bay Area Homeowners Say

Rated 5 out of 5

If you are looking for a family owned company that really cares and goes the extra mile for you. Then Genuine Custom Homes is the team you want to hire.

Brian and Dianna along with their team have been fabulous. Always on time and communication is key when you are remodeling your home.

Brian worked with me to stay on budget which is almost impossible, but he did it. We absolutely love our kitchen.

Thank you Brian, Dianna and team. Useful Funny Cool

M. M. - San Carlos, CA

Rated 5 out of 5

Brian and his company, Genuine Custom Homes were fantastic. I am a real estate agent and used them for a countertop installation at a listing I was bringing to market. They sourced the counters we were looking for, met with my client to show them the material, ordered, and installed.

All of this was done professionally while on an extremely tight timeline. Their work is spectacular and the finished product looks amazing.

If you are in the market for custom home work, Genuine Custom Homes are your answer!

Andrew B. - Burlingame, CA

Ready to Add Space to Your San Mateo County Home?

Let’s talk about your addition. Brian will come to your property, walk the site, look at what your lot and your city’s zoning will actually support, and give you a straight answer on what the project will take and what it will cost — before you commit to anything.

Call Brian at 650-966-4190 or request a free in-home consultation online.

Serving San Carlos, Redwood City, Burlingame, Belmont, Foster City, Millbrae, San Mateo, and all of San Mateo County.

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