Serving the Bay Area
ADU Builders in San Mateo: Add a Legal Unit, Add Real Value
Bayside Home Builders designs and builds detached ADUs, attached ADUs, and garage conversions in San Mateo with permits handled start to finish.
adu project · San MateoSan Mateo Lots Are Smaller Than They Look on Paper
Peninsula lots tend to be compact, and many San Mateo homes already use most of their rear yard for decks, landscaping, or existing structures. That does not automatically rule out an ADU, but it means your options need to be evaluated before you spend money on plans.
What you are living with
Homeowners who skip a real site assessment end up paying for drawings that do not match what the city will actually approve. Setback requirements, lot coverage limits, and utility easements can all shrink the buildable area. Finding that out after design is finished is an expensive way to learn it.
What you get instead
We start with a site review that looks at your lot dimensions, existing coverage, setbacks, and utility locations before we draw anything. If a detached ADU does not fit, we can evaluate a garage conversion or a JADU inside the existing footprint. You get a straight answer on what will work and what it will cost before committing to a full design contract.
Our Recent Work
Accessory Dwelling Units Built to Code, Start to Finish
An ADU adds a fully independent living space to your property, a detached backyard cottage, an attached unit, or a converted garage. We handle every piece of it: design, permits, foundation, framing, MEP rough-ins, finishes, and final inspection. One contractor accountable for the whole project.
Site Assessment and Feasibility Review
Before you spend money on drawings, we walk the property, check setbacks, utility access, and lot coverage limits so you know what is actually buildable on your specific parcel.
Custom Design and Construction Documents
Our in-house design team produces permit-ready architectural and structural drawings sized to your lot and matched to your goals, whether that is a rental unit, space for family, or a home office.
Permit Procurement
We prepare and submit the full permit package and manage agency responses, corrections, and resubmittals. You do not have to track down planners or decode comment letters.
Foundation and Structural Work
Depending on the ADU type, this means a new concrete foundation, reinforced slab, or engineered upgrades to an existing structure. All work is inspected and documented.
Full MEP Systems
Electrical panel upgrade or sub-panel, new plumbing supply and waste lines, HVAC or mini-split installation, and smoke and CO detection, all pulled under separate trade permits and signed off by inspectors.
Interior Finishes and Final Inspection
Kitchen, bath, flooring, doors, and trim completed to a livable, rentable standard. We walk the unit with the building inspector and hand you a passed final inspection card before we close out the job.
Why Homeowners Choose Bayside Home Builders for Their ADU
A lot of contractors will frame an ADU. Fewer can carry it from raw dirt to a certificate of occupancy without the homeowner having to manage three separate companies. Here is what sets our process apart.
Licensed Design-Build Under One Roof
We are a CA CSLB licensed, bonded, and insured contractor with in-house design staff. Your architect and your builder are on the same team from day one, which eliminates the finger-pointing that derails projects when design and construction are separate contracts.
Permit Experience Across Multiple Jurisdictions
ADU permit requirements vary significantly from one municipality to the next. We have worked through the planning and building departments in multiple cities and know how to prepare packages that move through review without unnecessary back-and-forth.
Family-Owned with Direct Superintendent Oversight
As a family-owned company, the people who sold you the project are the same people managing it in the field. You reach a decision-maker directly, not a call center, when you have a question mid-construction.
Transparent Scope Before You Sign
We provide a written scope of work that breaks out site work, structural, MEP, and finishes so you know what is included and what would constitute a change order. Vague contracts are the leading cause of ADU budget overruns.
What our clients say
We recently had our kitchen remodeled by Bayside Home Improvement, and overall, we’re really happy with how everything turned out. The kitchen looks completely refreshed, and the space is much more pleasing to walk into. The team was professional…
I had the pleasure of working with Samuel on my kitchen and home remodel and I am thrilled about how it came out. His team was great in getting the designs together and listening to our ideas and interests. We went with an open layout design which…
How an ADU Project Works With Us
ADUs involve more moving parts than a typical remodel. Here is the honest sequence of how we take a project from your first call to a finished, permitted unit.
Step 1: Site Visit and Feasibility Conversation
We schedule a visit to your property, review your parcel data, discuss what type of ADU fits your goals and your lot constraints, and give you a realistic picture of timeline and process before any money changes hands.
Step 2: Design, Pricing, and Contract
We produce a design that meets local zoning and building code requirements and prepare a fixed-scope proposal. Once you approve the design and sign the contract, we finalize construction documents and submit for permits.
Step 3: Permit Processing and Pre-Construction
We manage the permit queue and respond to any agency comments. While permits are in review, we lock in subcontractors and order long-lead materials so construction starts without delay once approvals are in hand.
Step 4: Construction, Inspections, and Closeout
We build to the approved plans with all required inspections scheduled and passed at each phase. At completion, we walk the unit with you, address any punch list items, and hand over your certificate of occupancy and all warranty documentation.
What Adu Costs
ADU costs in San Mateo depend on the project type, your lot, and what the city's permitting process adds to the timeline.
Attached ADU
Converting or adding living space within the existing footprint.
Garage conversion
Turning an existing garage into a permitted dwelling.
Detached DADU
A standalone backyard cottage, built to code and permitted.
San Mateo has its own permit fee schedule and review process separate from the county, so the clock and cost start there. The City of San Mateo reviews ADU applications through its planning and building departments, and older housing stock common in established San Mateo neighborhoods often adds scope once walls are opened. Garage conversions tend to move faster than new detached builds because the structure is already there, but condition of the existing slab and utilities still shapes the final number.
ADU Contractors Serving San Mateo, CA
San Mateo sits in one of the most expensive housing markets in the country. A permitted ADU adds a rentable unit to your property and real appraised value. We handle design, permits, and construction so you deal with one contractor, not three.
Permits & review
San Mateo City charges a flat fee for ADU permits, roughly $2,831 for a new unit and $2,031 for a remodel or conversion, based on current city fee schedules. You still need separate plan check, utility connection, and school fees on top of those figures. San Mateo County also runs an ADU Resource Center that can help homeowners understand the process before breaking ground. We pull all permits and manage the city review from submittal through final inspection.
Housing stock
San Mateo has a mix of postwar ranch homes, split-levels, and bungalows, many with attached or detached garages that convert well to ADUs. Lot sizes in established neighborhoods often have enough rear yard to support a detached unit without a variance. Older foundations and aging utilities sometimes need upgrades once walls open up, and we scope that work before you sign a contract.
Areas we serve
We serve San Mateo and the surrounding Peninsula cities including Foster City, Burlingame, Belmont, and San Carlos. If your property is in San Mateo County, call us and we can tell you quickly whether the project pencils out.
Common questions
How much does it cost to build a 1,200 sq ft ADU in San Mateo?
A 1,200 sq ft detached ADU in San Mateo typically runs $350,000 to $500,000 fully built out, depending on site conditions, finishes, and utility connections. San Mateo City charges a flat permit fee of roughly $2,800 for a new ADU unit, but total soft costs including engineering, architecture, and school fees commonly add $30,000 to $60,000 on top of construction. Garage conversions cost less, usually $150,000 to $250,000, because the structure already exists. Get a site-specific estimate before budgeting off any number you read online.
Can I build an ADU in my backyard in San Mateo?
Yes. California state law requires San Mateo to allow at least one detached ADU on any single-family lot. The city sets a maximum height of 16 feet for detached ADUs, requires 4-foot side and rear setbacks, and limits size to 850 sq ft for a studio or one-bedroom and up to 1,000 sq ft for two or more bedrooms. Lots with an existing structure may also qualify for a Junior ADU inside the main home. We pull your parcel data and confirm what applies before design starts.
What are the drawbacks of building an ADU?
The honest ones: upfront costs are high and financing options are limited compared to a standard home loan. Construction timelines in the Bay Area commonly run 12 to 18 months from permit application to move-in. If your utility laterals are undersized, San Mateo may require upgrades to water and sewer lines, which adds cost and time. Renting the unit also means becoming a landlord, with local tenant protections that apply once a tenant is established. These are real considerations, not reasons to walk away, but they should be part of your planning.
What are the new ADU laws in California for 2025 and beyond?
California has been expanding ADU rights steadily. As of recent legislation, owner-occupancy requirements cannot be enforced on most ADU permits, impact fees are waived for ADUs under 750 sq ft, and cities must allow ADUs on lots with multi-family buildings in addition to single-family lots. Bills moving through the legislature for 2026 continue to push local governments toward faster ministerial approval. We stay current on these changes so your project is designed to take full advantage of what the law allows in San Mateo today.
How long does it take to build an ADU in San Mateo?
Plan on 12 to 18 months for a detached ADU from the start of design through construction completion. San Mateo's building department targets plan check timelines of 4 to 8 weeks for ADUs under state law, but corrections and resubmittals can extend that. Construction itself on a new detached unit typically runs 4 to 8 months once permits are issued. Garage conversions are faster, often 6 to 10 months total. The biggest delays we see are owner decision-making at the design stage and utility company scheduling, not the build itself.
Do I need an architect for an ADU in San Mateo?
San Mateo requires stamped plans from a licensed architect or structural engineer for permit submission. Some contractors advertise pre-approved stock plans that skip this step, but those plans still require site-specific structural and Title 24 energy compliance drawings. As a design-build firm, we handle architecture and engineering in-house, which keeps handoffs tight and avoids the delays that happen when a contractor and a separate architect are not coordinating on your timeline.
Can I use my ADU as a short-term rental in San Mateo?
San Mateo's short-term rental rules limit platforms like Airbnb to owner-occupied primary residences, and the ADU itself cannot be the primary residence for this purpose. In practice, most ADUs in San Mateo are rented long-term. Long-term rental income for a one-bedroom ADU in San Mateo currently ranges from roughly $2,200 to $3,200 per month depending on size and finish level. That figure comes from current market listings, not a projection, and it can shift. We are not financial advisors, but we can connect you with a local property manager who can give you a realistic rental estimate for your specific address.
Keep exploring
Other services in San Mateo
Bathroom Remodeling→Kitchen Remodeling→Home Renovation→Home Addition→Home Builders→
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