With 15% pre-1940 in San Mateo, many properties have room for an ADU alongside the original structure. moisture-related wood rot on north-facing eaves and fascia of 1940s-50s homes. Original wood-frame windows in San Mateo Park show widespread seal failure and paint deterioration. often make ADU construction the ideal time to address deferred maintenance on the primary residence.
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ADU Builder in San Mateo
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For detailed ADU pricing factors, use our 2026 Bay Area ADU cost guide.
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Share your property and project goals. Our team will discuss feasibility, design, permits, and the next planning step.
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Responsive from consultation through project closeout
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ADU Services 
Start With the ADU Service Your Property Needs
Detached ADUs
A new backyard structure planned around the lot, setbacks, access, utilities, fire requirements, privacy, permits, and construction logistics.
ADU Design
Architect-led feasibility and plans shaped by the property, existing home, program, code constraints, utilities, and budget.
ADU Permits
Plan-check coordination for architecture, engineering, energy documents, utility requirements, agency comments, revisions, and inspections.
Garage Conversion ADUs
Conversion planning for the existing slab, framing, ceiling height, fire separation, insulation, utilities, windows, access, and permits.
San Mateo's housing stock spans 15% pre-1940, 30% 1940-1960, 25% 1960-1985. Architectural styles range from 1940s-50s ranch homes and Cape Cods in San Mateo Park and Hillsdale to pre-war Colonials and Tudors in Baywood. Each era and style presents different adu requirements — from material compatibility to structural considerations. Our crews know what to expect before they arrive on site.
Neighborhoods We Serve
ADU Across San Mateo Neighborhoods
Local planning starts with the site, neighborhood context, access, and applicable city review.
Baywood
Baywood homeowners frequently choose Hamilton Exteriors for adu. With 15% pre-1940 in San Mateo, many properties have room for an ADU alongside the original structure. With San Mateo's Moderate Peninsula: summer highs 70-82°F, winter lows 40-48°F, material selection in Baywood requires local expertise.
Hillsdale
In Hillsdale, homes are predominantly pre-war Colonials and Tudors in Baywood. moisture-related wood rot on north-facing eaves and fascia of 1940s-50s homes. Our crews are familiar with Hillsdale's architectural character and building requirements.
Aragon
Aragon lots support various ADU configurations including detached new builds and garage conversions. We evaluate each property's specific constraints and design an ADU that maximizes livable space within San Mateo's zoning requirements.
Fiesta Gardens
Fiesta Gardens offers opportunities for ADU construction on residential lots throughout this San Mateo neighborhood. Our design-build team handles everything from feasibility assessment through permitting and construction.
Beresford
Beresford's location near downtown San Mateo and Caltrain makes ADUs here particularly attractive to renters. We design efficient ADUs that make the most of available space while meeting all San Mateo County requirements.
Project Management How Hamilton Exteriors
How Hamilton Exteriors
Manages Your ADU Project
Property Feasibility
We review the lot, setbacks, existing structures, access, utilities, drainage, fire requirements, and likely permit path.
Design and Permit Coordination
Our team coordinates architecture, engineering, energy documents, utility requirements, submittals, comments, revisions, and approved construction documents.
ADU Construction
We manage site work, foundation, framing, building envelope, mechanical, electrical, plumbing, finishes, inspections, and documented changes.
Final Inspection and Closeout
We coordinate corrections and final inspections, complete the punch list, and organize closeout and applicable warranty information.
Google rating 4.6 ★ from 47 public reviews
ADU Project Reviews
Verified public reviews from homeowners who hired Hamilton Exteriors.
Y
Yaku Escobar
via Google
We worked with Hamilton Exteriors to build a small ADU in our backyard. from the beginning the process felt organized and clear. Alex Lee was very responsive and always kept us in the loop, especially during planning and inspections. The crew was consistent and respectful of the space which we appreciated since we were living there during the build. overall everything moved pretty smoothly and the final result looks exactly like what we had in mind. really happy with how it turned out.
R
Rafa Simons
via Google
My family hired Hamilton after seeing the neigbours new ADU, the whole team were super organized and tecnological and also very patient to undestand my needs before starting the construction. But what i like the best is that they are really preocupated with the final result and they kept us well inform with details and everything. Definitely recomend them.
H
H.T.
via Google
I had a great experience working with Hamilton exteriors. From start to finish, their team was professional, responsive, and detail-oriented. They made the entire process smooth and stress-free, keeping me updated every step of the way. The quality of their work really stands out. Everything was done efficiently and exceeded my expectations. If you’re looking for a reliable company that takes pride in what they do, I highly recommend them!
J
Jessica W.
via BBB
The team was professional, communicative, and completed the job in a timely manner. They kept me informed throughout the entire process and arrived as scheduled. The work was well supervised, and the crew paid close attention to every detail. They left the property clean and organized when the job was finished. Overall, the entire experience was smooth, stress-free, and exceeded my expectations. I would definitely recommend their services to others.
Google rating: 4.6/5 from 47 public reviews
Read reviews on Google Premium Market
ADU Costs in San Mateo
$210,000–$450,000
Median home value in San Mateo: $1,600,000
Prices reflect San Mateo's premium market and San Mateo County labor rates. Actual costs depend on home size, materials, and project complexity. We provide free, itemized estimates with no hidden fees.
Get Your EstimateCompare Broad ADU Project Paths
Swipe to compare all options.
| Feature | Detached ADU | Garage Conversion | Junior ADU |
|---|---|---|---|
| Location | Separate backyard structure | Within an existing garage | Within the primary home |
| Existing shell | New foundation and structure | Existing shell requires verification | Existing conditioned structure |
| Main feasibility focus | Lot, access, setbacks, and utilities | Structure, slab, height, and utilities | Interior layout and required shared systems |
| Privacy planning | Separate building and entrance | Entrance and yard relationship | Connection to the primary home |
| Dedicated guide | Detached ADUs | Garage conversions | Start with an ADU consultation |
ADU Financing
Plan Your ADU Project With Approved Credit
Financing may be available for eligible ADU construction projects with approved credit. During feasibility and scope planning, we explain current lender programs, terms, amounts, and payment timing that may apply to the design and construction path.
Ask which current options fit your ADU project scope.
Call Now (650) 977-3351 Frequently Asked Questions
Didn't find the answer to your question? Call Now (650) 977-3351
What areas do you serve?
We serve Northern California across three regions: the San Francisco Bay Area, Greater Sacramento, and the Central Valley. Our published guides cover 14 counties and 88 cities. Our Castro Valley and Sacramento offices support local estimating, permit coordination, and project planning across these regions.
How does billing and financing work?
Financing may be available for eligible projects with approved credit. Available terms depend on lender approval, project scope, and current programs. Your project manager can review the options that apply before work begins. We also accept checks, ACH, and major credit cards.
Are you licensed and insured?
Yes. Hamilton Exteriors holds CSLB License #1078806, active general liability coverage, and the required contractor bond. We also hold GAF Master Elite and James Hardie Elite Preferred credentials. We can provide current proof of coverage before work begins.
Do I need a permit for my exterior project?
Permit requirements depend on the project scope and local jurisdiction. Hamilton Exteriors coordinates required applications, plan documents, corrections, and inspections. Permit fees and allowances appear in the proposal before construction begins.
How much does an ADU cost in San Mateo?
ADU costs in San Mateo typically range from $150,000 to $350,000 depending on size, finishes, and whether it's a new build or garage conversion. With median home prices at $1,600,000, an ADU can add significant value. We provide free consultations with detailed estimates.
Can I build an ADU on my property in San Mateo?
California's ADU laws make it possible to build on most residential lots in San Mateo. San Mateo County has streamlined the permitting process. We evaluate your property's potential during our free consultation, including setbacks, utility access, and local zoning.
What types of ADUs can I build in San Mateo?
In San Mateo, you can build detached ADUs (up to 1,200 sq ft), garage conversions, attached additions, and junior ADUs (up to 500 sq ft). Many homeowners in Sugarloaf are choosing detached ADUs for rental income or multi-generational living.
What permits do I need for an ADU in San Mateo?
ADU permits in San Mateo go through San Mateo County's building department. California law requires 60-day approval timelines. Hamilton Exteriors handles all permit applications, plan submissions, and inspections. Our CSLB #1078806 license covers the full scope.
Can I rent out my ADU in San Mateo?
Yes, ADUs in San Mateo can be rented at market rates. San Mateo County does not impose rental restrictions on ADUs beyond standard landlord-tenant laws. Many San Mateo homeowners earn $2,500 to $4,000 monthly, making ADUs a strong investment.
Your Next Step
Plan Your Project With a Clear Scope
Financing may be available with approved credit for eligible projects.
Tell us about your project. We usually call within 3–5 minutes to schedule the next step.
21634 Redwood Rd Unit F, Castro Valley, CA 94546
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