With 27% pre-1940 in Mill Valley, many properties have room for an ADU alongside the original structure. moss often make ADU construction the ideal time to address deferred maintenance on the primary residence.
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ADU Builder in Mill Valley
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For local cost factors and planning, read our ADU construction cost guide for Mill Valley.
For detailed ADU pricing factors, use our 2026 Bay Area ADU cost guide.
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Start Your ADU Consultation Online
Share your property and project goals. Our team will discuss feasibility, design, permits, and the next planning step.
ADU certifications, brands, and contractor credentials
Customer-Service Obsessed
Responsive from consultation through project closeout
Architect and general contractor
One accountable team
Licensed, bonded, and insured
Verified business credentials
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.
Neighborhoods We Serve
ADU Across Mill Valley Neighborhoods
Local planning starts with the site, neighborhood context, access, and applicable city review.
Blithedale Canyon
Blithedale Canyon homeowners frequently choose Hamilton Exteriors for adu. With 27% pre-1940 in Mill Valley, many properties have room for an ADU alongside the original structure. With Mill Valley's Heavy fog May-September, burns off by mid-morning but returns late afternoon, material selection in Blithedale Canyon requires local expertise.
Tamalpais Valley
Tamalpais Valley's relatively flat lots and generous setbacks make it one of Mill Valley's most practical neighborhoods for detached ADU construction. We design efficient units that maximize rental potential while blending with the neighborhood's casual, family-oriented character.
Alto
Alto homeowners are building ADUs for rental income and multigenerational flexibility. We design hillside-adapted units with separate entries and private outdoor space that take advantage of Alto's elevated terrain and views.
Homestead Valley
Homestead Valley's mix of lot sizes accommodates both garage conversions and small detached ADUs. We navigate the specific permitting requirements for this unincorporated Marin County area and deliver move-in-ready units.
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 Mill Valley
$300,000–$600,000
Median home value in Mill Valley: $2,100,000
Prices reflect Mill Valley's premium market and Marin 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 Mill Valley?
A detached ADU in Mill Valley typically costs $200,000 to $375,000 for a 600 to 800 square foot unit with mid-range finishes (Hamilton Exteriors 2024-2026 project data). Garage conversions run $150,000 to $200,000. Junior ADUs start around $150,000. These numbers include design, engineering, permitting, and construction. Site-specific conditions like hillside grading, retaining walls, and long utility runs can push costs higher.
Do I need a permit to build an ADU in Mill Valley?
Yes. All ADUs in Mill Valley require a building permit from the Mill Valley Building Division. California state law (AB 68, SB 13) has streamlined the process, but you still need approved plans and inspections. We handle the entire permit process from application through final inspection.
How long does it take to build an ADU in Mill Valley?
Plan on 8 to 12 months total from design start to move-in. Design and permitting take 2 to 4 months. Construction runs 4 to 6 months for detached ADUs or 10 to 14 weeks for garage conversions. The permitting phase is the most variable, though state law requires a 60-day decision.
Can I build an ADU on a sloped lot in Mill Valley?
Yes, but sloped lots require additional engineering. Hillside properties in Mill Valley often need stepped footings, retaining walls, and engineered grading. We include a geotechnical soil report and structural engineering in every estimate for sloped lots.
What fire-safety requirements apply to ADUs in Mill Valley?
Mill Valley is in a High or Very High Fire Hazard Severity
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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