Looking for an addition contractor in Mill Valley, CA? Describe your project once and up to three addition contractors who work in Mill Valley and Marin County get back to you with a quote. It's free and there is no obligation.
Mill Valley is home to about 13,904 people (2024 Census Bureau estimate), a shrinking community (−2.3% since 2020). With 2,907 people per sq mi, it is a suburban area. Growing families here often add space instead of moving.
Mill Valley at a glance
- Population (Census Bureau estimate, 2024)
- 13,904 (−2.3% since 2020)
- Rank in California
- 335th of 482 cities and towns
- Land area
- 4.8 sq mi
- Population density
- 2,907 people per sq mi (suburban)
- Type
- City (Mill Valley city)
- Main county
- Marin County
What to check before hiring in Mill Valley
- Zoning: check Mill Valley's setbacks, lot coverage and height limits first.
- License and permit: in California, additions are built by licensed contractors under a permit.
- Structure: second stories need an engineer to check the existing house.
- Price drivers: size, foundation, roof tie-in, plumbing and finishes.
Useful official resources
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ICC: International Code Council
Official information and guidance.
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NAHB
Official information and guidance.
Services available in Mill Valley
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Room addition
Ground-floor addition with foundation, framing, roof and finishes.
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Second-story addition
New floor over the existing house, with structural engineering.
Frequently asked questions in Mill Valley
How much does an addition cost in Mill Valley, CA?
It depends on size, foundation and finishes. Describe your project: contractors who serve Mill Valley reply with a quote.
How long does it take?
Often a few months, plus design and permits.
Does an addition raise property taxes?
Usually, since it increases the assessed value.
Which addition contractors will contact me?
Up to three independent addition contractors who serve Mill Valley and the surrounding area, chosen for the service you need. Your details are shared only with your consent.
Source: U.S. Census Bureau, Population Estimates Program (Vintage 2024) and Gazetteer Files (public domain), imported on 10/31/2026. Figures describe the incorporated place or county as published; they are shown for information only.