The typical asking rate per listing is in the $50's per square foot per year. The Mill Valley region is experiencing a a rise in rates. In the last 3 months, the median rate increased $4 per sqft per year. That's a 8% increase in rent.
Compared to the rest of the Bay Area market, the Mill Valley submarket has more expensive rates, and you'll need to pay a premium to be in this hot submarket.
In the last 90 days, 11 listings have been listed, which means new listings most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. About 9% of the available space in the Mill Valley are subleases, so you will probably need to consider the implications of signing a direct lease with the building. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 250 sqft. Most properties in this region are approximately 2 floors. Most buildings in this region were built in 1964 and are Class A.