Most listings are in the $60's per square foot per year. The Mill Valley region is going through a decrease in rates. In the last 3 months, the median rate fell $10 per sqft per year. That's a 13% decline in rent.
Compared to the rest of the Bay Area market, the Mill Valley region has more expensive rates, and you'll have to negotiate above asking rent to be in this hot region.
In the last 90 days, 4 listings have been listed, which means new listings might be going fast. You'll want to be prepared to tour spaces as quick as possible. 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 area are approximately 2 stories. Most buildings in this area were built in 1964 and are Class A.