On average, listings lease in the $70's per square foot per year. The Mill Valley submarket is going through a a boost in rates. In the last 3 months, the median rate rose $17 per sqft per year. That's a 30% rise in rent.
Compared to the rest of the Bay Area market, the Mill Valley submarket has greater rates, and you'll have to negotiate above asking rent to be in this hot region.
In the last 90 days, 5 listings have come on the market, which means listings might be leasing quickly. 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 600 sqft. Most buildings in this market are around 2 floors. Most buildings in this market were built in 1964 and are Class A.