The typical asking rate per listing is in the $40's per square foot per year. The San Anselmo region is going through a a rise in rates. In the last 3 months, the median rate increased $13 per sqft per year. That's a 34% rise in rent.
Compared to the rest of the Bay Area market, the San Anselmo region has higher rates, and you'll need to make a great offer to be in this hot submarket.
In the last 90 days, 2 listings have come on the market, which means new listings most likley are leasing quickly. You'll want to be prepared to make an offer immediately. 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 1,200 sqft. Most buildings in this market were built in 2004 and are Class B.