The typical asking rate per listing is in the $30's per square foot per year. The San Anselmo 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 20% decrease in rent.
Compared to the rest of the Bay Area market, the San Anselmo submarket has less expensive rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 1 listings have come on the market, which means listings might be going fast. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet available, 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 region were built in 2004 and are Class B.