On average, listings lease in the $30's per square foot per year. The San Anselmo submarket is experiencing a a rise in rates. In the last 3 months, the median rate went up $10 per sqft per year. That's a 32% increase in rent.
Compared to the rest of the Bay Area market, the San Anselmo region has smaller rates, so you should have no trouble finding a great deal in this undervalued submarket.
In the last 90 days, 6 listings have been posted online, which means office spaces most likley are going fast. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet on the market, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,400 sqft. Most buildings in this market were built in 1962 and are Class B.