Most listings are in the $40's per square foot per year. The San Anselmo region is seeing a an increase in rates. In the last 3 months, the median rate increased $42 per sqft per year.
Compared to the rest of the Bay Area market, the San Anselmo submarket has lower rates, so you should have no trouble finding a great deal in this discounted region.
In the last 90 days, 6 listings have been listed, which means new availabilities are probably going fast. You'll want to be prepared to tour spaces as quick as possible. About 33% of the available leases in the San Anselmo are subleases, so you might be able to find a shorter term or below market rent. 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 780 sqft. Most buildings in this area were built in 1956 and are Class B.