The typical asking rate per listing is in the $40's per square foot per year. The San Anselmo submarket is going through a a boost in rates. In the last 3 months, the median rate went up $42 per sqft per year.
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 discounted region.
In the last 90 days, 6 listings have been listed, which means new listings might be leasing quickly. You'll want to be prepared to make an offer immediately. About 50% of the listings in the San Anselmo are subleases, so you might be able to sign a shorter term by taking over a previous lease. 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 990 sqft. Most properties in this region are around 2 stories. Most buildings in this area were built in 1956 and are Class B.