Most listings are in the $50's per square foot per year. The Mill Valley submarket is seeing a a rise in rates. In the last 3 months, the median rate rose $55 per sqft per year.
Compared to the rest of the Bay Area market, the Mill Valley submarket has higher rates, and you'll need to pay a premium to be in this hot submarket.
In the last 90 days, 11 listings have been posted online, which means spaces most likley are leasing quickly. You'll want to be prepared to make an offer immediately. About 9% of the available leases in the Mill Valley are subleases, so you will probably need to sign a longer term lease with the landlord directly. 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 250 sqft. Most buildings in this region are approximately 2 floors. Most buildings in this region were built in 1964 and are Class A.