Compared to the rest of the Bay Area market, the Mill Valley region has higher rates, and you'll have to negotiate above asking rent to be in this hot region.
In the last 90 days, 11 listings have been posted online, which means leases are probably leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 9% of the listings 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 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 250 sqft. Most properties in this market are around 2 stories. Most buildings in this market were built in 1964 and are Class A.