The typical asking rate per listing is in the $30's per square foot per year. The Napa region is experiencing a decline in rates. In the last 3 months, the median rate decreased $4 per sqft per year. That's a 11% decline in rent.
In the last 90 days, 38 listings have been posted online, which means new listings most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. 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 1,610 sqft. Most properties in this region are around 1 floors. Most buildings in this area were built in 1983 and are Class A.