On average, listings lease in the $50's per square foot per year. The Napa region is seeing a a rise in rates. In the last 3 months, the median rate went up $51 per sqft per year.
Compared to the rest of the Bay Area market, the Napa region has lower rates, so you should have no trouble finding a great deal in this undervalued submarket.
In the last 90 days, 23 listings have been made available, which means new listings most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 30% of the available space in the Napa 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 available, so finding a space for a small business might be tough.
Even though the median size is 2,990 sqft, there are still 0 listings below 2,000 sqft. Most buildings in this market are around 1 floors. Most buildings in this market were built in 1980 and are Class B.