Most listings are in the $30's per square foot per year. The Napa region is seeing a decrease in rates. In the last 3 months, the median rate decreased $3 per sqft per year. That's a 7% decline in rent.
Compared to the rest of the Bay Area market, the Napa region has less expensive rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 14 listings have been listed, which means office spaces are probably going fast. 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 1,320 sqft. Most buildings in this market are close to 1 stories. Most buildings in this region were built in 1978 and are Class B.