On average, listings lease in the $30's per square foot per year. The Napa region is going through a a boost in rates. In the last 3 months, the median rate went up $0 per sqft per year. That's a 1% rise in rent.
In the last 90 days, 33 listings have been listed, which means listings are probably 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 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 1,440 sqft. Most properties in this area are approximately 1 floors. Most buildings in this region were built in 1983 and are Class A.