On average, listings lease in the $20's per square foot per year. The Fairfax submarket is going through a a rise in rates. In the last 3 months, the median rate rose $30 per sqft per year.
Compared to the rest of the Washington D.C. market, the Fairfax region has lower rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 31 listings have been made available, which means spaces might be leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 139% of the office spaces in the Fairfax are subleases, so you might be able to snag a great spot on a short term. 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 1,100 sqft. Most buildings in this area are close to 3 stories. Most buildings in this region were built in 1985 and are Class B.