Most listings are in the $30's per square foot per year. The Fairfax region is going through a decline in rates. In the last 3 months, the median rate fell $1 per sqft per year. That's a 2% decrease in rent.
Compared to the rest of the Washington D.C. market, the Fairfax region has smaller rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 31 listings have been listed, which means new availabilities are probably leasing quickly. You'll want to be prepared to make an offer immediately. 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 1,150 sqft. Most properties in this region are around 3 stories. Most buildings in this area were built in 1985 and are Class B.