Most listings are in the $30's per square foot per year. The Fairfax County submarket is going through a an increase in rates. In the last 3 months, the median rate went up $1 per sqft per year. That's a 2% increase in rent.
In the last 90 days, 31 listings have been made available, which means leases most likley are going fast. You'll want to be prepared to make an offer immediately. About 190% of the available space in the Fairfax County 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,230 sqft. Most buildings in this area are approximately 3 stories. Most buildings in this region were built in 1985 and are Class B.