On average, listings lease in the $30's per square foot per year. The Fairfax County region is going through a a rise in rates. In the last 3 months, the median rate went up $31 per sqft per year.
In the last 90 days, 31 listings have been listed, which means leases are probably going fast. You'll want to be prepared to make an offer immediately. About 190% of the office spaces in the Fairfax County are subleases, so you might be able to find a shorter term or below market rent. 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 1,210 sqft. Most properties in this region are around 3 floors. Most buildings in this region were built in 1985 and are Class B.