On average, listings lease 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 $31 per sqft per year.
In the last 90 days, 31 listings have been made available, which means leases are probably leasing quickly. You'll want to be prepared to tour spaces as quick as possible. 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 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 close to 3 floors. Most buildings in this area were built in 1985 and are Class B.