On average, listings lease in the $30's per square foot per year. The Fairfax region is seeing a a rise in rates. In the last 3 months, the median rate rose $31 per sqft per year.
Compared to the rest of the Washington D.C. market, the Fairfax submarket has less expensive rates, so you should have no trouble finding a great deal in this discounted submarket.
In the last 90 days, 31 listings have come on the market, which means new listings most likley are leasing quickly. You'll want to be prepared to make an offer immediately. About 190% of the available leases in the Fairfax 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 1,210 sqft. Most properties in this market are around 3 floors. Most buildings in this area were built in 1985 and are Class B.