On average, listings lease in the $0's per square foot per year. The Cos Cob region is going through a decline in rates. In the last 3 months, the median rate decreased $46 per sqft per year. That's a 100% decrease in rent.
In the last 90 days, 1 listings have been posted online, which means office spaces most likley are leasing quickly. You'll want to be prepared to make an offer immediately. 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 1,850 sqft. Most buildings in this region are around 3 stories. Most buildings in this area were built in 1967 and are Class A.