Most listings are in the $20's per square foot per year. The Hampstead submarket is seeing a an increase in rates. In the last 3 months, the median rate increased $25 per sqft per year.
In the last 90 days, 2 listings have been listed, which means leases are probably going fast. You'll want to be prepared to tour spaces as quick as possible. 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 1,980 sqft.