On average, listings lease in the $20's per square foot per year. The Hampstead submarket is going through a an increase in rates. In the last 3 months, the median rate rose $25 per sqft per year.
In the last 90 days, 2 listings have come on the market, which means leases are probably leasing quickly. You'll want to be prepared to tour spaces as quick as possible. There aren't many listings below 2,000 square feet on the market, 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.