Most listings are in the $20's per square foot per year. The Portsmouth submarket is going through a decrease in rates. In the last 3 months, the median rate fell $7 per sqft per year. That's a 22% decrease in rent.
Compared to the rest of the Greater Boston market, the Portsmouth submarket has higher rates, and you'll have to negotiate above asking rent to be in this hot region.
In the last 90 days, 64 listings have been made available, so you'll want to check back regularly for new listings. About 2% of the available leases in the Portsmouth are subleases, so you will probably need to consider the implications of signing a direct lease with the building. 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 2,500 sqft. Most buildings in this market are close to 2 floors. Most buildings in this area were built in 1920 and are Class A.