Most listings are in the $20's per square foot per year. The Portsmouth region is going through a an increase in rates. In the last 3 months, the median rate rose $22 per sqft per year.
Compared to the rest of the Greater Boston market, the Portsmouth region has smaller rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 64 listings have come on the market, so you'll want to check back regularly for new listings. About 55% of the office spaces in the Portsmouth are subleases, so you might be able to find a shorter term or below market rent. 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 2,500 sqft. Most buildings in this area are around 2 stories. Most buildings in this market were built in 1942 and are Class A.