The typical asking rate per listing is in the $40's per square foot per year. The Mercer Island submarket is going through a a rise in rates. In the last 3 months, the median rate increased $45 per sqft per year.
Compared to the rest of the Seattle Metropolitan Area market, the Mercer Island region has higher rates, and you'll have to negotiate above asking rent to be in this hot submarket.
In the last 90 days, 1 listings have been made available, which means new availabilities are probably leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 300% of the available leases in the Mercer Island 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,460 sqft. Most properties in this region are around 2 stories. Most buildings in this market were built in 1979 and are Class A.