On average, listings lease in the $30's per square foot per year. The Burien submarket is going through a a boost in rates. In the last 3 months, the median rate rose $13 per sqft per year. That's a 70% rise in rent.
Compared to the rest of the Seattle Metropolitan Area market, the Burien region has greater rates, and you'll have to negotiate above asking rent to be in this hot region.
In the last 90 days, 2 listings have been made available, which means new listings are probably leasing quickly. You'll want to be prepared to make an offer immediately. 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 960 sqft. Most properties in this market are approximately 2 stories. Most buildings in this area were built in 1980 and are Class B.