When Target's west campus goes on the market this fall, it will be one of the biggest pieces of commercial real estate available in the Twin Cities.

With 400,000 square feet in nine stories, the building is comparable in size to CityPlace in Woodbury, a former regional headquarters of State Farm Insurance.

The metro area's biggest piece of empty office space is Northwest Airlines' former headquarters in Eagan. Its two buildings, totaling 552,000 square feet, were sold last month.

Coming next is the nearly 520,000-square-foot Ameriprise Client Service Center in downtown Minneapolis. That's followed by CityPlace and the Target west campus.

Transactions for such buildings can take years. For instance, Delta Air Lines, which bought Northwest in 2008, put the Eagan complex up for sale in 2009. Like the former Northwest campus, the Target property sits on a broader acreage that may serve to limit the number of potential buyers and, ultimately, tenants.

And Lincoln Corporate Center in Edina, formerly used by UnitedHealth Group, sat empty for a few years before Children's Hospitals and Clinics of Minnesota agreed earlier this year to lease most of its 300,000 square feet.

Buyer or not, Target will move its 1,300 employees next summer to a bigger, newer complex it has been building over the past decade in Brooklyn Park.