The 406,309-sf building sits on a 106,400-sf site at 15th and Wynkoop Streets. It's across the street from the LoDo Tattered Covered Bookstore - which has received national attention as one of the best and largest bookstores in the country. It's an easy walk to dozens of restaurants and lofts, some which have sold for more than $400 per sf. The annex also borders the Central Platte Valley, where billions of dollars in high-end residential is either under construction or on the drawing board and a class-A office project by Legacy Partners is about to break ground.

"Without a doubt, this is the biggest and most important site to go on the market in LoDo," says Art Seiden, who is listing the project with fellow Fuller and Co. broker Terry Matthews. Seiden expects the property to be redeveloped into a mixed-use project that could include housing, retail, office and perhaps a hotel.

The postal service is selling the property because it no longer needs the building and it expects to get an excellent price because of the hot LoDo market. While an asking price wasn't set, real estate experts estimate the building could fetch $8.5 million to $15.5 million. If the property would be razed and built out to its maximum density of almost 800,000 sf, it could have a completed value of about $181 million, real estate experts estimate.