The Washtenaw County Administration Building is a former post office located at 220 North Main Street in Ann Arbor, Michigan.

[1] In 1903, Congress authorized funds to construct this building to house the postal services for Ann Arbor.

Plans for the new building were based on those drawn up by the staff at the US Treasury Department, and architect Fremont Ward supervised the construction.

The builders C. Hoertz & Son of Grand Rapids, Michigan constructed the building.

The original 1906 building was only five bays wide; the 1932 addition was carefully designed to seamlessly integrate both parts together.