I have driven by this old building a million times but only recently became aware of what it used to be. I actually don't think many people realize it's there. Below gives an explanation:

"Unbelievably, this homely little house that sits at Constitution Avenue and 17th Street near the White House, looking for all the world like a grandiose public toilet, is one of the oldest surviving houses within Washington. Even more incredible, it once guarded the junction of what were at the time major institutional arteries -- the Potomac River and the Washington branch of the Chesapeake & Ohio Canal. Erected about 1835, it housed the keeper of the canal lock between the Washington Canal and the C&O branch, and sat on a narrow spit of land between the canal and the river. Later landfill would place this structure blocks from the Tidal Basin and nearly a half-mile from the Potomac proper."

From "Washington, D.C. Then and Now" by Alexander D. Mitchell IV, 2000 (an excellent reference for the old and new Washington)