What building is on the back of the 5 dollar bill?

There is not a building on the back of the one dollar bill. There are however an unfinished pyramid and an eagle. You must mean the pyramid in your question.

The reverse of the seal on the left features a barren landscape dominated by an unfinished pyramid of 13 steps, topped by the Eye of Providence within a triangle. At the base of the pyramid are engraved the Roman numerals MDCCLXXVI (1776), the date of American independence from Britain. At the top of the seal stands a Latin phrase , "ANNUIT COEPTIS," meaning "He (God) favors our undertaking." At the bottom of the seal is a semicircular banner proclaiming "NOVUS ORDO SECLORUM" meaning "New Order of the Ages," which is a reference to the new American era. To the left of this seal, a string of 13 pearls extends toward the edge of the bill.