The Imperial Senate was originally called the Presidential Palace or Palace of the Republic during the reign of the Galactic Republic. For most of the Republic's early years, until at least 10,000 BBY, it was used as the official residence of the Chancellor. Before and after that date, it served as a series of offices and chambers for the Galactic Senate.
It was included on the list of the Twenty Wonders of the Universe by the historian Vicendi in 10,000 BBY.
The Imperial Palace is currently the residence of Emperor Palpatine during his current reign over the Galactic Empire. He drastically rebuilt the Palace into a massive complex that has made it the tallest building on Coruscant.