Buildings built in the Federal style frequently don't appear very different from their Georgian predecessors. Some new elements were introduced such as small porticoes over doorways and occasionally larger projecting porticoes echoing Greek Temples. There were also interior innovations such as oval rooms like those found in both the Woodlands and Lemon Hill. Many of these features can be seen in America's greatest Federal house - The White House in Washington, DC. Less obvious differences are a matter of nuance - larger panes of glass surrounded by thinner muntin strips in the windows; the cornices don't overhang as much and are plainer (often without brackets) and windows may be larger with less wall space between. Overall, the effect on the exterior is one of a simpler and lighter elegance.

These small changes however were caused by a fundamental shift in architectural theory. The Georgian style was based on British interpretations of renaissance models which meant that Georgian architects saw the world through renaissance eyes. Palladio and other members of the Italian renaissance community had idealized ancient architecture so much that they had determined that there was a single correct Doric or Ionic order. Even when archeological variations showed up, renaissance architects assumed that the builders of the ancient building had somehow erred.

The English speaking architectural world changed when, in the 1750's, a pair of British gentleman - one a painter, James Stuart, and the other one an architect, Nicholas Revett, visited Greece and published a set of measured drawings of the Antiquities of Athens. This monumental work, perhaps the most influential architectural book in the English language, made it clear that there is no such thing as a single Doric or Ionic order but instead the ancients had freely interpreted Doric and Ionic proportions and details.
The Federal Style
At the same time that Stuart and Revett were traveling in Greece, there were also new archaeological discoveries in Pompeii which altered our understanding of Roman architecture, particularly of roman interiors. Two Scottish architects, Robert and James Adam, capitalized on these new ideas and began to produce lavish interiors mixing classical details with rich jewel like color schemes. In Philadelphia, the Museum of Art has a drawing room from Lansdowne House by the Adam brothers which is not to be missed.

What all this meant was that starting in about 1755 in England and in about 1790 in the new United States, architects began to build buildings based on the archaeological understanding of the ancient world rather than the ancient world seen through the eyes of the Italian renaissance. The Adam style in England and the Federal style in the United States were a transition between renaissance architecture and the long series of revival styles which persisted through the Victorian era and well into the 20th century.