Complex sentence: The exterior of the House has not been significantly changed since 1779 although, in the first decade of the 19th century, the "Egyptian Hall" was added beneath the "North Portico" as a secondary entrance.
Split into simpler sentences: The exterior of the House has not been significantly changed since 1779. In the first decade of the 19th century the "Egyptian Hall" was added beneath the "North Portico" as a secondary entrance.