Complex sentence: The decree 437/97 of the National Executive Branch declared the Avenue a National Historic Site in 1997 and, as a result, the aesthetics of the buildings could not be changed, including billboards and marquees.
Split into simpler sentences: The decree 437/97 of the National Executive Branch declared the Avenue a national historic site in 1997. As a result the aesthetics of the buildings could not be changed, including billboards and marquees.