The actual house did not exist and was built for the movie, then dismantled. In using Google Earth to see the house, enter 41.7142, -87.8901 in the window on the Fly To tab. This will take you to the center of the lake. Zoom to the clearing on the eastern shore. Then select April 29, 2005 as the date.
The name of the exclusive restaurant in the film is "Il Mare". "Il Mare" is also the name of the house and the International title of the Korean film upon which "The Lake House" is based, Siworae (2000).
The song featured at Kate's birthday party is "This Never Happened Before" by Paul McCartney. However, this song was released on his 2005 album Chaos and Creation in the Backyard, which at the time of her party (2004) had not yet been released.
The address of the building in Chicago at which Sandra Bullock lives is 1620 Racine. This would place it in the same block as the apartment belonging to Sean Connery in The Untouchables (1987) (Connery's character lived at 1634 Racine).
The location of the lake house, on Maple Lake, is located in the same Forest Preserve and is approximately 1 kilometer away from the site of the world's first nuclear reactor, CP-1, which was constructed during World War II's Manhattan Project in the Forest Preserve.
The tree in the Middle of the Lake House at the Beginning of the movie is leafless; we only see branches. When Kate looks back at the house one last time, it looks as though the tree is covered in red fall foliage.
The trivia item below may give away important plot points.
Simon Wyler's death date is listed as October 6, 2004. October 6 is also the birthday of Swiss architect Le Corbusier, one of the two architects mentioned as his influences when he designed the Lake House (the other is Frank Lloyd Wright).