I don’t usually address WH security at all, but I was e-mailed a question about the history of the perimeter fence and ended up finding a very interesting Secret Service report on WH security (hosted by the Federation of American Scientists). It was written at the time of some security incidents in the mid-1990s that resulted in the closing of Penn Ave in front of the mansion. It includes interesting tidbits like the history of presidential security and the number of trespassers in the then-recent past.
UPDATE: The question in question was graciously answered by William Bushong of the White House Historical Association. I reprint his excellent reply here with respect.
While not always restricted to the public, walls and fences have surrounded the White House grounds almost as long as the mansion has been occupied. When John Adams moved into the new house in 1800, construction sheds and debris littered the yard. Trespassers chopped down many of the older trees for firewood. Thomas Jefferson prepared plans in 1803 to cultivate the grounds into lawns, groves and gardens. He cordoned off eight of the 82 available acres within President’s Park erecting a split-rail fence to the north and an eight-foot-high stonewall to the south, thus introducing fences to the White House landscape by 1808.
Though the immediate South grounds became a vegetable and flower garden for the first family, the enclosed North grounds remained open, as did the White House itself —a symbol of Jefferson’s commitment to an open and free democracy. By 1820, President James Monroe had replaced the rail fence with one of black-painted iron, the same style still seen there today. Iron gates on stone posts remained open during daylight hours. However, the stone barrier on the south remained unchanged. Mischievous youths often painted their names on the wall until its removal in 1873.
President Grant built a new iron south fence at the end of his first administration. With time, the South grounds became accessible for public strolls and band concerts, as its gates were open daily. The introduction of May garden parties in the late nineteenth century brought ever more guests to the grounds, offering sweets, music, and a cherished visit by the president to those invited. One event, however, left the South gates closed for some time. During his second term, Grover Cleveland’s beloved daughter Ruth was plucked out of her baby carriage on the south lawn by a group of visiting women and passed around to receive their coddling and kisses. So alarmed was Mrs. Cleveland that the South gates were ordered locked, not to be reopened for some years. After the bombing of Pearl Harbor in 1941 and the start of World War II, the grounds were closed to all but those with appointments and guarded at their perimeters from newly installed gatehouses. The driveway was emptied and the custom of leaving calling cards at the North door was discontinued.
Today, copies of Monroe’s fence surround the White House and its 18 acres of lawn and gardens. While ever-present security concerns usually keep the grounds closed to all but invited guests, the public is still welcomed in three times a year for fall and spring garden tours and for one of the White House’s most historic traditions – the White House Easter Egg Roll.