Failure to submit to the searches, which can last an hour, disqualifies applicants from assistance.
The 10-year-old program was challenged by the American Civil Liberties Union on behalf of six single parents who were seeking assistance. The welfare applicants argued that the Fourth Amendment, which prohibits unreasonable searches, protects them from the home visits.
"When the investigator conducts the home inspection, no part of the home is off-limits," they said.
The 9th U.S. Circuit Court of Appeals, upholding the program, said the Supreme Court in 1971 allowed social workers to visit homes in New York to determine eligibility. The appeals court, in a 2-1 decision, said the visits do not even constitute a search under the Fourth Amendment in part because people are free to turn away the investigators.
In dissent, however, Judge Raymond Fisher said it was unlawful for an investigator from the district attorney's office to go "walking through the applicant's home in search of physical evidence of ineligibility that could lead to criminal prosecution either for welfare fraud or other crimes unrelated to the welfare application."
The local government said the high court should not step in.
"No applicant has been prosecuted for welfare fraud based upon anything observed or discovered during a home visit that contradicted information provided by the applicant," the county said in its brief for the Supreme Court.
Eight appeals court judges voted to have the full San Francisco-based court hear the case. Seven of those judges called the program "an attack on the poor."