How many housing units in San Francisco are not occupied by a permanent resident? And do such units further constrain the city’s tight housing market and drive up housing prices? In early 2014, SPUR assembled a task force to answer these questions. We reviewed all available data and compared San Francisco to other cities. We found that it was very difficult to exactly quantify the number of non-primary residences in San Francisco. Nevertheless it is possible to provide a rough picture of how many there are and how San Francisco compares to other cities. SPUR recommends that the city gather additional data and continue to track this issue.