The policy option of extending mandatory Social Security coverage to newly hired uncovered state and local workers is often included in packages to eliminate the program’s financing shortfall. The arguments for mandatory coverage go beyond financial considerations, though, as extending coverage would bring benefit protections that state and local workers currently lack and would improve equity by more broadly sharing the burden of Social Security’s legacy costs. The main argument against mandatory coverage is that it would raise costs to public employers and workers. The actual cost increase depends on the extent to which employers reduce their existing pensions when adopting Social Security. This paper estimates the costs under four different integration strategies: 1) no adjustment to existing pensions; 2) match the level of the first-year benefit; 3) match the lifetime benefit; and 4) match the benefit to levels in neighboring states with Social Security coverage. This analysis is conducted for 22 state-administered plans in 13 states that were identified as lacking coverage. The results show that the cost of adding Social Security varies significantly, with the smallest increase for the “match lifetime benefit” option and the largest increase for the “no adjustment” option. Presenting the additional costs as a percent of payroll may exaggerate their burden on the employer as the increases will likely be split between employer and employee. Perhaps a better way to gauge the size of the cost increase is as a share of a state’s budget; this measure shows only a very modest impact.