 Historically, there have been two types of rent control - vacancy control (where the rent level of a unit is controlled irrespective of whether the tenant remains in the unit or not) and vacancy decontrol (where the rent level is controlled only while the existing tenant remains in the unit).In California prior to 1997, both types were allowed (the Costa/Hawkins bill of that year phased out vacancy control provisions).A 1990 study of Santa Monica, CA showed that vacancy control in that city protected existing tenants (lower increases in rent and longer stability).