If block is used under the walls for a garage, there are specific openings needed to fit garage doors and service doors.  A service door requires a minimum block opening 2” larger than the door size.  Example: a standard 36” service door will need a 38” block opening.  A garage door will need an opening 4 ½” larger than the door size.  Example: a 16’ wide door will need a 16’ 4’1/2” opening.  This extra space allows for the frame and rough opening for the service door and additional framing lumber and jambs for the garage door.  These requirements can vary from builder to builder.