Ben B

From my research, there are generally two opinions on this topic Those that deal with non-solo 401k's will tell you that even with a Solo plan salary deferrals must be withheld from w2 and deadline is 12/31 Those that specialize in Solo plans will usually tell you that even for s corp employee contributions do not need to be withheld from payroll, and can be done lump sum by tax filing deadline (although must be elected by year end and reported on w2 which is usually by Jan of following year) This seems to be one of those areas where there's various IRS material and publications that can be interpreted either way regarding solo plans And to be clear I'm not referring just to internet opinions. I've got both opinions above from professional retirement plan managers, lawyers etc.