S Corp, single company, 401(k) withholding for owners

I have been reading the other threads and trying to apply my situation but I want to be certain so I apologize if this overlaps.  I have a solo company, landscape company.  It's an S Corp. Takeover plan, CPA has been allowing the owners to withhold after year end stating they have until 4/15 but then going back and adjusting their W-2's.  From what I have researched, if it's S Corp, they have to take the withholding from their reasonable compensation by 12/31.  Am I correct?

You are correct and the CPA is wrong.

seconded

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. 

15 minutes ago, Ben B said:

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 is still running it through payroll. The owner in this case is just giving themselves an extra paycheck that is large enough to cover the entire year's deferral in one shot. As you mentioned it still gets included on the W-2. It is also subject to withholding for FICA and other taxes.

And it's still a prohibited transaction if it's deposited late. A one-participant plan is not exempt from IRC 4975.

21 minutes ago, Ben B said:

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

Can you share any of the materials you feel are ambiguous?

5 hours ago, Ben B said:

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. 

I would love to see an actual opinion of someone that says "s corp employee contributions do not need to be withheld from payroll."