Late Form 8606 with our without Amending

[mattslegacy]

I realize now that I need to go back and File Form 8606 for 2017 (when I made a nondeductible contribution to an IRA), and also for 2018 (when I made additional nondeductible contributions, and converted it all over to Roth).

I have a pretty good handle on how to fill the Form out for each year, but my question is if I need to Amend my return for 2018. I've spent a long time on these forums and the consensus is that it's not necessary for 2017, when I simply made a contribution, but there I've gathered both YES and NO answers for if I need to amend my 2018 return, or simply submit the 8606.

Looking for clarity, please!

[dmertz]

With respect to the changes for 2018, since your Form 8606 must include Part II, I would include  2018 Form 1040X even if the taxable amount of the Roth conversion was zero (which only seems possible if you contributed for 2017 and 2018 simultaneously and immediately converted to Roth, otherwise I would expect as least a small amount of the Roth conversion to be taxable).  This will allow you to explicitly show that there is no change to your taxable income or tax liability.

[mattslegacy]

You are right--I was pretty on top of converted right away, but there was a .69 cent gain throughout 2018. 

So your suggestion would be to include 1040X--even if it is identical, just to cover all my bases?

[dmertz]

A $0.69 gain rounds up to $1, so your 2018 Form 8606 would show $1 as taxable and your Form 1040X needs to show that $1 increase in AGI and taxable income.  There is probably a 2% chance that this $1 of additional taxable income results in an increase in tax liability (due to the stepped nature of the tax tables), but your Form 1040X needs to show this, otherwise the IRS is likely to reject the Form 8606 if filed standalone.  Also, your 2018 tax return should have reported the Roth conversion on Form 8606, so you are not just using the 2018 Form 8606 to report the addition to your basis in nondeductible traditional IRA contributions.  I think the fact that the 2018 Form 8606 includes entries on Part II is also justification for the need to include Form 1040X for 2018.