FEIE & Solo 401k

Expat clients.

I know that taking the foreign earned income exclusion can create a wrinkle with IRAs. That is, if we wipe out all earned income with the FEIE, the client is not eligible to make an IRA contribution. At least, that's my understanding.

If a US citizen client owns a US corporate tax entity (C or S) and pays themselves salary via W-2 during the year, would the client be able to make both EE and ER contributions to a Solo 401k for the tax year if we wipe out the entire W-2 Box 1 via the FEIE?

I suspect we can. A traditional Solo 401k is easy to conceptualize in my mind.

What's a little harder is a Roth Solo 401k, which is perhaps the better planning opportunity.

If a client draws a $100k W-2 salary from the corp, and puts $20k as an EE contribution into a Roth Solo 401k, can we exclude the entire $100k W-2 under the FEIE, or can we only exclude $80k?

The 401(k) contribution would be deducted on the C or S tax return.
Employee deferrals would be noted on the W-2.
$100,000 wages with $20K ROTH 401(k) deferral would be $100K taxable wages in W-2 Box 1, so $100K eligible for foreign earned income exclusion.

Right, ER contribution is on the corp return.

Interesting. So they're paying 0% federal income tax on the Roth EE elective deferral. That's optimal. For some reason I was thinking that it might not be excludeable, and would therefore be subject to a pretty high marginal rate due to the FEIE dynamic.