401(k) plans for Household Employees

Can a household employer establish a 401(k) plan on behalf of the household employees & be exempt from the 4972 10% excise tax on nondeductible contributions if the plan restricts contributions to only elective deferrals? Elective deferrals are not taken into account for purposes of Sec 404 deduction limits, so I'm assuming this can be done.

A Simple plan would be the alternative, but they require employer contributions.

Household employee qualified plans have been around for years.  The deductible limits don't change, but the ability to take a deduction is restricted.  Hence, no 4972 that wouldn't otherwise apply.  Don't have time to look up chapter and verse.  If you need that maybe somebody else can provide.

An elective deferral only plan for household employees would be permitted & the ADP test & 410(b) coverage would apply?

Sure, but I've never seen one where a household employee earned over the HCE threshold.  Uncharted waters, there.