Internal Revenue Code of 1986 § 4972(c)(6)(B) relieves from counting as nondeductible contributions (for the extra § 4972(a) tax on them):
“so much of the contributions to a simple retirement account (within the meaning of section 408(p)), a simple plan (within the meaning of section 401(k)(11)), or a simplified employee pension (within the meaning of section 408(k)) which are not deductible when contributed solely because such contributions are not made in connection with a trade or business of the employer.”
http://uscode.house.gov/view.xhtml?req=(title:26%20section:4972%20edition:prelim)%20OR%20(granuleid:USC-prelim-title26-section4972)&f=treesort&edition=prelim&num=0&jumpTo=true
Congress enacted this, in Economic Growth and Tax Relief Reconciliation Act of 2001 § 637, to help make it feasible for an employer, even if the employer is not a trade or business, maintain some individual-account retirement plan for domestic workers.
You might advise your client about which of the three kinds of recognized plans, and which benefit structures, fit the needs and interests of the employer of the domestic workers.
For an explanation about how an employer of domestic workers might not be a part of the same § 414(b)-(c)-(m)-(n)-(o) employer as trades or businesses, even if commonly controlled by the same natural person, see Derrin Watson’s Who’s the employer? book.