20 CFR § 416.1131 - The one-third reduction rule.
(a) What the rule is. Instead of determining the actual dollar value of in-kind support and maintenance, we count one-third of the Federal benefit rate as additional income if you (or you and your eligible spouse)—
(1) Live in another person's household (see § 416.1132) for a full calendar month except for temporary absences (see § 416.1149), and
(2) Receive both food and shelter from the person in whose household you are living. (If you do not receive both food and shelter from this person, see § 416.1140.)
(b) How we apply the one-third reduction rule. The one-third reduction applies in full or not at all. When you are living in another person's household, and the one-third reduction rule applies, we do not apply any income exclusions to the reduction amount. However, we do apply appropriate exclusions to any other earned or unearned income you receive. If you have an eligible spouse we apply the rules described in § 416.1147.
(c) If you receive other support and maintenance. If the one-third reduction rule applies to you, we do not count any other in-kind support and maintenance you receive.