42 CFR § 409.11 - Bed and board.
(a) Semiprivate and ward accommodations. Except for applicable deductible and coinsurance amounts, Medicare Part A pays in full for bed and board and semiprivate (2 to 4 beds), or ward (5 or more beds) accommodations.
(b) Private accommodations -
(1) Conditions for payment in full. Except for applicable deductible and coinsurance amounts, Medicare Part A pays in full for a private room if -
(i) The patient's condition requires him or her to be isolated;
(iii) The hospital's or CAH's semiprivate and ward accommodations are fully occupied by other patients, were so occupied at the time the patient was admitted to the hospital or CAH, respectively, for treatment of a condition that required immediate inpatient hospital or inpatient CAH care, and have been so occupied during the interval.
(2) Period of payment. In the situations specified in paragraph (b)(1) (i) and (iii) of this section, Medicare pays for a private room until the patient's condition no longer requires isolation or until semiprivate or ward accommodations are available.
(3) Conditions for patient's liability. The hospital or CAH may charge the patient the difference between its customary charge for the private room and its most prevalent charge for a semiprivate room if -
(i) None of the conditions of paragraph (b)(1) of this section is met; and