24 C.F.R. § 982.505 : Housing and Urban Development — REGULATIONS RELATING TO HOUSING AND URBAN DEVELOPMENT — OFFICE OF ASSISTANT SECRETARY FOR PUBLIC AND INDIAN HOUSING, DEPARTMENT OF HOUSING AND URBAN DEVELOPMENT — SECTION 8 TENANT-BASED ASSISTANCE: HOUSING CHOICE VOUCHER PROGRAM — RENT AND HOUSING ASSISTANCE PAYMENT — How to calculate housing assistance payment

(a) Use of payment standard. A payment standard is used to calculate the monthly housing assistance payment for a family. The "payment standard" is the maximum monthly subsidy payment.
(b) Amount of monthly housing assistance payment. The PHA shall pay a monthly housing assistance payment on behalf of the family that is equal to the lower of:
(1) The payment standard for the family minus the total tenant payment; or
(2) The gross rent minus the total tenant payment.
(c) Payment standard for family.
(1) The payment standard for the family is the lower of:
(i) The payment standard amount for the family unit size; or
(ii) The payment standard amount for the size of the dwelling unit rented by the family.
(2) If the PHA has established a separate payment standard amount for a designated part of an FMR area in accordance with §982.503 (including an exception payment standard amount as determined in accordance with §982.503(b)(2) and §982.503(c)), and the dwelling unit is located in such designated part, the PHA must use the appropriate payment standard amount for such designated part to calculate the payment standard for the family. The payment standard for the family shall be calculated in accordance with this paragraph and paragraph (c)(1) of this SECTION.
(3) Decrease in the payment standard amount during the HAP contract term. If the amount on the payment standard schedule is decreased during the term of the HAP contract, the PHA is not required to reduce the payment standard amount used to calculate the subsidy for the families under HAP contract for as long as the HAP contract remains in effect.
(i) If the PHA chooses to reduce the payment standard for the families currently under HAP contract during the HAP contract term in accordance with their administrative plan, the initial reduction to the payment standard amount used to calculate the monthly housing assistance payment for the family may not be applied any earlier than the effective date of the family's second regular reexamination following the effective date of the decrease in the payment standard amount.
(ii) The PHA may choose to reduce the payment standard amount for families that remain under HAP contract to the current payment standard amount in effect on the PHA voucher payment standard schedule, or may reduce the payment standard amount to an amount that is higher than the normally applicable payment standard amount on the PHA voucher payment standard schedule. The PHA may further reduce the payment standard amount for the families during the term of the HAP contract, provided the subsequent reductions continue to result in a payment standard amount that meets or exceeds the normally applicable payment standard amount on the PHA voucher payment standard schedule.
(iii) The PHA must provide the family with at least 12 months' notice that the payment standard is being reduced during the term of the HAP contract before the effective date of the change.
(iv) The PHA shall administer decreases in the payment standard amount during the term of the HAP contract in accordance with the PHA policy as described in the PHA administrative plan. The PHA may establish different policies for designated areas within their jurisdiction ( e.g., for different zip code areas), but the PHA administrative policy on decreases to payment standards during the term of the HAP contract applies to all families under HAP contract at the time of the effective date of decrease in the payment standard within that designated area. The PHA may not limit or otherwise establish different protections or policies for certain families under HAP contract.
(4) Increase in the payment standard amount during the HAP contract term. If the payment standard amount is increased during the term of the HAP contract, the increased payment standard amount shall be used to calculate the monthly housing assistance payment for the family beginning at the effective date of the family's first regular reexamination on or after the effective date of the increase in the payment standard amount.
(5) Change in family unit size during the HAP contract term. Irrespective of any increase or decrease in the payment standard amount, if the family unit size increases or decreases during the HAP contract term, the new family unit size must be used to determine the payment standard amount for the family beginning at the family's first regular reexamination following the change in family unit size.
(d) PHA approval of higher payment standard for the family as a reasonable accommodation. If the family includes a person with disabilities and requires a payment standard above the basic range, as a reasonable accommodation for such person, in accordance with part 8 of this title, the PHA may establish a payment standard for the family of not more than 120 percent of the FMR. A PHA may establish a payment standard greater than 120 percent of the FMR by submitting a request to HUD.

24 C.F.R. §982.505

64 FR 26649, 5/14/1999, as amended at 64 FR 56914, 10/21/1999; 65 FR 16822, 3/30/2000; 65 FR 42509, 7/10/2000; 66 FR 30568, 6/6/2001; 67 FR 56689, 9/4/2002; 80 FR 8247, 2/17/2014; 81 FR 12376, 3/8/2016; 81 FR 80582, 1/17/2017