24 C.F.R. § 8.23

(a)  If alterations are undertaken to a project (including a public housing project as required by § 8.25(a)(2)) that has 15 or more units and the cost of the alterations is 75 percent or more of the replacement cost of the completed facility, then the provisions of § 8.22 shall apply.

Other alterations. to the maximum extent feasible (b)  (1) Subject to paragraph (b)(2) of this section, alterations to dwelling units in a multifamily housing project (including public housing) shall, to the maximum extent feasible, be made to be readily accessible to and usable by individuals with handicaps. If alterations of single elements or spaces of a dwelling unit, when considered together, amount to an alteration of a dwelling unit, the entire dwelling unit shall be made accessible. Once five percent of the dwelling units in a project are readily accessible to and usable by individuals with mobility impairments, then no additional elements of dwelling units, or entire dwelling units, are required to be accessible under this paragraph. Alterations to common areas or parts of facilities that affect accessibility of existing housing facilities shall, to the maximum extent feasible, be made to be accessible to and usable by individuals with handicaps. For purposes of this paragraph, the phrase  shall not be interpreted as requiring that a recipient (including a PHA) make a dwelling unit, common area, facility or element thereof accessible if doing so would impose undue financial and administrative burdens on the operation of the multifamily housing project.

(2) HUD may prescribe a higher percentage or number than that prescribed in paragraph (b)(1) of this section for any area upon request therefor by any affected recipient or by any State or local government or agency thereof based upon demonstration to the reasonable satisfaction of HUD of a need for a higher percentage or number, based on census data or other available current data (including a currently effective Housing Assistance Plan or Comprehensive Homeless Assistance Plan), or in response to evidence of a need for a higher percentage or number received in any other manner. In reviewing such request or otherwise assessing the existence of such needs, HUD shall take into account the expected needs of eligible persons with and without handicaps.